diff --git a/src/session/project-key.test.ts b/src/session/project-key.test.ts
index 17754aa0c..8af2bc6e3 100644
--- a/src/session/project-key.test.ts
+++ b/src/session/project-key.test.ts
@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, expect, test } from "bun:test";
-import { mkdir, rm, writeFile } from "node:fs/promises";
+import { mkdir, readFile, rm, writeFile } from "node:fs/promises";
 import { realpathSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
@@ -51,6 +51,73 @@ test("projectKeyFor shares nested dirs under the same git toplevel", async () =>
   expect(projectKeyFor(nested)).toBe(projectKeyFor(root));
 });
 
+test("projectRootFor spawns git rev-parse once for the same cwd", async () => {
+  initTemporaryGitRepo(root);
+  await commitReadme(root);
+
+  const traceFile = join(root, "git-trace2-events");
+  const previousTrace = process.env.GIT_TRACE2_EVENT;
+  process.env.GIT_TRACE2_EVENT = traceFile;
+  try {
+    const first = projectRootFor(root);
+    const second = projectRootFor(root);
+    expect(second).toBe(first);
+    expect(first).toBe(realpathSync(root));
+    const spawns = (await readFile(traceFile, "utf8"))
+      .split("\n")
+      .filter(
+        (line) =>
+          line.includes('"event":"start"') && line.includes("rev-parse"),
+      ).length;
+    expect(spawns).toBe(1);
+  } finally {
+    if (previousTrace === undefined) {
+      delete process.env.GIT_TRACE2_EVENT;
+    } else {
+      process.env.GIT_TRACE2_EVENT = previousTrace;
+    }
+  }
+});
+
+test("projectRootFor resolves a directory created after a missing lookup", async () => {
+  initTemporaryGitRepo(root);
+  await commitReadme(root);
+
+  const parent = realpathSync(root);
+  const child = join(parent, "child");
+  expect(projectRootFor(child)).toBe(child);
+  await mkdir(child);
+  expect(projectRootFor(child)).toBe(parent);
+});
+
+test("projectRootFor retries a nested dir after git was unavailable", async () => {
+  initTemporaryGitRepo(root);
+  await commitReadme(root);
+
+  const nested = join(root, "nested");
+  await mkdir(nested);
+  const parent = realpathSync(root);
+  // Bun 1.3 ignores PATH rewrites and does not forward process.env mutations
+  // unless the child is given `env`. A missing GIT_DIR is not enough: git may
+  // keep discovering the work tree. An existing file that is not a git dir
+  // makes rev-parse exit non-zero.
+  const notGitDir = join(root, "not-a-git-dir");
+  await writeFile(notGitDir, "not a git dir\n");
+
+  const previousGitDir = process.env.GIT_DIR;
+  process.env.GIT_DIR = notGitDir;
+  try {
+    expect(projectRootFor(nested)).toBe(realpathSync(nested));
+  } finally {
+    if (previousGitDir === undefined) {
+      delete process.env.GIT_DIR;
+    } else {
+      process.env.GIT_DIR = previousGitDir;
+    }
+  }
+  expect(projectRootFor(nested)).toBe(parent);
+});
+
 test("linked worktrees have distinct project roots and keys from main and each other", async () => {
   initTemporaryGitRepo(root);
   await commitReadme(root);
diff --git a/src/session/project-key.ts b/src/session/project-key.ts
index bfc434a6e..1f76d6b90 100644
--- a/src/session/project-key.ts
+++ b/src/session/project-key.ts
@@ -29,31 +29,58 @@ function slugSegment(name: string): string {
   return slug.length > 0 ? slug : "project";
 }
 
-/**
- * Checkout project root for session identity.
- *
- * Nested cwd under a git toplevel shares that root (and therefore the same
- * project key). Linked worktrees each have their own `--show-toplevel`.
- */
-export function projectRootFor(cwd: string): string {
+// Repeated boot lookups of one cwd must not each spawn git. Only a successful
+// toplevel is stored: a missing directory or a failed git lookup must be tried
+// again, or the session key stays on the fallback for the process lifetime.
+const projectRootByResolvedCwd = new Map<string, string>();
+
+function resolvedCwd(cwd: string): string {
   try {
+    return realpathSync(cwd);
+  } catch {
+    return cwd;
+  }
+}
+
+function gitToplevel(cwd: string): string | undefined {
+  try {
+    // Pass `env` explicitly. Bun 1.3's execFileSync otherwise keeps the
+    // startup environ, so later process.env changes never reach git.
     const toplevelRaw = execFileSync(
       "git",
       ["rev-parse", "--path-format=absolute", "--show-toplevel"],
       {
         cwd,
         encoding: "utf8",
+        env: process.env,
         stdio: ["ignore", "pipe", "ignore"],
       },
     ).trim();
-    if (toplevelRaw.length > 0) {
-      const toplevelAbs = isAbsolute(toplevelRaw)
-        ? toplevelRaw
-        : resolve(cwd, toplevelRaw);
-      return realpathOr(toplevelAbs);
-    }
+    if (toplevelRaw.length === 0) return undefined;
+    const toplevelAbs = isAbsolute(toplevelRaw)
+      ? toplevelRaw
+      : resolve(cwd, toplevelRaw);
+    return realpathOr(toplevelAbs);
   } catch {
     // Not a git checkout (or git unavailable) — use the workspace path.
+    return undefined;
+  }
+}
+
+/**
+ * Checkout project root for session identity.
+ *
+ * Nested cwd under a git toplevel shares that root (and therefore the same
+ * project key). Linked worktrees each have their own `--show-toplevel`.
+ */
+export function projectRootFor(cwd: string): string {
+  const key = resolvedCwd(cwd);
+  const cached = projectRootByResolvedCwd.get(key);
+  if (cached !== undefined) return cached;
+  const toplevel = gitToplevel(cwd);
+  if (toplevel !== undefined) {
+    projectRootByResolvedCwd.set(key, toplevel);
+    return toplevel;
   }
   return realpathOr(cwd);
 }