diff --git a/apps/web/src/components/ChatView.logic.test.ts b/apps/web/src/components/ChatView.logic.test.ts
index c1f68878e9644..06c01a222c7d0 100644
--- a/apps/web/src/components/ChatView.logic.test.ts
+++ b/apps/web/src/components/ChatView.logic.test.ts
@@ -1,5 +1,17 @@
 import { shouldShowOpenInPicker } from "./chat/OpenInPicker.logic";
 import {
+  addPreparingComposerImages,
+  removePreparingComposerImages,
+} from "./chat/preparingComposerImages";
+import { pendingDraftWork } from "./chat/pendingDraftWork";
+import {
+  composerTargetKey,
+  useComposerDraftStore,
+  type ComposerThreadDraftState,
+} from "../composerDraftStore";
+import {
+  composerDraftUserContentSignature,
+  composerDraftChangedDuringPreparation,
   boundAttachmentIds,
   buildExpiredTerminalContextToastCopy,
   mergeDirectPreviewAnnotation,
@@ -14,6 +26,7 @@ import { deriveProviderInstanceEntries } from "../providerInstances";
 import { ensureInlineContextReferences } from "../lib/composerContextReferences";
 import { terminalContextReference } from "../lib/composerContextRecords";
 import {
+  ComposerContextId,
   CommandId,
   EnvironmentId,
   MessageId,
@@ -1217,6 +1230,183 @@ describe("deriveAvailableRevertTurnCounts", () => {
     expect(deriveAvailableRevertTurnCounts(null).size).toBe(0);
   });
 });
+describe("composerDraftUserContentSignature", () => {
+  const target = {
+    environmentId: EnvironmentId.make("send-preparation"),
+    threadId: ThreadId.make("draft"),
+  };
+  const empty: ComposerThreadDraftState = {
+    prompt: "Send this",
+    images: [],
+    files: [],
+    nonPersistedImageIds: [],
+    persistedAttachments: [],
+    terminalContexts: [],
+    previewAnnotations: [],
+    reviewComments: [],
+    threadContexts: [],
+    environmentContexts: [],
+    modelSelectionByProvider: {},
+    activeProvider: null,
+    runtimeMode: null,
+    interactionMode: null,
+  };
+
+  it("detects prompt edits but accepts upload metadata writes", () => {
+    const store = useComposerDraftStore.getState();
+    try {
+      store.setPrompt(target, "Send this");
+      store.addFiles(target, [
+        {
+          type: "file",
+          id: "report",
+          name: "report.txt",
+          mimeType: "text/plain",
+          sizeBytes: 3,
+          file: new File(["abc"], "report.txt"),
+        },
+      ]);
+      const before = composerDraftUserContentSignature(store.getComposerDraft(target));
+      store.setFileUpload(target, "report", target.environmentId, "server-upload");
+      expect(composerDraftUserContentSignature(store.getComposerDraft(target))).toBe(before);
+      const key = composerTargetKey(target);
+      addPreparingComposerImages(key, [
+        { id: "compressing", name: "image.png", file: new File(["image"], "image.png") },
+      ]);
+      try {
+        expect(composerDraftChangedDuringPreparation(target, before)).toBe(true);
+        expect(
+          composerDraftChangedDuringPreparation(
+            { ...target, threadId: ThreadId.make("other") },
+            composerDraftUserContentSignature(null),
+          ),
+        ).toBe(false);
+      } finally {
+        removePreparingComposerImages(key, new Set(["compressing"]));
+      }
+      pendingDraftWork.begin(key);
+      try {
+        expect(composerDraftChangedDuringPreparation(target, before)).toBe(true);
+      } finally {
+        pendingDraftWork.end(key);
+      }
+      expect(composerDraftChangedDuringPreparation(target, before)).toBe(false);
+      store.setPrompt(target, "Send this plus my new edit");
+      expect(composerDraftUserContentSignature(store.getComposerDraft(target))).not.toBe(before);
+      expect(store.getComposerDraft(target)?.prompt).toBe("Send this plus my new edit");
+    } finally {
+      store.clearComposerContent(target);
+    }
+  });
+
+  it("detects attachments and same-ID context edits", () => {
+    const draft: ComposerThreadDraftState = {
+      ...empty,
+      terminalContexts: [
