diff --git a/.github/workflows/check.yml b/.github/workflows/check.yml
index ee48b77..a634c61 100644
--- a/.github/workflows/check.yml
+++ b/.github/workflows/check.yml
@@ -13,7 +13,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
-          node-version: "20"
+          node-version: "24"
       - run: npm ci
       - run: npx biome ci
   client-build:
@@ -23,7 +23,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
-          node-version: "20"
+          node-version: "24"
       - run: npm ci
       - run: cd client && bash build.sh
   server-build:
@@ -33,6 +33,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
-          node-version: "20"
+          node-version: "24"
       - run: npm ci
       - run: cd server && npm run build
+      - run: cd server && npm run typecheck
diff --git a/Dockerfile b/Dockerfile
index ca9c6f3..e5e6a78 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM node:20 AS builder
+FROM node:24 AS builder
 WORKDIR /app
 
 COPY package*.json ./
@@ -10,7 +10,7 @@ COPY common ./common
 COPY server ./server
 RUN npm run build:server
 
-FROM node:20-slim
+FROM node:24-slim
 WORKDIR /app
 
 RUN apt-get update && apt-get install -y openssl && rm -rf /var/lib/apt/lists/*
diff --git a/client/src/pages/Home.tsx b/client/src/pages/Home.tsx
index 88cbe76..52700f3 100644
--- a/client/src/pages/Home.tsx
+++ b/client/src/pages/Home.tsx
@@ -7,7 +7,7 @@ import Footer from "../components/Footer";
 import Header from "../components/Header";
 import { briefProjectReviver } from "../revivers";
 import type { BriefProject } from "../types";
-import { API_ENDPOINT } from "../utils";
+import { API_ENDPOINT, readErrorMessage } from "../utils";
 
 const client = hc<AppType>(API_ENDPOINT);
 
@@ -29,16 +29,7 @@ export default function HomePage() {
           const parsedData = data.map((p) => briefProjectReviver(p));
           setInvolvedProjects(parsedData);
         } else {
-          let errorMessage = "イベントの取得に失敗しました。";
-          try {
-            const data = await res.json();
-            const err = data as unknown as { message: string }; // Middleware のレスポンスは Hono RPC の型に乗らない
-            if (typeof err.message === "string" && err.message.trim()) {
-              errorMessage = err.message.trim();
-            }
-          } catch (_) {
-            // レスポンスがJSONでない場合は無視
-          }
+          const errorMessage = await readErrorMessage(res, "イベントの取得に失敗しました。");
           setToast({
             message: errorMessage,
             variant: "error",
diff --git a/client/src/pages/Project.tsx b/client/src/pages/Project.tsx
index 4f236fb..498d491 100644
--- a/client/src/pages/Project.tsx
+++ b/client/src/pages/Project.tsx
@@ -24,7 +24,7 @@ import { EXTERNAL_LINKS } from "../constants/links";
 import dayjs from "../lib/dayjs";
 import { projectReviver } from "../revivers";
 import type { Project } from "../types";
-import { API_ENDPOINT, FRONTEND_ORIGIN } from "../utils";
+import { API_ENDPOINT, FRONTEND_ORIGIN, readErrorMessage } from "../utils";
 
 const client = hc<AppType>(API_ENDPOINT);
 
@@ -59,15 +59,7 @@ export default function ProjectPage() {
         const parsedData = projectReviver(data);
         setProject(parsedData);
       } else {
-        let errorMessage = "プロジェクトの取得に失敗しました。";
-        try {
-          const data = await res.json();
-          if (data && typeof data.message === "string" && data.message.trim()) {
-            errorMessage = data.message.trim();
-          }
-        } catch (_) {
-          // レスポンスがJSONでない場合は無視
-        }
+        const errorMessage = await readErrorMessage(res, "プロジェクトの取得に失敗しました。");
         setToast({
           message: errorMessage,
           variant: "error",
@@ -254,17 +246,10 @@ export default function ProjectPage() {
         });
         setTimeout(() => setToast(null), 3000);
       } else {
-        let errorMessage = "更新に失敗しました。";
-        try {
-          const data = await res.json();
-          if (data && typeof data.message === "string" && data.message.trim()) {
-            errorMessage = data.message.trim();
-          } else if (res.status === 403) {
-            errorMessage = "権限がありません。";
-          }
-        } catch (_) {
-          if (res.status === 403) errorMessage = "権限がありません。";
-        }
+        const errorMessage = await readErrorMessage(
+          res,
+          (res.status as number) === 403 ? "権限がありません。" : "更新に失敗しました。",
+        );
         setToast({
           message: errorMessage,
           variant: "error",
diff --git a/client/src/pages/eventId/Submission.tsx b/client/src/pages/eventId/Submission.tsx
index c78f267..ae99d81 100644
--- a/client/src/pages/eventId/Submission.tsx
+++ b/client/src/pages/eventId/Submission.tsx
@@ -21,7 +21,7 @@ import { Calendar, type Highlight } from "../../components/Calendar";
 import Header from "../../components/Header";
 import { projectReviver } from "../../revivers";
 import type { Project, Slot } from "../../types";
-import { API_ENDPOINT } from "../../utils";
+import { API_ENDPOINT, readErrorMessage } from "../../utils";
 
 const client = hc<AppType>(API_ENDPOINT);
 
@@ -103,15 +103,7 @@ export default function SubmissionPage() {
         const parsedData = projectReviver(data);
         setProject(parsedData);
       } else {
-        let errorMessage = "プロジェクトの取得に失敗しました。";
-        try {
-          const data = await res.json();
-          if (data && typeof data.message === "string" && data.message.trim()) {
-            errorMessage = data.message.trim();
-          }
-        } catch (_) {
-          // レスポンスがJSONでない場合は無視
-        }
+        const errorMessage = await readErrorMessage(res, "プロジェクトの取得に失敗しました。");
         setToast({
           message: errorMessage,
           variant: "error",
diff --git a/client/src/utils.ts b/client/src/utils.ts
index f128965..1b3a1d4 100644
--- a/client/src/utils.ts
+++ b/client/src/utils.ts
@@ -1,2 +1,18 @@
 export const API_ENDPOINT = import.meta.env.VITE_API_ENDPOINT || "http://localhost:3000";
 export const FRONTEND_ORIGIN = import.meta.env.VITE_FRONTEND_ORIGIN || "http://localhost:5173";
+
+/**
+ * エラーレスポンスの `{ message }` を読む。読めなければ `fallback` を返す。
+ * サーバーの onError やミドルウェアが返すエラーは Hono RPC の型に乗らないので、型を付けずに読む。
+ */
+export async function readErrorMessage(res: { json(): Promise<unknown> }, fallback: string): Promise<string> {
+  try {
+    const data = await res.json();
+    if (data && typeof data === "object" && "message" in data && typeof data.message === "string") {
+      return data.message.trim() || fallback;
+    }
+  } catch (_) {
+    // レスポンスがJSONでない場合は無視
+  }
+  return fallback;
+}
diff --git a/package-lock.json b/package-lock.json
index d40398c..c5c2180 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -241,23 +241,23 @@
       }
     },
     "node_modules/@babel/helpers": {
-      "version": "7.29.7",
-      "resolved": "https://registry.npmjs.org/@babel/helpers/-/helpers-7.29.7.tgz",
-      "integrity": "sha512-1k2lAGRMfHTcwuNYcCNUmaUffmQv8KWMfh2iJUUeRlwlwH4FdNG7mfPI10NPfLHJFThE4Tyr4mv7kTNZOiPuBg==",
+      "version": "7.29.10",
+      "resolved": "https://registry.npmjs.org/@babel/helpers/-/helpers-7.29.10.tgz",
+      "integrity": "sha512-EMCx3AGZ3WfAl6ht7AyQoPm2Lh+BGYEMdc4Q6Hf7uSCDOZFuLq44qNwL+iaim8VlyYhRAo4X43+weadK8ZB4Yg==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
         "@babel/template": "^7.29.7",
-        "@babel/types": "^7.29.7"
+        "@babel/types": "^7.29.8"
       },
       "engines": {
         "node": ">=6.9.0"
       }
     },
     "node_modules/@babel/parser": {
-      "version": "7.29.8",
-      "resolved": "https://registry.npmjs.org/@babel/parser/-/parser-7.29.8.tgz",
-      "integrity": "sha512-E8lTAYNB1KW+FH+VGJuZM1ioAx2E6oVlvQFRrf5P8ZZmsiJXYAD9vTFV7yyEURNzgh1dFqMZuO6tUwcARbqFCA==",
+      "version": "7.29.9",
+      "resolved": "https://registry.npmjs.org/@babel/parser/-/parser-7.29.9.tgz",
+      "integrity": "sha512-CjXrNHTnvqBVqHgdBysY3vk2T8tpJHb5/RMeHJBTyVa9xgugCB0CJTx/3oO8RV2QRQP391RWpB7D6hLjm8V9uA==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
@@ -318,16 +318,16 @@
       }
     },
     "node_modules/@babel/traverse": {
-      "version": "7.29.8",
-      "resolved": "https://registry.npmjs.org/@babel/traverse/-/traverse-7.29.8.tgz",
-      "integrity": "sha512-I5z7H3bf/41ktsNVLtpN0wAa336HkqIHQ5BuPLEhTkt1jVSyZpeNKIzTgEWmlxjdg81R0IgUCcaE+Ok3NvrfZg==",
+      "version": "7.29.10",
+      "resolved": "https://registry.npmjs.org/@babel/traverse/-/traverse-7.29.10.tgz",
+      "integrity": "sha512-B3lYAs2nkKskbgQ1nfy8Ma9ao9Q1rntTAQHrzzyDuj+TSO+X3le6PfWwqDvodsb+OumE10eMFfuMCTQSQ5iuIQ==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
         "@babel/code-frame": "^7.29.7",
         "@babel/generator": "^7.29.8",
         "@babel/helper-globals": "^7.29.7",
-        "@babel/parser": "^7.29.8",
+        "@babel/parser": "^7.29.9",
         "@babel/template": "^7.29.7",
         "@babel/types": "^7.29.8",
         "debug": "^4.3.1"
@@ -1170,9 +1170,9 @@
       "license": "MIT"
     },
     "node_modules/@rollup/rollup-android-arm-eabi": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-android-arm-eabi/-/rollup-android-arm-eabi-4.63.1.tgz",
-      "integrity": "sha512-UZ8sUxPTiHWYX9QNdJedb1kDZSpS1t/VPWBWGSgqHNi9w3Cu6IXvu2mzbhiTiPvtrqgTQJ+zqiAq2iPIPilpaQ==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-android-arm-eabi/-/rollup-android-arm-eabi-4.64.12.tgz",
+      "integrity": "sha512-Z8lz+5UOiCJLonjeqwJlhUtHA/Sr6R1AidkccQxdWF0nigNzP81aLoU+udgOaGfL5s1uzFXzcux/pCwGAMu0lw==",
       "cpu": [
         "arm"
       ],
@@ -1183,9 +1183,9 @@
       ]
     },
     "node_modules/@rollup/rollup-android-arm64": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-android-arm64/-/rollup-android-arm64-4.63.1.tgz",
-      "integrity": "sha512-cQ4nFQABN5cDvDpbvJ7bMStCpnaVxynZrRMfUJYgxcIk9Sh54FIO1vtfkg0B69REjER77ioZ/ov+eAApx/KmLQ==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-android-arm64/-/rollup-android-arm64-4.64.12.tgz",
+      "integrity": "sha512-+tKxNa2psFmiTb0KfXlXs4Yj7vkEhzAvZ0DrQ5+0gWHQJM10jSleQ3l1FIVm5uvD53YM8lHfM4mgCLDDN9f1Gg==",
       "cpu": [
         "arm64"
       ],
@@ -1196,9 +1196,9 @@
       ]
     },
     "node_modules/@rollup/rollup-darwin-arm64": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-darwin-arm64/-/rollup-darwin-arm64-4.63.1.tgz",
-      "integrity": "sha512-FQNqd1lRy/0QhDk3xeRIkSBiCpXCiDnZO3YLVdcDKN1UBiKToNftCzcXYNLshmPDUMlu2TdeS8tGcsU6f3YF1Q==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-darwin-arm64/-/rollup-darwin-arm64-4.64.12.tgz",
+      "integrity": "sha512-iiripdfvyRBugV55/Dk7GplNXQtN/PG85OcKeveU1z2UkTg0MofbBF1qk//qgPWiqJbVvQcXAVCfE56oDGO4EQ==",
       "cpu": [
         "arm64"
       ],
@@ -1209,9 +1209,9 @@
       ]
     },
     "node_modules/@rollup/rollup-darwin-x64": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-darwin-x64/-/rollup-darwin-x64-4.63.1.tgz",
-      "integrity": "sha512-pvD16V939D3CloK0+qikpGaxiPrDUXTe7Y5cWOMkMSy7m1cawa8EGy/kXYi/G/cKAC4HDAbSnzCIk1WmsoOKXg==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-darwin-x64/-/rollup-darwin-x64-4.64.12.tgz",
+      "integrity": "sha512-BmATib8swSMAagWNIfDZMnb+k34RXR4bbGOUfZgK8lLlniuMyVoSDDXCOGkdhtUmBXL4aygDh7bhjQ+TmhTvIw==",
       "cpu": [
         "x64"
       ],
@@ -1222,9 +1222,9 @@
       ]
     },
     "node_modules/@rollup/rollup-freebsd-arm64": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-freebsd-arm64/-/rollup-freebsd-arm64-4.63.1.tgz",
-      "integrity": "sha512-pcFGeL2345VwdTnJhA6zLbew+YgWB0qBG2+dMtXjCicf6+rm6kO6cOoh5VnTe0ZMrMRgRyuHmCJxZWrIdzYuOw==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-freebsd-arm64/-/rollup-freebsd-arm64-4.64.12.tgz",
+      "integrity": "sha512-GzrO0CiUq8vcAI/hYJuIcp6wNm3T3YhOpumYtf9Z3Fz+cfN1VESCVfy7EPcM3/m2K8l+Gn2U+4XtNZjwhUXctQ==",
       "cpu": [
         "arm64"
       ],
@@ -1235,9 +1235,9 @@
       ]
     },
     "node_modules/@rollup/rollup-freebsd-x64": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-freebsd-x64/-/rollup-freebsd-x64-4.63.1.tgz",
-      "integrity": "sha512-mRJlqSRulVzcKq/LKA6ICSIc3K/l4fzlVn/gePn2nXIHy8seRi5z/eeRE0d/XMBxcMldiXtQTSpRj0tkkC3g8Q==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-freebsd-x64/-/rollup-freebsd-x64-4.64.12.tgz",
+      "integrity": "sha512-Dpta6gigm4o6SYioXtTkZBlGLA/Sl6OpzgLqY+51uafJE2blPejAtR1w82CVLPN+F9cvo1+/em92x/zcOgY0uQ==",
       "cpu": [
         "x64"
       ],
@@ -1248,9 +1248,9 @@
       ]
     },
     "node_modules/@rollup/rollup-linux-arm-gnueabihf": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-arm-gnueabihf/-/rollup-linux-arm-gnueabihf-4.63.1.tgz",
-      "integrity": "sha512-YDUNvVM85TI3g/1OpnqKP1h4NeW/j64DfWMf+G3M809xNk1bJSnpFp4sh83NpmVE5DXnkh8ULor4LTVZKoYLHw==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-arm-gnueabihf/-/rollup-linux-arm-gnueabihf-4.64.12.tgz",
+      "integrity": "sha512-YpeDwQ5uFaogRvytNC8RK0keDO65kI5wNEhGSs+4GiJc3KHhxj4j5cnQh2nCu9ybJj6RP0/ycB6bkN0yFJ9I+w==",
       "cpu": [
         "arm"
       ],
@@ -1264,9 +1264,9 @@
       ]
     },
     "node_modules/@rollup/rollup-linux-arm-musleabihf": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-arm-musleabihf/-/rollup-linux-arm-musleabihf-4.63.1.tgz",
-      "integrity": "sha512-7Mcn71p9ZuQFAj+h+dhQXy/yeLePRS2yKRnmW1DijA9thKO5qap0GNOIQK4yQ6iP3SU0Mrb/yWo8h8vgRba8lw==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-arm-musleabihf/-/rollup-linux-arm-musleabihf-4.64.12.tgz",
+      "integrity": "sha512-LQrB3/sImIr1ycrlEyaApOLH1rAy8JV4/dH4eoK817aWATdcs4fESXz53/w72NwDnwPM5zEUpoL7socH4ZDnwA==",
       "cpu": [
         "arm"
       ],
@@ -1280,9 +1280,9 @@
       ]
     },
     "node_modules/@rollup/rollup-linux-arm64-gnu": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-arm64-gnu/-/rollup-linux-arm64-gnu-4.63.1.tgz",
-      "integrity": "sha512-4YiLQTX6U4CSl0L9cluep9A9W6UmTfqBDc2/CH6wlu54pl4E7Jn3cOD8oxzvBDEGk/JMKgJ47C8g+radF7mwvg==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-arm64-gnu/-/rollup-linux-arm64-gnu-4.64.12.tgz",
+      "integrity": "sha512-iAF2iYu9S4SBfNsb4k0ewNBjO83WxHEltIuKQBGMzEKFgrYfmJn+gatBDXR2t/0zJN0UW0kzR4rfvQ1UHzkZ1A==",
       "cpu": [
         "arm64"
       ],
@@ -1296,9 +1296,9 @@
       ]
     },
     "node_modules/@rollup/rollup-linux-arm64-musl": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-arm64-musl/-/rollup-linux-arm64-musl-4.63.1.tgz",
-      "integrity": "sha512-2ra8F7w8OquwZN9z2/fKFnli69wa8PLwaVzRMIPGb13ByMJwC28Fbp8YcVGoUhlYMTt7j5j9bNgpysrN2UM+vw==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-arm64-musl/-/rollup-linux-arm64-musl-4.64.12.tgz",
+      "integrity": "sha512-0MQdRtdbirJCXPIVeW/hd/LXLR8Qz96pc+2EBptuuV5q/zSGOzND7SZt8gBTE58NC9cUG/8UxcUXQiKRZ2WzzA==",
       "cpu": [
         "arm64"
       ],
@@ -1312,9 +1312,9 @@
       ]
     },
     "node_modules/@rollup/rollup-linux-loong64-gnu": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-loong64-gnu/-/rollup-linux-loong64-gnu-4.63.1.tgz",
-      "integrity": "sha512-Sy20ncyhjmBP0Ml+UvQbimjlk6VFgjW5uNP+qqwHB00mTE8Bl2C1TuHTlRwK2YoXeZbee5lP2XevBWVkAQAtSQ==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-loong64-gnu/-/rollup-linux-loong64-gnu-4.64.12.tgz",
+      "integrity": "sha512-yGDEwn0AGnrY/KCz2oY8R9KKNiQnTcwNuCWXeSXF0VLxXzlqoCgaS5QHsf1lbQvKWwPuN2Czkjk1wXRCBONTOQ==",
       "cpu": [
         "loong64"
       ],
@@ -1328,9 +1328,9 @@
       ]
     },
     "node_modules/@rollup/rollup-linux-loong64-musl": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-loong64-musl/-/rollup-linux-loong64-musl-4.63.1.tgz",
-      "integrity": "sha512-noITLp8oNjYliPnGWmLyelIHwULGqbHloQHGw1rtxbWhTuWooRpnZarZQJ1y9EUC4szuCusCc+HEpUtxpIwYvA==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-loong64-musl/-/rollup-linux-loong64-musl-4.64.12.tgz",
+      "integrity": "sha512-AjlbD0efZEhHaJ3bsNjxSFDoguNq6VZvC+FKGJGBCA+ZN7BrZq2TuuSWFa6AI8IpOeuvhp2LoYcP7jiZwmtU0Q==",
       "cpu": [
         "loong64"
       ],
@@ -1344,9 +1344,9 @@
       ]
     },
     "node_modules/@rollup/rollup-linux-ppc64-gnu": {
-      "version": "4.63.1",
-      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-ppc64-gnu/-/rollup-linux-ppc64-gnu-4.63.1.tgz",
-      "integrity": "sha512-hlxxXd+F1mWiAcaFR7Sv9ZQT6m6UfI8+Vy/kFJzztq2pDMU/0wZ9sish0iszNZvsQDo8Gc0i5yuFEOz5dDf6fA==",
+      "version": "4.64.12",
+      "resolved": "https://registry.npmjs.org/@rollup/rollup-linux-ppc64-gnu/-/rollup-linux-ppc64-gnu-4.64.12.tgz",
+      "integrity": "sha512-sDW0CiPoUCEEUV25uBG5sJ2gKe75DJqLpj91m1eK1vKffywyY7v4i6q1450ZuR6QW4NArXg7JpbMGgKU8di3HQ==",
       "cpu": [
         "ppc64"
       ],
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+      "version": "0.28.0",
+      "resolved": "https://registry.npmjs.org/scheduler/-/scheduler-0.28.0.tgz",
+      "integrity": "sha512-juorfCmIkIw8tT+p5BXSm6PJjQF/ycEYmKyzURCIt/RaZIhL+PulbQ9Yu2z1HdOJDdqDTlxA1+xKBmHXJsczAw==",
       "license": "MIT"
     },
     "node_modules/semver": {
@@ -3140,14 +3235,21 @@
       "license": "MIT"
     },
     "node_modules/source-map-js": {
-      "version": "1.2.1",
-      "resolved": "https://registry.npmjs.org/source-map-js/-/source-map-js-1.2.1.tgz",
-      "integrity": "sha512-UXWMKhLOwVKb728IUtQPXxfYU+usdybtUrK/8uGE8CQMvrhOpwvzDBwj0QhSL7MQc7vIsISBG8VQ8+IDQxpfQA==",
+      "version": "1.2.2",
+      "resolved": "https://registry.npmjs.org/source-map-js/-/source-map-js-1.2.2.tgz",
+      "integrity": "sha512-KGj/8Y43x35aZVDtt+J4mK1hoLGHULMYfSkODJNQjNDC3oW1PqPoxMwo0pLUsWM/UEGzON/NxeHywEfNXNP3Vw==",
       "license": "BSD-3-Clause",
       "engines": {