+        {
+          id: "terminal",
+          threadId: target.threadId,
+          terminalId: "default",
+          terminalLabel: "Terminal",
+          lineStart: 1,
+          lineEnd: 1,
+          text: "before",
+          createdAt: "2026-10-20T00:00:00Z",
+        },
+      ],
+      reviewComments: [
+        {
+          id: "review",
+          sectionId: "diff",
+          sectionTitle: "Changes",
+          filePath: "file.ts",
+          startIndex: 0,
+          endIndex: 0,
+          rangeLabel: "L1",
+          text: "before",
+          diff: "+line",
+        },
+      ],
+      previewAnnotations: [
+        {
+          id: "preview",
+          pageUrl: "http://localhost/",
+          pageTitle: null,
+          comment: "before",
+          elements: [],
+          regions: [],
+          strokes: [],
+          styleChanges: [],
+          screenshot: null,
+          createdAt: "2026-10-20T00:00:00Z",
+        },
+      ],
+      threadContexts: [
+        {
+          version: 1,
+          kind: "thread",
+          contextId: ComposerContextId.make("thread-context"),
+          label: "Thread",
+          environmentId: target.environmentId,
+          threadId: target.threadId,
+          title: "before",
+        },
+      ],
+      environmentContexts: [
+        {
+          version: 1,
+          kind: "environment",
+          contextId: ComposerContextId.make("environment-context"),
+          label: "Before",
+          environmentId: target.environmentId,
+        },
+      ],
+    };
+    const before = composerDraftUserContentSignature(draft);
+    const edits: ComposerThreadDraftState[] = [
+      {
+        ...draft,
+        files: [
+          {
+            type: "file",
+            id: "new",
+            name: "new.txt",
+            mimeType: "text/plain",
+            sizeBytes: 1,
+            file: null,
+          },
+        ],
+      },
+      {
+        ...draft,
+        terminalContexts: draft.terminalContexts.map((entry) => ({ ...entry, text: "after" })),
+      },
+      {
+        ...draft,
+        reviewComments: draft.reviewComments.map((entry) => ({ ...entry, text: "after" })),
+      },
+      {
+        ...draft,
+        previewAnnotations: draft.previewAnnotations.map((entry) => ({
+          ...entry,
+          comment: "after",
+        })),
+      },
+      {
+        ...draft,
+        threadContexts: draft.threadContexts.map((entry) => ({ ...entry, title: "after" })),
+      },
+      {
+        ...draft,
+        environmentContexts: draft.environmentContexts.map((entry) => ({
+          ...entry,
+          label: "After",
+        })),
+      },
+    ];
+    for (const edited of edits) expect(composerDraftUserContentSignature(edited)).not.toBe(before);
+  });
+});
 
 describe("Open in for the host environment", () => {
   it("shows both local environments when the primary has a separate WSL home", () => {
diff --git a/apps/web/src/components/ChatView.logic.ts b/apps/web/src/components/ChatView.logic.ts
index d35b6ce28c433..2dad3ef83b7ef 100644
--- a/apps/web/src/components/ChatView.logic.ts
+++ b/apps/web/src/components/ChatView.logic.ts
@@ -66,10 +66,16 @@ import {
 import {
   type ComposerFileAttachment,
   type ComposerImageAttachment,
+  type ComposerThreadDraftState,
+  type ComposerThreadTarget,
+  composerTargetKey,
+  useComposerDraftStore,
   type DraftId,
   type DraftThreadState,
 } from "../composerDraftStore";
 import * as Schema from "effect/Schema";
+import { countPreparingComposerImages } from "./chat/preparingComposerImages";
+import { pendingDraftWork } from "./chat/pendingDraftWork";
 import { appAtomRegistry } from "../rpc/atomRegistry";
 import { environmentThreadDetails } from "../state/threads";
 import {
@@ -1096,6 +1102,35 @@ export function shouldRefocusComposerOnWindowFocus(
   );
 }
 
+// Upload completion replaces file records without changing the content the user intends to send.