         "node": ">=0.10.0"
       }
     },
+    "node_modules/std-env": {
+      "version": "4.3.0",
+      "resolved": "https://registry.npmjs.org/std-env/-/std-env-4.3.0.tgz",
+      "integrity": "sha512-OtU/EgQ1kIm5KwqQpBC6ZEMXrZRui11w8zgfTWp8cdO9B8OaPsbA8bTHO2P+HNo1VlUTGMVBwPhydu6poeXiag==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/tailwindcss": {
       "version": "4.3.3",
       "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.3.3.tgz",
@@ -3173,6 +3275,16 @@
       "integrity": "sha512-Eet/eeMhkO6TX8mnUteS9zgPbUMQa4I6Kkp5ORiBD5476/m+PIRiumP5tmh5ioJpH7k51Kehawy2UDfsnxxY8Q==",
       "license": "MIT"
     },
+    "node_modules/tinybench": {
+      "version": "6.2.2",
+      "resolved": "https://registry.npmjs.org/tinybench/-/tinybench-6.2.2.tgz",
+      "integrity": "sha512-nIZ2N2dFpjBsg2hgitKu7mA+FvsIJuGJOwysRngmVRRmWgR/UyPjwJj8c/fT/dhm8EkHnF5EOogRf+0Z2qrimA==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=20.0.0"
+      }
+    },
     "node_modules/tinyexec": {
       "version": "1.3.1",
       "resolved": "https://registry.npmjs.org/tinyexec/-/tinyexec-1.3.1.tgz",
@@ -3206,9 +3318,9 @@
       "license": "MIT"
     },
     "node_modules/tsx": {
-      "version": "4.23.13",
-      "resolved": "https://registry.npmjs.org/tsx/-/tsx-4.23.13.tgz",
-      "integrity": "sha512-BL5MGkRln6aDYhb0xbQlEAGw743BaZYWdbWtdJOBriYJboKgUUYCadFp2/FpBBZquBC/ezNBn7wMMPx7FDZUDw==",
+      "version": "4.23.15",
+      "resolved": "https://registry.npmjs.org/tsx/-/tsx-4.23.15.tgz",
+      "integrity": "sha512-Yiex1Ovn8z2xPpOWckIiysV1SSyRMY9BkLF++q0yKiDxCqRhosKfMg3janKkiLBwZ5c/YryloKwGZcrEmtwxKw==",
       "devOptional": true,
       "license": "MIT",
       "dependencies": {
@@ -3251,16 +3363,16 @@
       }
     },
     "node_modules/undici-types": {
-      "version": "6.21.0",
-      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-6.21.0.tgz",
-      "integrity": "sha512-iwDZqg0QAGrg9Rav5H4n0M64c3mkR59cJ6wQp+7C4nI0gsmExaedaYLNO44eT4AtBBwjbTiGPMlt2Md0T9H9JQ==",
+      "version": "7.24.6",
+      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-7.24.6.tgz",
+      "integrity": "sha512-WRNW+sJgj5OBN4/0JpHFqtqzhpbnV0GuB+OozA9gCL7a993SmU+1JBZCzLNxYsbMfIeDL+lTsphD5jN5N+n0zg==",
       "devOptional": true,
       "license": "MIT"
     },
     "node_modules/update-browserslist-db": {
-      "version": "1.3.2",
-      "resolved": "https://registry.npmjs.org/update-browserslist-db/-/update-browserslist-db-1.3.2.tgz",
-      "integrity": "sha512-UQ+MSxlhRm1bzjhU+DcuXfjFO1FzNtqhK5+9Yvlp90ItDLk5vT932A0rFu619nf7RVS+Y/VeaUW1jaRDqZ8VJw==",
+      "version": "1.3.4",
+      "resolved": "https://registry.npmjs.org/update-browserslist-db/-/update-browserslist-db-1.3.4.tgz",
+      "integrity": "sha512-t8wonIOdQyOjtPd2VktlBu6QjTVTks6v8bSi7kd0r2Vll8TK1JYDZtRRdbmCHOAMphkZgrzK+L+vXK4pyZ8t4A==",
       "dev": true,
       "funding": [
         {
@@ -3289,9 +3401,9 @@
       }
     },
     "node_modules/vite": {
-      "version": "6.4.3",
-      "resolved": "https://registry.npmjs.org/vite/-/vite-6.4.3.tgz",
-      "integrity": "sha512-NTKlcQjlAK7MlQoyb6LgaqHc8sso/pVyUJYWMws3jg21uTJw/LddqIFPcPqP6PzpgbIcZyKI85sFE4HBrQDA8A==",
+      "version": "6.4.5",
+      "resolved": "https://registry.npmjs.org/vite/-/vite-6.4.5.tgz",
+      "integrity": "sha512-4enwjFfi4IwtEh0w+jPzpLVOFQiPHmmzPxcHXQOyEur47o6u0jE573DmGpjA+7irXoMLYRZD5ZaFUEneS3abPg==",
       "license": "MIT",
       "dependencies": {
         "esbuild": "^0.25.0",
@@ -3819,6 +3931,140 @@
         "@esbuild/win32-x64": "0.25.12"
       }
     },
+    "node_modules/vitest": {
+      "version": "5.0.5",
+      "resolved": "https://registry.npmjs.org/vitest/-/vitest-5.0.5.tgz",
+      "integrity": "sha512-cPODOMF8YnkiiYtDdHo6n4wWmBrOBBbOIYH2d33M4l3CdNn72h+qMmO6uXffUN6A5AZFXNND/d+krS9mgkXRfg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/chai": "^5.2.2",
+        "@vitest/mocker": "5.0.5",
+        "chai": "^6.3.0",
+        "es-module-lexer": "^2.3.2",
+        "expect-type": "^1.4.0",
+        "magic-string": "^1.4.3",
+        "obug": "^3.0.0",
+        "picomatch": "^4.0.7",
+        "std-env": "^4.3.0",
+        "tinybench": "^6.2.2",
+        "tinyexec": "^1.3.1",
+        "tinyglobby": "^0.2.17",
+        "why-is-node-running": "3.2.1"
+      },
+      "bin": {
+        "vitest": "vitest.mjs"
+      },
+      "engines": {
+        "node": "^22.12.0 || ^24.0.0 || >=26.0.0"
+      },
+      "funding": {
+        "url": "https://opencollective.com/vitest"
+      },
+      "peerDependencies": {
+        "@edge-runtime/vm": "*",
+        "@opentelemetry/api": "^1.9.0",
+        "@types/node": "^22.0.0 || >=24.0.0",
+        "@vitest/browser-playwright": "5.0.5",
+        "@vitest/browser-preview": "5.0.5",
+        "@vitest/browser-webdriverio": "^5.0.0-beta.5 || >=5.0.0",
+        "@vitest/coverage-istanbul": "5.0.5",
+        "@vitest/coverage-v8": "5.0.5",
+        "@vitest/ui": "5.0.5",
+        "happy-dom": "*",
+        "jsdom": "*",
+        "vite": "^6.4.0 || ^7.0.0 || ^8.0.0"
+      },
+      "peerDependenciesMeta": {
+        "@edge-runtime/vm": {
+          "optional": true
+        },
+        "@opentelemetry/api": {
+          "optional": true
+        },
+        "@types/node": {
+          "optional": true
+        },
+        "@vitest/browser-playwright": {
+          "optional": true
+        },
+        "@vitest/browser-preview": {
+          "optional": true
+        },
+        "@vitest/browser-webdriverio": {
+          "optional": true
+        },
+        "@vitest/coverage-istanbul": {
+          "optional": true
+        },
+        "@vitest/coverage-v8": {
+          "optional": true
+        },
+        "@vitest/ui": {
+          "optional": true
+        },
+        "happy-dom": {
+          "optional": true
+        },
+        "jsdom": {
+          "optional": true
+        },
+        "vite": {
+          "optional": false
+        }
+      }
+    },
+    "node_modules/vitest/node_modules/@vitest/mocker": {
+      "version": "5.0.5",
+      "resolved": "https://registry.npmjs.org/@vitest/mocker/-/mocker-5.0.5.tgz",
+      "integrity": "sha512-xLL3lkDegJZw+MGoGaCCsFayDZthGgpAtr3Vziyfc4p7xH89K7KRPcRBBh/kEYm0Aqkt5DtTmkYpBtRV/FrHZg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@jridgewell/trace-mapping": "0.3.31",
+        "@vitest/spy": "5.0.5",
+        "estree-walker": "^3.0.3",
+        "magic-string": "^1.4.3"
+      },
+      "funding": {
+        "url": "https://opencollective.com/vitest"
+      },
+      "peerDependencies": {
+        "msw": "^2.4.9 || ^3.0.0",
+        "vite": "^6.0.0 || ^7.0.0 || ^8.0.0"
+      },
+      "peerDependenciesMeta": {
+        "msw": {
+          "optional": true
+        },
+        "vite": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/vitest/node_modules/magic-string": {
+      "version": "1.4.3",
+      "resolved": "https://registry.npmjs.org/magic-string/-/magic-string-1.4.3.tgz",
+      "integrity": "sha512-z12OxmaPGE0F4xlpGdjuAUckipLpQlTAuAmyGhNoD7ODpYUzvDfvtxUbjM/jwznyP178oju/KDdyi220wNJ0RQ==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@jridgewell/sourcemap-codec": "^1.6.0"
+      }
+    },
+    "node_modules/why-is-node-running": {
+      "version": "3.2.1",
+      "resolved": "https://registry.npmjs.org/why-is-node-running/-/why-is-node-running-3.2.1.tgz",
+      "integrity": "sha512-Tb2FUhB4vUsGQlfSquQLYkApkuPAFQXGFzxWKHHumVz2dK+X1RUm/HnID4+TfIGYJ1kTcwOaCk/buYCEJr6YjQ==",
+      "dev": true,
+      "license": "MIT",
+      "bin": {
+        "why-is-node-running": "cli.js"
+      },
+      "engines": {
+        "node": ">=20.11"
+      }
+    },
     "node_modules/yallist": {
       "version": "3.1.1",
       "resolved": "https://registry.npmjs.org/yallist/-/yallist-3.1.1.tgz",
@@ -3860,10 +4106,11 @@
         "zod": "^3.24.2"
       },
       "devDependencies": {
-        "@types/node": "^22.13.10",
+        "@types/node": "^24.19.2",
         "prisma": "^6.5.0",
         "tsx": "^4.19.3",
-        "typescript": "^5.8.2"
+        "typescript": "^5.8.2",
+        "vitest": "^5.0.5"
       }
     },
     "server/node_modules/typescript": {
diff --git a/server/.env.sample b/server/.env.sample
index 67c5cde..f18140c 100644
--- a/server/.env.sample
+++ b/server/.env.sample
@@ -1,5 +1,7 @@
 DATABASE_URL=postgresql://postgres:password@localhost:5432/itsuhima_dev
+PORT=3000
 CORS_ALLOW_ORIGINS=http://localhost:5173
-DOMAIN=localhost
+DOMAIN=localhost # browserId の Cookie を送るドメイン
 NODE_ENV=dev # dev or production
-COOKIE_SECRET=your-random-secret-key-for-development
+COOKIE_SECRET=your-random-secret-key-for-development # Cookie の署名鍵
+# COOKIE_SECRET_PREVIOUS= # 鍵のローテーション時、前の COOKIE_SECRET をカンマ区切りで指定する。省略可
diff --git a/server/package.json b/server/package.json
index 509717e..fd4d13e 100644
--- a/server/package.json
+++ b/server/package.json
@@ -6,7 +6,9 @@
   "scripts": {
     "dev": "tsx watch src/main.ts",
     "prebuild": "prisma generate",
-    "build": "tsc"
+    "build": "tsc -p tsconfig.build.json",
+    "typecheck": "tsc",
+    "test": "vitest run"
   },
   "keywords": [],
   "author": "",
@@ -22,9 +24,10 @@
     "zod": "^3.24.2"
   },
   "devDependencies": {
-    "@types/node": "^22.13.10",
+    "@types/node": "^24.19.2",
     "prisma": "^6.5.0",
     "tsx": "^4.19.3",
-    "typescript": "^5.8.2"
+    "typescript": "^5.8.2",
+    "vitest": "^5.0.5"
   }
 }
diff --git a/server/src/app.ts b/server/src/app.ts
new file mode 100644
index 0000000..44411b8
--- /dev/null
+++ b/server/src/app.ts
@@ -0,0 +1,44 @@
+import { Hono } from "hono";
+import { cors } from "hono/cors";
+import { browserIdMiddleware } from "./middleware/browserId.js";
+import projectsRoutes from "./routes/projects.js";
+import { ServiceError, type ServiceErrorReason } from "./services/errors.js";
+
+const allowedOrigins = process.env.CORS_ALLOW_ORIGINS?.split(",") || [];
+
+const serviceErrorStatus = {
+  project_not_found: 404,
+  submission_not_found: 404,
+  not_host: 403,
+  already_submitted: 409,
+  options_empty: 400,
+  option_in_use: 400,
+  invalid_slot: 400,
+} as const satisfies Record<ServiceErrorReason, number>;
+
+export type AppVariables = {
+  browserId: string;
+};
+
+export const app = new Hono<{ Variables: AppVariables }>()
+  .use(
+    "*",
+    cors({
+      origin: allowedOrigins,
+      credentials: true,
+    }),
+  )
+  .use("*", browserIdMiddleware)
+  .get("/", (c) => {
+    return c.json({ message: "Hello! イツヒマ？" });
+  })
+  .route("/projects", projectsRoutes)
+  .onError((err, c) => {
+    if (err instanceof ServiceError) {
+      return c.json({ message: err.message }, serviceErrorStatus[err.reason]);
+    }
+    console.error(err);
+    return c.json({ message: "Internal Server Error" }, 500);
+  });
+
+export type AppType = typeof app;
diff --git a/server/src/config.ts b/server/src/config.ts
new file mode 100644
index 0000000..1be5185
--- /dev/null
+++ b/server/src/config.ts
@@ -0,0 +1,10 @@
+const isProduction = process.env.NODE_ENV === "production";
+
+export const cookieOptions = {
+  path: "/",
+  domain: process.env.DOMAIN || "localhost", // /home へのリダイレクトのためフロントエンドにも送る
+  httpOnly: true,
+  secure: isProduction,
+  sameSite: "lax",
+  maxAge: 60 * 60 * 24 * 365, // Express だとミリ秒だったが、Hono では秒らしい
+} as const;
diff --git a/server/src/db.ts b/server/src/db.ts
new file mode 100644
index 0000000..901f3a0
--- /dev/null
+++ b/server/src/db.ts
@@ -0,0 +1,3 @@
+import { PrismaClient } from "@prisma/client";
+
+export const prisma = new PrismaClient();
diff --git a/server/src/lib/id.ts b/server/src/lib/id.ts
new file mode 100644
index 0000000..2c7f356
--- /dev/null
+++ b/server/src/lib/id.ts
@@ -0,0 +1,6 @@
+import { customAlphabet } from "nanoid";
+
+/**
+ * ハイフン・アンダースコアを含まない Nano ID 形式。
+ */
+export const nanoid = customAlphabet("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789", 21);
diff --git a/server/src/main.ts b/server/src/main.ts
index 69f937f..242d30c 100644
--- a/server/src/main.ts
+++ b/server/src/main.ts
@@ -1,45 +1,10 @@
+import "dotenv/config";
 import { serve } from "@hono/node-server";
-import { PrismaClient } from "@prisma/client";
-import dotenv from "dotenv";
-import { Hono } from "hono";
-import { cors } from "hono/cors";
-import { customAlphabet } from "nanoid";
-import { browserIdMiddleware } from "./middleware/browserId.js";
-import projectsRoutes from "./routes/projects.js";
+import { app } from "./app.js";
 
-dotenv.config();
-
-/**
- * ハイフン・アンダースコアを含まない Nano ID 形式。
- */
-export const nanoid = customAlphabet("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789", 21);
-
-export const prisma = new PrismaClient();
+export type { AppType } from "./app.js";
 
 const port = Number(process.env.PORT) || 3000;
-const allowedOrigins = process.env.CORS_ALLOW_ORIGINS?.split(",") || [];
-
-export type AppVariables = {
-  browserId: string;
-};
-
-const app = new Hono<{ Variables: AppVariables }>()
-  .use(
-    "*",
-    cors({
-      origin: allowedOrigins,
-      credentials: true,
-    }),
-  )
-  .use("*", browserIdMiddleware)
-  .get("/", (c) => {
-    return c.json({ message: "Hello! イツヒマ？" });
-  })
-  .route("/projects", projectsRoutes)
-  .onError((err, c) => {
-    console.error(err);
-    return c.json({ message: "Internal Server Error" }, 500);
-  });
 
 serve(
   {
@@ -51,16 +16,3 @@ serve(
     console.log(`Server listening on 0.0.0.0:${port}`);
   },
 );
-
-const isProduction = process.env.NODE_ENV === "production";
-
-export const cookieOptions = {
-  path: "/",
-  domain: process.env.DOMAIN || "localhost", // /home へのリダイレクトのためフロントエンドにも送る
-  httpOnly: true,
-  secure: isProduction,
-  sameSite: "lax",
-  maxAge: 60 * 60 * 24 * 365, // Express だとミリ秒だったが、Hono では秒らしい
-} as const;
-
-export type AppType = typeof app;
diff --git a/server/src/middleware/browserId.ts b/server/src/middleware/browserId.ts
index 06d7101..8e303b7 100644
--- a/server/src/middleware/browserId.ts
+++ b/server/src/middleware/browserId.ts
@@ -1,75 +1,56 @@
-import crypto from "node:crypto";
 import type { Context, MiddlewareHandler } from "hono";
 import { getCookie, getSignedCookie, setSignedCookie } from "hono/cookie";
-import { cookieOptions } from "../main.js";
+import { cookieOptions } from "../config.js";
 
 const COOKIE_NAME = "browserId";
 