+export function composerDraftUserContentSignature(
+  draft: ComposerThreadDraftState | null | undefined,
+): string {
+  return JSON.stringify({
+    prompt: draft?.prompt ?? "",
+    images: draft?.images.map((image) => image.id) ?? [],
+    files: draft?.files.map((file) => file.id) ?? [],
+    terminalContexts: draft?.terminalContexts ?? [],
+    previewAnnotations: draft?.previewAnnotations ?? [],
+    reviewComments: draft?.reviewComments ?? [],
+    threadContexts: draft?.threadContexts ?? [],
+    environmentContexts: draft?.environmentContexts ?? [],
+  });
+}
+
+export function composerDraftChangedDuringPreparation(
+  target: ComposerThreadTarget,
+  signature: string,
+): boolean {
+  const key = composerTargetKey(target);
+  return (
+    countPreparingComposerImages(key) > 0 ||
+    pendingDraftWork.has(key) ||
+    composerDraftUserContentSignature(useComposerDraftStore.getState().getComposerDraft(target)) !==
+      signature
+  );
+}
+
 /** Includes the annotation screenshot only within the attachment cap; exceeding it rejects the whole turn. */
 export function mergeDirectPreviewAnnotation(input: {
   readonly images: ReadonlyArray<ComposerImageAttachment>;
diff --git a/apps/web/src/components/chat/useComposerSend.ts b/apps/web/src/components/chat/useComposerSend.ts
index cc68f956aea5f..e7646026535cb 100644
--- a/apps/web/src/components/chat/useComposerSend.ts
+++ b/apps/web/src/components/chat/useComposerSend.ts
@@ -1,6 +1,8 @@
 import { isClearComposerCommand, parseSideChatCommand } from "@t3tools/shared/composerTrigger";
 import { armFreshSession, disarmFreshSession, readFreshSessionArm } from "./freshSessionArms";
 import {
+  composerDraftUserContentSignature,
+  composerDraftChangedDuringPreparation,
   mergeDirectPreviewAnnotation,
   resolveBackgroundDraftWorkspaceOptions,
   shouldDockDraftHeroForSubmission,
@@ -834,6 +836,13 @@ export function useComposerSend(input: ComposerSendInput) {
       multipleTargets.push(resolved);
     }
 
+    const draftContentForSend = composerDraftUserContentSignature(
+      useComposerDraftStore.getState().getComposerDraft(composerDraftTarget),
+    );
+    const draftChangedDuringPreparation = () =>
+      composerDraftChangedDuringPreparation(composerDraftTarget, draftContentForSend);
+    const changedDraftError =
+      "Draft changed while preparing this message. Review it and send again.";
     sendInFlightRef.current = true;
     const sendGeneration = ++composerSendGenerationRef.current;
     const attachmentCapabilitiesBeforeUpload = readLiveAttachmentCapabilities();
@@ -887,6 +896,15 @@ export function useComposerSend(input: ComposerSendInput) {
       await dockStarted;
     }
 
+    if (draftChangedDuringPreparation()) {
+      sendInFlightRef.current = false;
+      setThreadError(threadIdForSend, changedDraftError);
+      setDockedDraftHeroThreadKey((currentThreadKey) =>
+        currentThreadKey === activeThreadKey ? null : currentThreadKey,
+      );
+      return;
+    }
+
     const attachmentCapabilitiesBeforeDispatch = readLiveAttachmentCapabilities();
     if (
       !readEnvironmentScope(environmentId, AuthOrchestrationOperateScope) ||
@@ -915,9 +933,11 @@ export function useComposerSend(input: ComposerSendInput) {
         (getUploadedAttachments({ environmentId, images: composerAttachmentsSnapshot }) === null
           ? "Retry or remove failed uploads before sending."
           : null);
-      if (uploadBlockReason !== null) {
+      const blockReason =
+        uploadBlockReason ?? (draftChangedDuringPreparation() ? changedDraftError : null);
+      if (blockReason !== null) {
         sendInFlightRef.current = false;
-        setThreadError(threadIdForSend, uploadBlockReason);
+        setThreadError(threadIdForSend, blockReason);
         setDockedDraftHeroThreadKey((currentThreadKey) =>
           currentThreadKey === activeThreadKey ? null : currentThreadKey,
         );
diff --git a/apps/web/src/components/chat/useMultipleModelSend.ts b/apps/web/src/components/chat/useMultipleModelSend.ts
index 8ff3bf7fb1392..fb62ffd137f61 100644
--- a/apps/web/src/components/chat/useMultipleModelSend.ts
+++ b/apps/web/src/components/chat/useMultipleModelSend.ts
@@ -27,6 +27,8 @@ import { buildThreadRouteParams } from "../../threadRoutes";
 import { DEFAULT_INTERACTION_MODE } from "../../types";
 import { stackedThreadToast, toastManager } from "../ui/toast";
 import {
+  composerDraftUserContentSignature,
+  composerDraftChangedDuringPreparation,
   boundAttachmentIds,
   type ComposerSendSnapshot,
   type makeThreadErrorSetter,
@@ -114,6 +116,9 @@ export function useMultipleModelSend(input: MultipleModelSendInput) {
     /** False once a newer send from this composer started. */
     readonly isCurrentSend: () => boolean;
   }) => {
+    const draftContentForSend = composerDraftUserContentSignature(
+      useComposerDraftStore.getState().getComposerDraft(composerDraftTarget),
+    );
     const { attachments: attachmentsSnapshot, messageText } = sendSnapshot;
     const failedSelections: ModelSelection[] = [];
     let clearedDraft = false;
@@ -124,6 +129,9 @@ export function useMultipleModelSend(input: MultipleModelSendInput) {
     // fails every selection; the draft restore below still runs.
     const sendResult = await settlePromise(async () => {
       const attachments = await attachmentsPromise;
+      if (composerDraftChangedDuringPreparation(composerDraftTarget, draftContentForSend)) {
+        throw new Error("Draft changed while preparing this message. Review it and send again.");
+      }
       const fileBlockReason = readFileBlockReason();
       if (fileBlockReason !== null) throw new Error(fileBlockReason);
       const context = buildOutgoingMessageContext(