 /**
- * Express の署名付きクッキー（s:value.signature 形式）を検証して値を取り出す
+ * 署名鍵のローテーションに対応するため、現行の鍵、前の鍵の順に検証する。
+ * @returns 検証できた browserId。Cookie が無ければ undefined。Cookie はあるがどの鍵でも検証できなければ false
  */
-function unsignExpressCookie(signedValue: string, secret: string): string | null {
-  if (!signedValue.startsWith("s:")) return null;
-
-  const raw = signedValue.slice(2); // `s:` を削除
-  const lastDotIndex = raw.lastIndexOf(".");
-  if (lastDotIndex === -1) return null;
-
-  const value = raw.slice(0, lastDotIndex); // 署名前の値（uuid）
-  const signature = raw.slice(lastDotIndex + 1);
-
-  // Express と同じアルゴリズムで署名を検証
-  const expectedSignature = crypto.createHmac("sha256", secret).update(value).digest("base64").replace(/=+$/, "");
-
-  if (signature === expectedSignature) {
-    return value;
+async function readBrowserId(c: Context, secrets: string[]) {
+  if (getCookie(c, COOKIE_NAME) === undefined) {
+    return undefined;
   }
 
-  return null;
+  for (const secret of secrets) {
+    const value = await getSignedCookie(c, secret, COOKIE_NAME);
+    if (value) {
+      return value;
+    }
+  }
+  return false;
 }
 
-/**
- * Express → Hono の移行で signed cookie 形式が変わったため両方に対応するミドルウェア
- */
 export const browserIdMiddleware: MiddlewareHandler = async (c: Context, next) => {
   const cookieSecret = process.env.COOKIE_SECRET;
   if (!cookieSecret) {
     console.error("COOKIE_SECRET is not set");
     return c.json({ message: "サーバー設定エラー" }, 500);
   }
+  const previousSecrets = (process.env.COOKIE_SECRET_PREVIOUS ?? "")
+    .split(",")
+    .map((s) => s.trim())
+    .filter(Boolean);
 
-  // 新形式 （Hono） を試す
-  const browserIdHono = (await getSignedCookie(c, cookieSecret, COOKIE_NAME)) || undefined;
-  if (browserIdHono) {
-    // Cookie を更新
-    await setSignedCookie(c, COOKIE_NAME, browserIdHono, cookieSecret, cookieOptions);
-    c.set("browserId", browserIdHono);
-    return next();
-  }
+  const browserId = await readBrowserId(c, [cookieSecret, ...previousSecrets]);
 
-  // "browserId" という Cookie が存在しない場合は新規発行
-  const rawCookie = getCookie(c, COOKIE_NAME);
-  if (!rawCookie) {
-    const browserId = crypto.randomUUID();
-    await setSignedCookie(c, COOKIE_NAME, browserId, cookieSecret, cookieOptions);
-    c.set("browserId", browserId);
-    return next();
+  // Cookie はあるがどの鍵でも検証できなかった
+  if (browserId === false) {
+    console.warn("browserId Cookie の署名を検証できませんでした");
+    return c.json({ message: "ブラウザのCookie設定に問題があります。" }, 400);
   }
 
-  // 旧形式（Express）を試す
-  const browserIdExpress = unsignExpressCookie(rawCookie, cookieSecret);
-  if (browserIdExpress) {
-    // 旧形式が有効な場合は新形式で再発行
-    await setSignedCookie(c, COOKIE_NAME, browserIdExpress, cookieSecret, cookieOptions);
-    c.set("browserId", browserIdExpress);
+  // 初回アクセスでの新規発行
+  if (browserId === undefined) {
+    const newBrowserId = crypto.randomUUID();
+    await setSignedCookie(c, COOKIE_NAME, newBrowserId, cookieSecret, cookieOptions);
+    c.set("browserId", newBrowserId);
     return next();
   }
 
-  // ここまで来たら Cookie が不正
-  return c.json(
-    {
-      message: "ブラウザのCookie設定に問題があります。",
-    },
-    400,
-  );
+  // 2回目以降のアクセスでの再発行 (新しい鍵で署名・有効期限の更新)
+  await setSignedCookie(c, COOKIE_NAME, browserId, cookieSecret, cookieOptions);
+  c.set("browserId", browserId);
+  return next();
 };
diff --git a/server/src/routes/projects.ts b/server/src/routes/projects.ts
index d4b0bc7..621eb6e 100644
--- a/server/src/routes/projects.ts
+++ b/server/src/routes/projects.ts
@@ -1,248 +1,51 @@
 import { zValidator } from "@hono/zod-validator";
-import dotenv from "dotenv";
 import { Hono } from "hono";
 import { z } from "zod";
 import { editReqSchema, projectReqSchema, submitReqSchema } from "../../../common/validators.js";
-import { type AppVariables, nanoid, prisma } from "../main.js";
-
-dotenv.config();
+import type { AppVariables } from "../app.js";
+import {
+  createProject,
+  deleteProject,
+  getProject,
+  listMyProjects,
+  submitAvailability,
+  updateMyAvailability,
+  updateProject,
+} from "../services/projects.js";
 
 const projectIdParamsSchema = z.object({ projectId: z.string().length(21) });
 
 const router = new Hono<{ Variables: AppVariables }>()
   // プロジェクト作成
   .post("/", zValidator("json", projectReqSchema), async (c) => {
-    const browserId = c.get("browserId");
-    const input = c.req.valid("json");
-
-    const project = await prisma.project.create({
-      data: {
-        id: nanoid(),
-        name: input.name,
-        description: input.description.trim() || null,
-        startDate: new Date(input.startDate),
-        endDate: new Date(input.endDate),
-        allowedRanges: {
-          create: input.allowedRanges.map((range) => ({
-            startTime: new Date(range.startTime),
-            endTime: new Date(range.endTime),
-          })),
-        },
-        hosts: {
-          create: {
-            browserId,
-          },
-        },
-        participationOptions: {
-          create: input.participationOptions.map((opt) => ({
-            id: opt.id,
-            label: opt.label,
-            color: opt.color,
-          })),
-        },
-      },
-      select: {
-        id: true,
-        name: true,
-      },
-    });
-
+    const project = await createProject(c.get("browserId"), c.req.valid("json"));
     return c.json({ id: project.id, name: project.name }, 201);
   })
 
   // 自分が関連するプロジェクト取得
   .get("/mine", async (c) => {
-    const browserId = c.get("browserId");
-
-    const projects = await prisma.project.findMany({
-      where: {
-        OR: [
-          { hosts: { some: { browserId } } },
-          {
-            guests: {
-              some: { browserId },
-            },
-          },
-        ],
-      },
-      include: {
-        hosts: {
-          select: { browserId: true },
-        },
-      },
-    });
-
-    return c.json(
-      projects.map((p) => ({
-        id: p.id,
-        name: p.name,
-        description: p.description ?? "",
-        startDate: p.startDate,
-        endDate: p.endDate,
-        isHost: p.hosts.some((host) => host.browserId === browserId),
-      })),
-      200,
-    );
+    const projects = await listMyProjects(c.get("browserId"));
+    return c.json(projects, 200);
   })
 
   // プロジェクト取得
   .get("/:projectId", zValidator("param", projectIdParamsSchema), async (c) => {
-    const browserId = c.get("browserId");
     const { projectId } = c.req.valid("param");
-
-    const project = await prisma.project.findUnique({
-      where: { id: projectId },
-      include: {
-        allowedRanges: true,
-        participationOptions: true,
-        guests: {
-          include: {
-            slots: true,
-          },
-        },
-        hosts: true,
-      },
-    });
-
-    if (!project) {
-      return c.json({ message: "イベントが見つかりません。" }, 404);
-    }
-
-    const guest = project.guests.find((g) => g.browserId === browserId);
-    const meAsGuest = guest ? (({ browserId, ...rest }) => rest)(guest) : null;
-
-    return c.json(
-      {
-        id: project.id,
-        name: project.name,
-        description: project.description ?? "",
-        startDate: project.startDate,
-        endDate: project.endDate,
-        allowedRanges: project.allowedRanges,
-        participationOptions: project.participationOptions,
-        hosts: project.hosts.map(({ browserId, ...rest }) => rest),
-        guests: project.guests.map(({ browserId, ...rest }) => rest),
-        isHost: project.hosts.some((h) => h.browserId === browserId),
-        meAsGuest,
-      },
-      200,
-    );
+    const project = await getProject(c.get("browserId"), projectId);
+    return c.json(project, 200);
   })
 
   // プロジェクト編集
   .put("/:projectId", zValidator("param", projectIdParamsSchema), zValidator("json", editReqSchema), async (c) => {
-    const browserId = c.get("browserId");
     const { projectId } = c.req.valid("param");
-    const input = c.req.valid("json");
-
-    const [host, existingGuest] = await Promise.all([
-      prisma.host.findFirst({
-        where: {
-          browserId,
-          projectId: projectId,
-        },
-      }),
-      prisma.guest.findFirst({
-        where: { projectId: projectId },
-      }),
-    ]);
-
-    if (!host) {
-      return c.json({ message: "アクセス権限がありません。" }, 403);
-    }
-
-    // 参加形態の更新
-    if (input.participationOptions) {
-      if (input.participationOptions.length === 0) {
-        return c.json({ message: "参加形態は最低1つ必要です。" }, 400);
-      }
-
-      // 削除対象の参加形態に Slot が紐づいているかチェック
-      const existingOptions = await prisma.participationOption.findMany({
-        where: { projectId },
-        include: { slots: { select: { id: true } } },
-      });
-      const newOptionIds = input.participationOptions.map((o) => o.id);
-      const optionsToDelete = existingOptions.filter((o) => !newOptionIds.includes(o.id));
-      const undeletableOptions = optionsToDelete.filter((o) => o.slots.length > 0);
-      if (undeletableOptions.length > 0) {
-        const labels = undeletableOptions.map((o) => o.label).join(", ");
-        return c.json(
-          {
-            message: `以下の参加形態は日程が登録されているため削除できません: ${labels}`,
-          },
-          400,
-        );
-      }
-
-      await prisma.$transaction([
-        // 既存の参加形態で、新しいリストにないものを削除
-        prisma.participationOption.deleteMany({
-          where: {
-            projectId,
-            id: {
-              notIn: newOptionIds,
-            },
-          },
-        }),
-        // 既存の参加形態を更新または新規作成
-        ...input.participationOptions.map((opt) =>
-          prisma.participationOption.upsert({
-            where: { id: opt.id },
-            update: { label: opt.label, color: opt.color },
-            create: { id: opt.id, label: opt.label, color: opt.color, projectId },
-          }),
-        ),
-      ]);
-    }
-
-    const updatedProject = await prisma.project.update({
-      where: { id: projectId },
-      data: existingGuest
-        ? {
-            name: input.name,
-            description: input.description?.trim() || null,
-          }
-        : {
-            name: input.name,
-            description: input.description?.trim() || null,
-            startDate: input.startDate ? new Date(input.startDate) : undefined,
-            endDate: input.endDate ? new Date(input.endDate) : undefined,
-            allowedRanges: {
-              deleteMany: {}, // 既存削除
-              create: input.allowedRanges?.map((r) => ({
-                startTime: new Date(r.startTime),
-                endTime: new Date(r.endTime),
-              })),
-            },
-          },
-      include: { allowedRanges: true, participationOptions: true },
-    });
-
-    return c.json({ event: updatedProject }, 200);
+    const event = await updateProject(c.get("browserId"), projectId, c.req.valid("json"));
+    return c.json({ event }, 200);
   })
 
   // プロジェクト削除
   .delete("/:projectId", zValidator("param", projectIdParamsSchema), async (c) => {
-    const browserId = c.get("browserId");
     const { projectId } = c.req.valid("param");
-
-    const host = await prisma.host.findUnique({
-      where: {
-        browserId_projectId: {
-          browserId,
-          projectId,
-        },
-      },
-    });
-
-    if (!host) {
-      return c.json({ message: "削除権限がありません。" }, 403);
-    }
-
-    await prisma.project.delete({
-      where: { id: projectId },
-    });
+    await deleteProject(c.get("browserId"), projectId);
     return c.json(204);
   })
 
@@ -252,39 +55,9 @@ const router = new Hono<{ Variables: AppVariables }>()
     zValidator("param", projectIdParamsSchema),
     zValidator("json", submitReqSchema),
     async (c) => {
-      const browserId = c.get("browserId");
       const { projectId } = c.req.valid("param");
       const { name, comment, slots } = c.req.valid("json");
-
-      const existingGuest = await prisma.guest.findUnique({
-        where: {
-          browserId_projectId: {
-            browserId,
-            projectId,
-          },
-        },
-      });
-      if (existingGuest) {
-        return c.json({ message: "提出済みです。" }, 403);
-      }
-
-      await prisma.guest.create({
-        data: {
-          name,
-          comment: comment?.trim() || null,
-          browserId,
-          project: { connect: { id: projectId } },
-          slots: {
-            create: slots?.map((slot) => ({
-              from: slot.start,
-              to: slot.end,
-              projectId,
-              participationOptionId: slot.participationOptionId,
-            })),
-          },
-        },
-        include: { slots: true },
-      });
+      await submitAvailability(c.get("browserId"), projectId, { name, comment, slots });
       return c.json("日程が提出されました。", 201);
     },
   )
@@ -295,37 +68,9 @@ const router = new Hono<{ Variables: AppVariables }>()
     zValidator("param", projectIdParamsSchema),
     zValidator("json", submitReqSchema),
     async (c) => {
-      const browserId = c.get("browserId");
       const { projectId } = c.req.valid("param");
       const { name, comment, slots } = c.req.valid("json");
-
-      const existingGuest = await prisma.guest.findUnique({
-        where: { browserId_projectId: { browserId, projectId } },
-        include: { slots: true },
-      });
-
-      if (!existingGuest) {
-        return c.json({ message: "既存の日程が見つかりません。" }, 404);
-      }
-      const slotData = slots?.map((slot) => ({
-        from: slot.start,
-        to: slot.end,
-        projectId,
-        participationOptionId: slot.participationOptionId,
-      }));
-
-      await prisma.slot.deleteMany({ where: { guestId: existingGuest.id } });
-
-      const guest = await prisma.guest.update({
-        where: { id: existingGuest.id },
-        data: {
-          slots: { create: slotData },
-          name,
-          comment: comment?.trim() || null,
-        },
-        include: { slots: true },
-      });
-
+      const guest = await updateMyAvailability(c.get("browserId"), projectId, { name, comment, slots });
       return c.json({ message: "日程が更新されました。", guest }, 200);
     },
   );
diff --git a/server/src/services/errors.ts b/server/src/services/errors.ts
new file mode 100644
index 0000000..8d22d5f
--- /dev/null
+++ b/server/src/services/errors.ts
@@ -0,0 +1,25 @@
+/**
+ * サービス層のエラーの理由。HTTP のステータスへの変換は呼び出す側 (`app.ts` の `onError`) が行う。
+ */
+export type ServiceErrorReason =
+  | "project_not_found"
+  | "submission_not_found"
+  | "not_host"
+  | "already_submitted"
+  | "options_empty"
+  | "option_in_use"
+  | "invalid_slot";
+
+/**
+ * サービス層が投げる業務エラー。message は Web UI にそのまま表示する既定の文で、
+ * 呼び出し元 (P2 の MCP など) が reason を見て差し替えてよい。
+ */
+export class ServiceError extends Error {
+  constructor(
+    readonly reason: ServiceErrorReason,
+    message: string,
+  ) {
+    super(message);
+    this.name = "ServiceError";
+  }
+}
diff --git a/server/src/services/projects.ts b/server/src/services/projects.ts
new file mode 100644
index 0000000..0095990
--- /dev/null
+++ b/server/src/services/projects.ts
@@ -0,0 +1,300 @@
+import type { z } from "zod";
+import type { editReqSchema, projectReqSchema, submitReqSchema } from "../../../common/validators.js";
+import { prisma } from "../db.js";
+import { nanoid } from "../lib/id.js";
+import { ServiceError } from "./errors.js";
+import { validateSlots } from "./slots.js";
+
+export type CreateProjectInput = z.infer<typeof projectReqSchema>;
+export type UpdateProjectInput = z.infer<typeof editReqSchema>;
+export type SubmissionInput = Omit<z.infer<typeof submitReqSchema>, "projectId">;
+
+export async function createProject(browserId: string, input: CreateProjectInput) {
+  return prisma.project.create({
+    data: {
+      id: nanoid(),
+      name: input.name,
+      description: input.description.trim() || null,
+      startDate: new Date(input.startDate),
+      endDate: new Date(input.endDate),
+      allowedRanges: {
+        create: input.allowedRanges.map((range) => ({
+          startTime: new Date(range.startTime),
+          endTime: new Date(range.endTime),
+        })),
+      },
+      hosts: {
+        create: {
+          browserId,
+        },
+      },
+      participationOptions: {
+        create: input.participationOptions.map((opt) => ({
+          id: opt.id,
+          label: opt.label,
+          color: opt.color,
+        })),
+      },
+    },
+    select: {
+      id: true,
+      name: true,
+    },
+  });
+}
+
+/** ホストまたはゲストとして関わるプロジェクト */
+export async function listMyProjects(browserId: string) {
+  const projects = await prisma.project.findMany({
+    where: {
+      OR: [
+        { hosts: { some: { browserId } } },
+        {
+          guests: {
+            some: { browserId },
+          },
+        },
+      ],
+    },
+    include: {
+      hosts: {
+        select: { browserId: true },
+      },
+    },
+  });
+
+  return projects.map((p) => ({
+    id: p.id,
+    name: p.name,
+    description: p.description ?? "",
+    startDate: p.startDate,
+    endDate: p.endDate,
+    isHost: p.hosts.some((host) => host.browserId === browserId),
+  }));
+}
+
+/** プロジェクトの詳細。URL (プロジェクト ID) を知っていれば誰でも見られる。 */
+export async function getProject(browserId: string, projectId: string) {
+  const project = await prisma.project.findUnique({
+    where: { id: projectId },
+    include: {
+      allowedRanges: true,
+      participationOptions: true,
+      guests: {
+        include: {
+          slots: true,
+        },
+      },
+      hosts: true,
+    },
+  });
+
+  if (!project) {
+    throw new ServiceError("project_not_found", "イベントが見つかりません。");
+  }
+
+  const guest = project.guests.find((g) => g.browserId === browserId);
+
+  return {
+    id: project.id,
+    name: project.name,
+    description: project.description ?? "",
+    startDate: project.startDate,
+    endDate: project.endDate,
+    allowedRanges: project.allowedRanges,
+    participationOptions: project.participationOptions,
+    hosts: project.hosts.map(omitBrowserId),
+    guests: project.guests.map(omitBrowserId),
+    isHost: project.hosts.some((h) => h.browserId === browserId),
+    meAsGuest: guest ? omitBrowserId(guest) : null,
+  };
+}
+
+function omitBrowserId<T extends { browserId: string }>(entity: T): Omit<T, "browserId"> {
+  const { browserId: _, ...rest } = entity;
+  return rest;
+}
+
+export async function updateProject(browserId: string, projectId: string, input: UpdateProjectInput) {
+  const [host, existingGuest] = await Promise.all([
+    prisma.host.findFirst({
+      where: {
+        browserId,
+        projectId: projectId,
+      },
+    }),
+    prisma.guest.findFirst({
+      where: { projectId: projectId },
+    }),
+  ]);
+
+  if (!host) {
+    throw new ServiceError("not_host", "アクセス権限がありません。");
+  }
+
+  // 参加形態の更新
+  if (input.participationOptions) {
+    if (input.participationOptions.length === 0) {
+      throw new ServiceError("options_empty", "参加形態は最低1つ必要です。");
+    }
+
+    // 削除対象の参加形態に Slot が紐づいているかチェック
+    const existingOptions = await prisma.participationOption.findMany({
+      where: { projectId },
+      include: { slots: { select: { id: true } } },
+    });
+    const newOptionIds = input.participationOptions.map((o) => o.id);
+    const optionsToDelete = existingOptions.filter((o) => !newOptionIds.includes(o.id));
+    const undeletableOptions = optionsToDelete.filter((o) => o.slots.length > 0);
+    if (undeletableOptions.length > 0) {
+      const labels = undeletableOptions.map((o) => o.label).join(", ");
+      throw new ServiceError("option_in_use", `以下の参加形態は日程が登録されているため削除できません: ${labels}`);
+    }
+
+    await prisma.$transaction([
+      // 既存の参加形態で、新しいリストにないものを削除
+      prisma.participationOption.deleteMany({
+        where: {
+          projectId,
+          id: {
+            notIn: newOptionIds,
+          },
+        },
+      }),
+      // 既存の参加形態を更新または新規作成
+      ...input.participationOptions.map((opt) =>
+        prisma.participationOption.upsert({
+          where: { id: opt.id },
+          update: { label: opt.label, color: opt.color },
+          create: { id: opt.id, label: opt.label, color: opt.color, projectId },
+        }),
+      ),
+    ]);
+  }
+
+  // ゲストの回答があれば、日程と時間帯は変えない
+  return prisma.project.update({
+    where: { id: projectId },
+    data: existingGuest
+      ? {
+          name: input.name,
+          description: input.description?.trim() || null,
+        }
+      : {
+          name: input.name,
+          description: input.description?.trim() || null,
+          startDate: input.startDate ? new Date(input.startDate) : undefined,
+          endDate: input.endDate ? new Date(input.endDate) : undefined,
+          allowedRanges: {
+            deleteMany: {}, // 既存削除
+            create: input.allowedRanges?.map((r) => ({
+              startTime: new Date(r.startTime),
+              endTime: new Date(r.endTime),
+            })),
+          },
+        },
+    include: { allowedRanges: true, participationOptions: true },
+  });
+}
+
+export async function deleteProject(browserId: string, projectId: string) {
+  const host = await prisma.host.findUnique({
+    where: {
+      browserId_projectId: {
+        browserId,
+        projectId,
+      },
+    },
+  });
+
+  if (!host) {
+    throw new ServiceError("not_host", "削除権限がありません。");
+  }
+
+  await prisma.project.delete({
+    where: { id: projectId },
+  });
+}
+
+/** Slot の検証に必要な分だけプロジェクトを読む */
+async function findProjectForSlots(projectId: string) {
+  const project = await prisma.project.findUnique({
+    where: { id: projectId },
+    select: {
+      startDate: true,
+      endDate: true,
+      allowedRanges: { select: { startTime: true, endTime: true } },
+      participationOptions: { select: { id: true } },
+    },
+  });
+  if (!project) {
+    throw new ServiceError("project_not_found", "イベントが見つかりません。");
+  }
+  return project;
+}
+
+export async function submitAvailability(browserId: string, projectId: string, input: SubmissionInput) {
+  const existingGuest = await prisma.guest.findUnique({
+    where: {
+      browserId_projectId: {
+        browserId,
+        projectId,
+      },
+    },
+  });
+  if (existingGuest) {
+    throw new ServiceError("already_submitted", "提出済みです。");
+  }
+
+  validateSlots(await findProjectForSlots(projectId), input.slots);
+
+  return prisma.guest.create({
+    data: {
+      name: input.name,
+      comment: input.comment?.trim() || null,
+      browserId,
+      project: { connect: { id: projectId } },
+      slots: {
+        create: input.slots.map((slot) => ({
+          from: slot.start,
+          to: slot.end,
+          projectId,
+          participationOptionId: slot.participationOptionId,
+        })),
+      },
+    },
+    include: { slots: true },
+  });
+}
+
+export async function updateMyAvailability(browserId: string, projectId: string, input: SubmissionInput) {
+  const existingGuest = await prisma.guest.findUnique({
+    where: { browserId_projectId: { browserId, projectId } },
+    include: { slots: true },
+  });
+
+  if (!existingGuest) {
+    throw new ServiceError("submission_not_found", "既存の日程が見つかりません。");
+  }
+
+  validateSlots(await findProjectForSlots(projectId), input.slots);
+
+  const slotData = input.slots.map((slot) => ({
+    from: slot.start,
+    to: slot.end,
+    projectId,
+    participationOptionId: slot.participationOptionId,
+  }));
+
+  await prisma.slot.deleteMany({ where: { guestId: existingGuest.id } });
+
+  return prisma.guest.update({
+    where: { id: existingGuest.id },
+    data: {
+      slots: { create: slotData },
+      name: input.name,
+      comment: input.comment?.trim() || null,
+    },
+    include: { slots: true },
+  });
+}
diff --git a/server/src/services/slots.ts b/server/src/services/slots.ts
new file mode 100644
index 0000000..2572f66
--- /dev/null
+++ b/server/src/services/slots.ts
@@ -0,0 +1,90 @@
+import { ServiceError } from "./errors.js";
+
+/**
+ * イベントの日付と時刻は、このタイムゾーンの壁時計で解釈する (AllowedRange は日付部分を使わず、時刻だけを使う)。
+ * M3 で event.timezone に置き換える。
+ */
+const TIMEZONE = "Asia/Tokyo";
+
+const MINUTES_PER_DAY = 24 * 60;
+
+const wallClockFormat = new Intl.DateTimeFormat("en-CA", {
+  timeZone: TIMEZONE,
+  year: "numeric",
+  month: "2-digit",
+  day: "2-digit",
+  hour: "2-digit",
+  minute: "2-digit",
+  hourCycle: "h23",
+});
+
+/** TIMEZONE での日付 ("YYYY-MM-DD") と、0 時からの経過分 */
+function toWallClock(value: Date): { date: string; minutes: number } {
+  const parts = Object.fromEntries(wallClockFormat.formatToParts(value).map((p) => [p.type, p.value]));
+  return {
+    date: `${parts.year}-${parts.month}-${parts.day}`,
+    minutes: Number(parts.hour) * 60 + Number(parts.minute),
+  };
+}
+
+function formatMinutes(minutes: number): string {
+  return `${String(Math.floor(minutes / 60)).padStart(2, "0")}:${String(minutes % 60).padStart(2, "0")}`;
+}
+
+/** 秒以下が 0 で、分が 15 の倍数か。TIMEZONE のオフセットは 15 分の倍数なので UTC で判定してよい。 */
+function isQuarterHour(value: Date): boolean {
+  return value.getTime() % (15 * 60 * 1000) === 0;
+}
+
+type SlotInput = { start: Date; end: Date; participationOptionId: string };
+
+type Project = {
+  startDate: Date;
+  endDate: Date;
+  allowedRanges: { startTime: Date; endTime: Date }[];
+  participationOptions: { id: string }[];
+};
+
+/**
+ * Slot がイベントの日程の範囲・時間帯・15 分単位に収まり、イベントの参加形態を使っているかを検証する。
+ * Web UI はカレンダーの構造上これを満たすが、API を直接呼ぶと満たさない Slot を作れてしまう (#91)。
+ */
+export function validateSlots(project: Project, slots: SlotInput[]): void {
+  const firstDate = toWallClock(project.startDate).date;
+  const lastDate = toWallClock(project.endDate).date;
+  // 時間帯は現在 1 つだけ。無ければ終日とみなす
+  const range = project.allowedRanges[0];
+  const rangeStart = range ? toWallClock(range.startTime).minutes : 0;
+  const rangeEnd = range ? toWallClock(range.endTime).minutes : MINUTES_PER_DAY;
+  const optionIds = new Set(project.participationOptions.map((o) => o.id));
+
+  slots.forEach((slot, i) => {
+    const invalid = (message: string) => new ServiceError("invalid_slot", `${i + 1} 件目の時間帯: ${message}`);
+
+    if (slot.start.getTime() >= slot.end.getTime()) {
+      throw invalid("開始時刻は終了時刻より前にしてください。");
+    }
+    if (!isQuarterHour(slot.start) || !isQuarterHour(slot.end)) {
+      throw invalid("時刻は 15 分単位で指定してください。");
+    }
+
+    const start = toWallClock(slot.start);
+    const end = toWallClock(slot.end);
+    // [start, end) の最後の瞬間の日付。24:00 は翌日の 0:00 として届くので、こうしないと日をまたいだことになる
+    const lastInstantDate = toWallClock(new Date(slot.end.getTime() - 1)).date;
+    if (lastInstantDate !== start.date) {
+      throw invalid("日をまたいでいます。日付ごとに分けてください。");
+    }
+    if (start.date < firstDate || start.date > lastDate) {
+      throw invalid(`イベントの日程の範囲 (${firstDate}〜${lastDate}) の外です。`);
+    }
+    const endMinutes = end.date === start.date ? end.minutes : MINUTES_PER_DAY;
+    if (start.minutes < rangeStart || endMinutes > rangeEnd) {
+      throw invalid(`イベントの時間帯 (${formatMinutes(rangeStart)}〜${formatMinutes(rangeEnd)}) の外です。`);
+    }
+
+    if (!optionIds.has(slot.participationOptionId)) {
+      throw invalid("このイベントに無い参加形態です。");
+    }
+  });
+}
diff --git a/server/test/browserId.test.ts b/server/test/browserId.test.ts
new file mode 100644
index 0000000..160543b
--- /dev/null
+++ b/server/test/browserId.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, test, vi } from "vitest";
+import { app } from "../src/app.js";
+
+function browserIdCookie(res: Response) {
+  return res.headers.get("Set-Cookie")?.split(";")[0];
+}
+
+function requestWithCookie(cookie?: string) {
+  return app.request("/", { headers: cookie ? { Cookie: cookie } : {} });
+}
+
+describe("browserIdMiddleware", () => {
+  test("署名が不正な Cookie には 400 を返す", async () => {
+    const cookie = browserIdCookie(await requestWithCookie());
+
+    const res = await requestWithCookie(`${cookie}x`);
+
+    expect(res.status).toBe(400);
+    expect(res.headers.get("Set-Cookie")).toBeNull();
+  });
+
+  test("前の鍵で署名された Cookie は、同じ browserId を現行の鍵で署名し直して発行する", async () => {
+    vi.stubEnv("COOKIE_SECRET", "old-secret");
+    const oldCookie = browserIdCookie(await requestWithCookie());
+    vi.stubEnv("COOKIE_SECRET", "new-secret");
+    vi.stubEnv("COOKIE_SECRET_PREVIOUS", "older-secret, old-secret");
+
+    const res = await requestWithCookie(oldCookie);
+
+    expect(res.status).toBe(200);
+    const newCookie = browserIdCookie(res);
+    expect(newCookie).not.toBe(oldCookie);
+    expect(newCookie?.split(".")[0]).toBe(oldCookie?.split(".")[0]);
+
+    // 署名し直した Cookie は、前の鍵を外しても通る
+    vi.stubEnv("COOKIE_SECRET_PREVIOUS", undefined);
+    expect((await requestWithCookie(newCookie)).status).toBe(200);
+  });
+});
diff --git a/server/test/env.ts b/server/test/env.ts
new file mode 100644
index 0000000..224ce7a
--- /dev/null
+++ b/server/test/env.ts
@@ -0,0 +1,3 @@
+/** テスト用の DB。開発用の DB (itsuhima_dev) を消さないよう、別のデータベースにする */
+export const testDatabaseUrl =
+  process.env.TEST_DATABASE_URL ?? "postgresql://postgres:password@localhost:5432/itsuhima_test";
diff --git a/server/test/globalSetup.ts b/server/test/globalSetup.ts
new file mode 100644
index 0000000..5f6d0f8
--- /dev/null
+++ b/server/test/globalSetup.ts
@@ -0,0 +1,14 @@
+import { execFileSync } from "node:child_process";
+import { testDatabaseUrl } from "./env.js";
+
+export default function setup() {
+  // setup.ts で毎回全テーブルを空にするので、誤って開発用や本番の DB に向けないようにする
+  const dbName = new URL(testDatabaseUrl).pathname.slice(1);
+  if (!dbName.includes("test")) {
+    throw new Error(`テスト用の DB 名には "test" を含めてください: ${dbName}`);
+  }
+  execFileSync("npx", ["prisma", "migrate", "deploy"], {
+    env: { ...process.env, DATABASE_URL: testDatabaseUrl },
+    stdio: "inherit",
+  });
+}
diff --git a/server/test/helpers.ts b/server/test/helpers.ts
new file mode 100644
index 0000000..d14dcc3
--- /dev/null
+++ b/server/test/helpers.ts
@@ -0,0 +1,72 @@
+import { serializeSigned } from "hono/utils/cookie";
+import { app } from "../src/app.js";
+
+/**
+ * 1 つのブラウザを再現する。browserId を決めて署名済みの Cookie を作り、すべてのリクエストに付ける。
+ * サーバーは同じ browserId で Cookie を再発行するだけなので、レスポンスの Set-Cookie は見ない。
+ */
+export function createBrowser() {
+  const browserId = crypto.randomUUID();
+  // 署名は非同期なので、最初のリクエストのときに作る
+  let cookie: Promise<string> | undefined;
+
+  const request = async (path: string, init: RequestInit = {}) => {
+    cookie ??= serializeSigned("browserId", browserId, process.env.COOKIE_SECRET as string);
+    const headers = new Headers(init.headers);
+    headers.set("Cookie", await cookie);
+    return app.request(path, { ...init, headers });
+  };
+  const send = (method: string, path: string, body?: unknown) =>
+    request(path, {
+      method,
+      headers: { "Content-Type": "application/json" },
+      body: body === undefined ? undefined : JSON.stringify(body),
+    });
+
+  return {
+    browserId,
+    get: (path: string) => request(path),
+    post: (path: string, body?: unknown) => send("POST", path, body),
+    put: (path: string, body?: unknown) => send("PUT", path, body),
+    delete: (path: string) => send("DELETE", path),
+  };
+}
+
+export type Browser = ReturnType<typeof createBrowser>;
+
+/**
+ * 2026/11/02〜11/04 (JST)、08:00〜23:00 のイベント。Web UI と同じく、日時は ISO 文字列 (UTC) で送る。
+ */
+export function projectInput(overrides: Record<string, unknown> = {}) {
+  return {
+    name: "テストイベント",
+    description: "説明",
+    startDate: "2026-11-01T15:00:00.000Z",
+    endDate: "2026-11-04T14:59:59.999Z",
+    allowedRanges: [{ startTime: "2026-11-01T23:00:00.000Z", endTime: "2026-11-04T14:00:00.000Z" }],
+    participationOptions: [{ id: crypto.randomUUID(), label: "参加", color: "#0F82B1" }],
+    ...overrides,
+  };
+}
+
+/** イベントを作成して、その id と参加形態の id を返す */
+export async function createProject(host: Browser, overrides: Record<string, unknown> = {}) {
+  const input = projectInput(overrides);
+  const res = await host.post("/projects", input);
+  if (res.status !== 201) throw new Error(`イベントの作成に失敗しました: ${res.status} ${await res.text()}`);
+  const { id } = (await res.json()) as { id: string };
+  return { id, input, optionId: input.participationOptions[0].id };
+}
+
+/** 2026/11/02 (JST) 10:00〜12:00 */
+export const slotOnNov2 = { start: "2026-11-02T01:00:00.000Z", end: "2026-11-02T03:00:00.000Z" };
+
+export function submissionInput(projectId: string, optionId: string, overrides: Record<string, unknown> = {}) {
+  return {
+    name: "ゲスト",
+    projectId,
+    comment: "コメント",
+    slots: [{ ...slotOnNov2, participationOptionId: optionId }],
+    ...overrides,
+  };
+}
diff --git a/server/test/projects.test.ts b/server/test/projects.test.ts
new file mode 100644
index 0000000..d7cec00
--- /dev/null
+++ b/server/test/projects.test.ts
@@ -0,0 +1,506 @@
+import { describe, expect, test } from "vitest";
+import { type Browser, createBrowser, createProject, projectInput, slotOnNov2, submissionInput } from "./helpers.js";
+
+const missingProjectId = "a".repeat(21);
+
+async function getProject(browser: Browser, projectId: string) {
+  const res = await browser.get(`/projects/${projectId}`);
+  expect(res.status).toBe(200);
+  // biome-ignore lint/suspicious/noExplicitAny: レスポンスの形そのものを検証するため
+  return (await res.json()) as any;
+}
+
+async function submit(guest: Browser, projectId: string, optionId: string, overrides: Record<string, unknown> = {}) {
+  const res = await guest.post(`/projects/${projectId}/submissions`, submissionInput(projectId, optionId, overrides));
+  expect(res.status).toBe(201);
+}
+
+describe("POST /projects", () => {
+  test("作成すると 201 で id と name を返し、作成者がホストになる", async () => {
+    const host = createBrowser();
+    const input = projectInput({ description: "  説明  " });
+
+    const res = await host.post("/projects", input);
+
+    expect(res.status).toBe(201);
+    const body = await res.json();
+    expect(body).toEqual({ id: expect.stringMatching(/^[0-9A-Za-z]{21}$/), name: "テストイベント" });
+
+    const project = await getProject(host, body.id);
+    expect(project).toEqual({
+      id: body.id,
+      name: "テストイベント",
+      description: "説明",
+      startDate: "2026-11-01T15:00:00.000Z",
+      endDate: "2026-11-04T14:59:59.999Z",
+      allowedRanges: [
+        {
+          id: expect.any(String),
+          projectId: body.id,
+          startTime: "2026-11-01T23:00:00.000Z",
+          endTime: "2026-11-04T14:00:00.000Z",
+        },
+      ],
+      participationOptions: [{ ...input.participationOptions[0], projectId: body.id }],
+      hosts: [{ id: expect.any(String), projectId: body.id }],
+      guests: [],
+      isHost: true,
+      meAsGuest: null,
+    });
+  });
+
+  test("空白だけの説明は空文字として返す", async () => {
+    const host = createBrowser();
+    const { id } = await createProject(host, { description: "   " });
+
+    expect((await getProject(host, id)).description).toBe("");
+  });
+
+  test("名前が空なら 400", async () => {
+    const res = await createBrowser().post("/projects", projectInput({ name: "" }));
+
+    expect(res.status).toBe(400);
+  });
+
+  test("開始日が終了日より後なら 400", async () => {
+    const res = await createBrowser().post(
+      "/projects",
+      projectInput({ startDate: "2026-11-05T15:00:00.000Z", endDate: "2026-11-04T14:59:59.999Z" }),
+    );
+
+    expect(res.status).toBe(400);
+  });
+
+  test("参加形態が 0 件なら 400", async () => {
+    const res = await createBrowser().post("/projects", projectInput({ participationOptions: [] }));
+
+    expect(res.status).toBe(400);
+  });
+});
+
+describe("GET /projects/mine", () => {
+  test("ホストまたはゲストとして関わるイベントだけを返す", async () => {
+    const me = createBrowser();
+    const other = createBrowser();
+    const hosted = await createProject(me, { name: "主催" });
+    const joined = await createProject(other, { name: "参加" });
+    await createProject(other, { name: "無関係" });
+    await submit(me, joined.id, joined.optionId);
+
+    const res = await me.get("/projects/mine");
+
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { name: string }[];
+    expect(body.sort((a, b) => a.name.localeCompare(b.name))).toEqual([
+      {
+        id: hosted.id,
+        name: "主催",
+        description: "説明",
+        startDate: "2026-11-01T15:00:00.000Z",
+        endDate: "2026-11-04T14:59:59.999Z",
+        isHost: true,
+      },
+      {
+        id: joined.id,
+        name: "参加",
+        description: "説明",
+        startDate: "2026-11-01T15:00:00.000Z",
+        endDate: "2026-11-04T14:59:59.999Z",
+        isHost: false,
+      },
+    ]);
+  });
+
+  test("関わるイベントが無ければ空配列", async () => {
+    const res = await createBrowser().get("/projects/mine");
+
+    expect(res.status).toBe(200);
+    expect(await res.json()).toEqual([]);
+  });
+});
+
+describe("GET /projects/:projectId", () => {
+  test("存在しなければ 404", async () => {
+    const res = await createBrowser().get(`/projects/${missingProjectId}`);
+
+    expect(res.status).toBe(404);
+    expect(await res.json()).toEqual({ message: "イベントが見つかりません。" });
+  });
+
+  test("id が 21 文字でなければ 400", async () => {
+    const res = await createBrowser().get("/projects/short");
+
+    expect(res.status).toBe(400);
+  });
+
+  test("ゲストから見ると isHost は false で、自分の回答が meAsGuest に入る", async () => {
+    const host = createBrowser();
+    const guest = createBrowser();
+    const { id, optionId } = await createProject(host);
+    await submit(guest, id, optionId, { comment: "  よろしく  " });
+
+    const project = await getProject(guest, id);
+
+    const expectedGuest = {
+      id: expect.any(String),
+      name: "ゲスト",
+      comment: "よろしく",
+      projectId: id,
+      slots: [
+        {
+          id: expect.any(String),
+          from: slotOnNov2.start,
+          to: slotOnNov2.end,
+          projectId: id,
+          guestId: expect.any(String),
+          participationOptionId: optionId,
+        },
+      ],
+    };
+    expect(project.isHost).toBe(false);
+    expect(project.meAsGuest).toEqual(expectedGuest);
+    expect(project.guests).toEqual([expectedGuest]);
+  });
+
+  test("回答していない人から見ると meAsGuest は null", async () => {
+    const host = createBrowser();
+    const { id, optionId } = await createProject(host);
+    await submit(createBrowser(), id, optionId);
+
+    const project = await getProject(createBrowser(), id);
+
+    expect(project.isHost).toBe(false);
+    expect(project.meAsGuest).toBeNull();
+    expect(project.guests).toHaveLength(1);
+  });
+
+  test("ホストとゲストの browserId をレスポンスに含めない", async () => {
+    const host = createBrowser();
+    const guest = createBrowser();
+    const { id, optionId } = await createProject(host);
+    await submit(guest, id, optionId);
+
+    const res = await guest.get(`/projects/${id}`);
+    const text = await res.text();
+
+    expect(text).not.toContain("browserId");
+    expect(text).not.toContain(host.browserId);
+    expect(text).not.toContain(guest.browserId);
+  });
+});
+
+describe("PUT /projects/:projectId", () => {
+  test("ホスト以外は 403", async () => {
+    const { id } = await createProject(createBrowser());
+
+    const res = await createBrowser().put(`/projects/${id}`, { name: "乗っ取り" });
+
+    expect(res.status).toBe(403);
+    expect(await res.json()).toEqual({ message: "アクセス権限がありません。" });
+  });
+
+  test("存在しないイベントは 403", async () => {
+    const res = await createBrowser().put(`/projects/${missingProjectId}`, { name: "x" });
+
+    expect(res.status).toBe(403);
+  });
+
+  test("ゲストがいなければ、名前・説明・日程・時間帯を更新できる", async () => {
+    const host = createBrowser();
+    const { id } = await createProject(host);
+
+    const res = await host.put(`/projects/${id}`, {
+      name: "新しい名前",
+      description: "  新しい説明  ",
+      startDate: "2026-11-09T15:00:00.000Z",
+      endDate: "2026-11-10T14:59:59.999Z",
+      allowedRanges: [{ startTime: "2026-11-10T00:00:00.000Z", endTime: "2026-11-10T09:00:00.000Z" }],
+    });
+
+    expect(res.status).toBe(200);
+    const { event } = await res.json();
+    expect(event).toMatchObject({
+      id,
+      name: "新しい名前",
+      description: "新しい説明",
+      startDate: "2026-11-09T15:00:00.000Z",
+      endDate: "2026-11-10T14:59:59.999Z",
+      allowedRanges: [{ startTime: "2026-11-10T00:00:00.000Z", endTime: "2026-11-10T09:00:00.000Z" }],
+    });
+    expect(event.participationOptions).toHaveLength(1);
+  });
+
+  test("ゲストがいると、名前と説明だけ更新し、日程と時間帯は変えない", async () => {
+    const host = createBrowser();
+    const { id, optionId } = await createProject(host);
+    await submit(createBrowser(), id, optionId);
+
+    const res = await host.put(`/projects/${id}`, {
+      name: "新しい名前",
+      startDate: "2026-11-09T15:00:00.000Z",
+      endDate: "2026-11-10T14:59:59.999Z",
+      allowedRanges: [{ startTime: "2026-11-10T00:00:00.000Z", endTime: "2026-11-10T09:00:00.000Z" }],
+    });
+
+    expect(res.status).toBe(200);
+    const project = await getProject(host, id);
+    expect(project).toMatchObject({
+      name: "新しい名前",
+      startDate: "2026-11-01T15:00:00.000Z",
+      endDate: "2026-11-04T14:59:59.999Z",
+      allowedRanges: [{ startTime: "2026-11-01T23:00:00.000Z", endTime: "2026-11-04T14:00:00.000Z" }],
+    });
+  });
+
+  // 現状の挙動: 省略したフィールドは「変更しない」ではなく、消える
+  test("description を省略すると、説明が消える", async () => {
+    const host = createBrowser();
+    const { id } = await createProject(host);
+
+    await host.put(`/projects/${id}`, { name: "新しい名前" });
+
+    expect((await getProject(host, id)).description).toBe("");
+  });
+
+  // 現状の挙動: 省略したフィールドは「変更しない」ではなく、消える
+  test("ゲストがいないときに allowedRanges を省略すると、時間帯が消える", async () => {
+    const host = createBrowser();
+    const { id } = await createProject(host);
+
+    await host.put(`/projects/${id}`, { name: "新しい名前" });
+
+    expect((await getProject(host, id)).allowedRanges).toEqual([]);
+  });
+
+  test("参加形態を追加・変更・削除できる", async () => {
+    const host = createBrowser();
+    const { id } = await createProject(host, {
+      participationOptions: [
+        { id: crypto.randomUUID(), label: "参加", color: "#0F82B1" },
+        { id: crypto.randomUUID(), label: "消す", color: "#000000" },
+      ],
+    });
+    const [kept] = (await getProject(host, id)).participationOptions.filter(
+      (o: { label: string }) => o.label === "参加",
+    );
+    const added = { id: crypto.randomUUID(), label: "オンライン", color: "#FF0000" };
+
+    const res = await host.put(`/projects/${id}`, {
+      participationOptions: [{ id: kept.id, label: "対面", color: "#00FF00" }, added],
+    });
+
+    expect(res.status).toBe(200);
+    const options = (await getProject(host, id)).participationOptions;
+    expect(options.sort((a: { label: string }, b: { label: string }) => a.label.localeCompare(b.label))).toEqual([
+      { ...added, projectId: id },
+      { id: kept.id, label: "対面", color: "#00FF00", projectId: id },
+    ]);
+  });
+
+  test("Slot が紐づく参加形態は削除できない", async () => {
+    const host = createBrowser();
+    const { id, optionId } = await createProject(host);
+    await submit(createBrowser(), id, optionId);
+
+    const res = await host.put(`/projects/${id}`, {
+      participationOptions: [{ id: crypto.randomUUID(), label: "別", color: "#000000" }],
+    });
+
+    expect(res.status).toBe(400);
+    expect(await res.json()).toEqual({ message: "以下の参加形態は日程が登録されているため削除できません: 参加" });
+    expect((await getProject(host, id)).participationOptions).toHaveLength(1);
+  });
+
+  test("参加形態を 0 件にはできない", async () => {
+    const host = createBrowser();
+    const { id } = await createProject(host);
+
+    const res = await host.put(`/projects/${id}`, { participationOptions: [] });
+
+    expect(res.status).toBe(400);
+  });
+});
+
+describe("DELETE /projects/:projectId", () => {
+  test("ホスト以外は 403", async () => {
+    const { id } = await createProject(createBrowser());
+
+    const res = await createBrowser().delete(`/projects/${id}`);
+
+    expect(res.status).toBe(403);
+    expect(await res.json()).toEqual({ message: "削除権限がありません。" });
+  });
+
+  // 現状の挙動: `c.json(204)` なので、ステータスは 200 で本文が 204 になる
+  test("ホストは、ゲストの回答ごと削除できる", async () => {
+    const host = createBrowser();
+    const { id, optionId } = await createProject(host);
+    await submit(createBrowser(), id, optionId);
+
+    const res = await host.delete(`/projects/${id}`);
+
+    expect(res.status).toBe(200);
+    expect(await res.json()).toBe(204);
+    expect((await host.get(`/projects/${id}`)).status).toBe(404);
+  });
+});
+
+describe("POST /projects/:projectId/submissions", () => {
+  test("回答すると 201", async () => {
+    const { id, optionId } = await createProject(createBrowser());
+    const guest = createBrowser();
+
+    const res = await guest.post(`/projects/${id}/submissions`, submissionInput(id, optionId));
+
+    expect(res.status).toBe(201);
+    expect(await res.json()).toBe("日程が提出されました。");
+    expect((await getProject(guest, id)).meAsGuest).toMatchObject({ name: "ゲスト", comment: "コメント" });
+  });
+
+  test("空白だけのコメントは null になる", async () => {
+    const { id, optionId } = await createProject(createBrowser());
+    const guest = createBrowser();
+
+    await submit(guest, id, optionId, { comment: "  " });
+
+    expect((await getProject(guest, id)).meAsGuest.comment).toBeNull();
+  });
+
+  test("同じブラウザから 2 回目は 409", async () => {
+    const { id, optionId } = await createProject(createBrowser());
+    const guest = createBrowser();
+    await submit(guest, id, optionId);
+
+    const res = await guest.post(`/projects/${id}/submissions`, submissionInput(id, optionId));
+
+    expect(res.status).toBe(409);
+    expect(await res.json()).toEqual({ message: "提出済みです。" });
+  });
+
+  test("ホストも回答できる", async () => {
+    const host = createBrowser();
+    const { id, optionId } = await createProject(host);
+
+    await submit(host, id, optionId);
+
+    const project = await getProject(host, id);
+    expect(project.isHost).toBe(true);
+    expect(project.meAsGuest).not.toBeNull();
+  });
+
+  test("名前が空なら 400", async () => {
+    const { id, optionId } = await createProject(createBrowser());
+
+    const res = await createBrowser().post(`/projects/${id}/submissions`, submissionInput(id, optionId, { name: "" }));
+
+    expect(res.status).toBe(400);
+  });
+
+  test("存在しないイベントに回答すると 404", async () => {
+    const res = await createBrowser().post(
+      `/projects/${missingProjectId}/submissions`,
+      submissionInput(missingProjectId, crypto.randomUUID(), { slots: [] }),
+    );
+
+    expect(res.status).toBe(404);
+    expect(await res.json()).toEqual({ message: "イベントが見つかりません。" });
+  });
+});
+
+// 検証の中身は slots.test.ts で確かめる。ここでは、提出と更新で検証が効いていることだけを確かめる
+describe("日程の提出・更新での Slot の検証", () => {
+  test("イベントの期間外なら 400", async () => {
+    const { id, optionId } = await createProject(createBrowser());
+
+    const res = await createBrowser().post(
+      `/projects/${id}/submissions`,
+      submissionInput(id, optionId, {
+        // 11/01 10:00〜12:00
+        slots: [
+          { start: "2026-11-01T01:00:00.000Z", end: "2026-11-01T03:00:00.000Z", participationOptionId: optionId },
+        ],
+      }),
+    );
+
+    expect(res.status).toBe(400);
+    expect(await res.json()).toEqual({ message: expect.stringContaining("日程の範囲") });
+  });
+
+  test("別のイベントの参加形態なら 400", async () => {
+    const { id } = await createProject(createBrowser());
+    const other = await createProject(createBrowser());
+
+    const res = await createBrowser().post(`/projects/${id}/submissions`, submissionInput(id, other.optionId));
+
+    expect(res.status).toBe(400);
+  });
+
+  test("回答の更新でも検証し、弾いたときは元の回答を変えない", async () => {
+    const { id, optionId } = await createProject(createBrowser());
+    const guest = createBrowser();
+    await submit(guest, id, optionId);
+
+    const res = await guest.put(
+      `/projects/${id}/submissions/mine`,
+      submissionInput(id, optionId, {
+        name: "新しい名前",
+        // 11/01 10:00〜12:00
+        slots: [
+          { start: "2026-11-01T01:00:00.000Z", end: "2026-11-01T03:00:00.000Z", participationOptionId: optionId },
+        ],
+      }),
+    );
+
+    expect(res.status).toBe(400);
+    const { meAsGuest } = await getProject(guest, id);
+    expect(meAsGuest).toMatchObject({ name: "ゲスト", slots: [{ from: slotOnNov2.start, to: slotOnNov2.end }] });
+  });
+});
+
+describe("PUT /projects/:projectId/submissions/mine", () => {
+  test("回答していなければ 404", async () => {
+    const { id, optionId } = await createProject(createBrowser());
+
+    const res = await createBrowser().put(`/projects/${id}/submissions/mine`, submissionInput(id, optionId));
+
+    expect(res.status).toBe(404);
+    expect(await res.json()).toEqual({ message: "既存の日程が見つかりません。" });
+  });
+
+  test("名前・コメント・Slot を置き換える", async () => {
+    const { id, optionId } = await createProject(createBrowser());
+    const guest = createBrowser();
+    await submit(guest, id, optionId);
+    const newSlot = {
+      start: "2026-11-03T05:00:00.000Z",
+      end: "2026-11-03T06:00:00.000Z",
+      participationOptionId: optionId,
+    };
+
+    const res = await guest.put(
+      `/projects/${id}/submissions/mine`,
+      submissionInput(id, optionId, { name: "新しい名前", comment: "", slots: [newSlot] }),
+    );
+
+    expect(res.status).toBe(200);
+    expect(await res.json()).toMatchObject({
+      message: "日程が更新されました。",
+      guest: { name: "新しい名前", comment: null, slots: [{ from: newSlot.start, to: newSlot.end }] },
+    });
+    const { meAsGuest } = await getProject(guest, id);
+    expect(meAsGuest.slots).toEqual([expect.objectContaining({ from: newSlot.start, to: newSlot.end })]);
+  });
+
+  test("他のゲストの回答は変えない", async () => {
+    const { id, optionId } = await createProject(createBrowser());
+    const guest = createBrowser();
+    const other = createBrowser();
+    await submit(guest, id, optionId);
+    await submit(other, id, optionId, { name: "他の人" });
+
+    await guest.put(`/projects/${id}/submissions/mine`, submissionInput(id, optionId, { slots: [] }));
+
+    const { meAsGuest } = await getProject(other, id);
+    expect(meAsGuest).toMatchObject({ name: "他の人", slots: [expect.anything()] });
+  });
+});
diff --git a/server/test/setup.ts b/server/test/setup.ts
new file mode 100644
index 0000000..65ccc54
--- /dev/null
+++ b/server/test/setup.ts
@@ -0,0 +1,12 @@
+import { afterAll, beforeEach } from "vitest";
+import { prisma } from "../src/db.js";
+
+beforeEach(async () => {
+  await prisma.$executeRawUnsafe(
+    'TRUNCATE "Project", "AllowedRange", "Slot", "Host", "Guest", "ParticipationOption" CASCADE',
+  );
+});
+
+afterAll(async () => {
+  await prisma.$disconnect();
+});
diff --git a/server/test/slots.test.ts b/server/test/slots.test.ts
new file mode 100644
index 0000000..4d10917
--- /dev/null
+++ b/server/test/slots.test.ts
@@ -0,0 +1,146 @@
+import { describe, expect, test } from "vitest";
+import { ServiceError } from "../src/services/errors.js";
+import { validateSlots } from "../src/services/slots.js";
+
+// 2026/11/02〜11/04 (JST)、08:00〜23:00 のイベント。時刻はコメントに JST で書く
+const project = {
+  startDate: new Date("2026-11-01T15:00:00.000Z"),
+  endDate: new Date("2026-11-04T14:59:59.999Z"),
+  allowedRanges: [{ startTime: new Date("2026-11-01T23:00:00.000Z"), endTime: new Date("2026-11-04T14:00:00.000Z") }],
+  participationOptions: [{ id: "option" }],
+};
+const allDayProject = { ...project, allowedRanges: [] };
+
+function slot(start: string, end: string, participationOptionId = "option") {
+  return { start: new Date(start), end: new Date(end), participationOptionId };
+}
+
+function validationError(fn: () => void): ServiceError {
+  try {
+    fn();
+  } catch (err) {
+    if (err instanceof ServiceError) return err;
+    throw err;
+  }
+  throw new Error("ServiceError が投げられませんでした");
+}
+
+describe("validateSlots", () => {
+  test("期間と時間帯に収まる Slot は通す", () => {
+    expect(() =>
+      validateSlots(project, [
+        // 11/02 08:00〜10:00 (初日、時間帯の始まりちょうどから)
+        slot("2026-11-01T23:00:00.000Z", "2026-11-02T01:00:00.000Z"),
+        // 11/04 21:00〜23:00 (最終日、時間帯の終わりちょうどまで)
+        slot("2026-11-04T12:00:00.000Z", "2026-11-04T14:00:00.000Z"),
+      ]),
+    ).not.toThrow();
+  });
+
+  test("Slot が 0 件なら通す", () => {
+    expect(() => validateSlots(project, [])).not.toThrow();
+  });
+
+  test("期間の前日と翌日は弾く", () => {
+    // 11/01 10:00〜12:00
+    const before = validationError(() =>
+      validateSlots(project, [slot("2026-11-01T01:00:00.000Z", "2026-11-01T03:00:00.000Z")]),
+    );
+    // 11/05 10:00〜12:00
+    const after = validationError(() =>
+      validateSlots(project, [slot("2026-11-05T01:00:00.000Z", "2026-11-05T03:00:00.000Z")]),
+    );
+
+    expect(before.reason).toBe("invalid_slot");
+    expect(before.message).toBe("1 件目の時間帯: イベントの日程の範囲 (2026-11-02〜2026-11-04) の外です。");
+    expect(after.reason).toBe("invalid_slot");
+  });
+
+  test("時間帯より前に始まる Slot は弾く", () => {
+    // 11/02 07:45〜09:00
+    const err = validationError(() =>
+      validateSlots(project, [slot("2026-11-01T22:45:00.000Z", "2026-11-02T00:00:00.000Z")]),
+    );
+
+    expect(err.message).toBe("1 件目の時間帯: イベントの時間帯 (08:00〜23:00) の外です。");
+  });
+
+  test("時間帯より後に終わる Slot は弾く", () => {
+    // 11/02 22:00〜23:15
+    const err = validationError(() =>
+      validateSlots(project, [slot("2026-11-02T13:00:00.000Z", "2026-11-02T14:15:00.000Z")]),
+    );
+
+    expect(err.reason).toBe("invalid_slot");
+  });
+
+  test("15 分単位でない Slot は弾く", () => {
+    // 11/02 10:10〜12:00
+    const minutes = validationError(() =>
+      validateSlots(project, [slot("2026-11-02T01:10:00.000Z", "2026-11-02T03:00:00.000Z")]),
+    );
+    // 11/02 10:00:30〜12:00
+    const seconds = validationError(() =>
+      validateSlots(project, [slot("2026-11-02T01:00:30.000Z", "2026-11-02T03:00:00.000Z")]),
+    );
+
+    expect(minutes.reason).toBe("invalid_slot");
+    expect(seconds.reason).toBe("invalid_slot");
+  });
+
+  test("開始が終了より前でない Slot は弾く", () => {
+    // 11/02 12:00〜12:00
+    const empty = validationError(() =>
+      validateSlots(project, [slot("2026-11-02T03:00:00.000Z", "2026-11-02T03:00:00.000Z")]),
+    );
+    // 11/02 12:00〜10:00
+    const reversed = validationError(() =>
+      validateSlots(project, [slot("2026-11-02T03:00:00.000Z", "2026-11-02T01:00:00.000Z")]),
+    );
+
+    expect(empty.reason).toBe("invalid_slot");
+    expect(reversed.reason).toBe("invalid_slot");
+  });
+
+  test("日をまたぐ Slot は弾く", () => {
+    // 11/02 23:00〜11/03 01:00
+    const err = validationError(() =>
+      validateSlots(allDayProject, [slot("2026-11-02T14:00:00.000Z", "2026-11-02T16:00:00.000Z")]),
+    );
+
+    expect(err.message).toBe("1 件目の時間帯: 日をまたいでいます。日付ごとに分けてください。");
+  });
+
+  // 時間帯が無いと、カレンダーは 23:45〜24:00 のセルを描画し、終わりを翌日の 0:00 として送る
+  test("時間帯が無ければ終日とみなし、最終日の 23:45〜24:00 も通す", () => {
+    expect(() =>
+      validateSlots(allDayProject, [
+        // 11/02 00:00〜01:00
+        slot("2026-11-01T15:00:00.000Z", "2026-11-01T16:00:00.000Z"),
+        // 11/04 23:45〜11/05 00:00
+        slot("2026-11-04T14:45:00.000Z", "2026-11-04T15:00:00.000Z"),
+      ]),
+    ).not.toThrow();
+  });
+
+  test("イベントに無い参加形態は弾く", () => {
+    const err = validationError(() =>
+      validateSlots(project, [slot("2026-11-02T01:00:00.000Z", "2026-11-02T03:00:00.000Z", "other")]),
+    );
+
+    expect(err.message).toBe("1 件目の時間帯: このイベントに無い参加形態です。");
+  });
+
+  test("何件目の Slot が違反したかをメッセージに含める", () => {
+    const err = validationError(() =>
+      validateSlots(project, [
+        // 11/02 10:00〜12:00
+        slot("2026-11-02T01:00:00.000Z", "2026-11-02T03:00:00.000Z"),
+        // 11/01 10:00〜12:00
+        slot("2026-11-01T01:00:00.000Z", "2026-11-01T03:00:00.000Z"),
+      ]),
+    );
+
+    expect(err.message).toMatch(/^2 件目の時間帯: /);
+  });
+});
diff --git a/server/tsconfig.build.json b/server/tsconfig.build.json
new file mode 100644
index 0000000..8869140
--- /dev/null
+++ b/server/tsconfig.build.json
@@ -0,0 +1,7 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "noEmit": false
+  },
+  "include": ["src/**/*"]
+}
diff --git a/server/tsconfig.json b/server/tsconfig.json
index d79ab90..e0dbd80 100644
--- a/server/tsconfig.json
+++ b/server/tsconfig.json
@@ -7,7 +7,8 @@
     "strict": true,
     "esModuleInterop": true,
     "skipLibCheck": true,
-    "forceConsistentCasingInFileNames": true
+    "forceConsistentCasingInFileNames": true,
+    "noEmit": true
   },
-  "include": ["src/**/*"]
+  "include": ["src/**/*", "test/**/*", "vitest.config.ts"]
 }
diff --git a/server/vitest.config.ts b/server/vitest.config.ts
new file mode 100644
index 0000000..54af0e1
--- /dev/null
+++ b/server/vitest.config.ts
@@ -0,0 +1,18 @@
+import { defineConfig } from "vitest/config";
+import { testDatabaseUrl } from "./test/env.js";
+
+export default defineConfig({
+  test: {
+    globalSetup: ["./test/globalSetup.ts"],
+    setupFiles: ["./test/setup.ts"],
+    // 全テストで 1 つの DB を共有するので、ファイルを並列に流さない
+    fileParallelism: false,
+    unstubEnvs: true,
+    restoreMocks: true,
+    env: {
+      DATABASE_URL: testDatabaseUrl,
+      COOKIE_SECRET: "test-secret",
+      DOMAIN: "localhost",
+    },
+  },
+});