@@ -162,6 +162,17 @@
         "produceDrawElementFrame",
       ],
     },
+    // External-conflict persistence is the #2990 stack boundary. The coordinator
+    // consumes these exports in child PR #2991; keep the primitive independently reviewable.
+    {
+      "file": "packages/studio/src/utils/externalConflictStorage.ts",
+      "exports": [
+        "persistExternalConflictSnapshot",
+        "persistExternalFailureSnapshot",
+        "loadExternalConflictSnapshot",
+        "deleteExternalConflictSnapshot",
+      ],
+    },
     // CLI command files: every command exports a const `examples` per the
     // convention documented in CLAUDE.md. This is a namespace barrel, not a
     // collision.
 
@@ -85,6 +85,7 @@
     "@types/react-dom": "19",
     "@vitejs/plugin-react": "^4.0.0",
     "autoprefixer": "^10.4.0",
+    "fake-indexeddb": "^6.2.5",
     "postcss": "^8.4.0",
     "puppeteer-core": "^25.2.1",
     "tailwindcss": "^3.4.0",
 
@@ -27,9 +27,13 @@ export function FileManagerProvider({
     editingPathRef,
     projectIdRef,
     saveRafRef,
+    flushPendingSourceSave,
+    discardPendingSourceSave,
+    getPendingSourceCandidate,
     importedFontAssetsRef,
     readProjectFile,
     writeProjectFile,
+    overwriteExternalConflict,
     readOptionalProjectFile,
     observeProjectFileVersion,
     updateEditingFileContent,
@@ -67,9 +71,13 @@ export function FileManagerProvider({
       editingPathRef,
       projectIdRef,
       saveRafRef,
+      flushPendingSourceSave,
+      discardPendingSourceSave,
+      getPendingSourceCandidate,
       importedFontAssetsRef,
       readProjectFile,
       writeProjectFile,
+      overwriteExternalConflict,
       readOptionalProjectFile,
       observeProjectFileVersion,
       updateEditingFileContent,
@@ -101,9 +109,13 @@ export function FileManagerProvider({
       editingPathRef,
       projectIdRef,
       saveRafRef,
+      flushPendingSourceSave,
+      discardPendingSourceSave,
+      getPendingSourceCandidate,
       importedFontAssetsRef,
       readProjectFile,
       writeProjectFile,
+      overwriteExternalConflict,
       readOptionalProjectFile,
       observeProjectFileVersion,
       updateEditingFileContent,
 
@@ -20,15 +20,65 @@ vi.mock("./useEditorSave", () => ({
   useEditorSave: () => ({
     saveRafRef: { current: null },
     handleContentChange: vi.fn(),
+    getPendingCandidate: vi.fn(() => null),
+    flushPendingSave: vi.fn(async () => ({ status: "clean" as const })),
+    discardPendingSave: vi.fn(),
   }),
 }));
 
 import { useFileManager } from "./useFileManager";
+import { resetStudioWriteTokens, studioFileContentVersion } from "../utils/studioFileVersion";
+import { StudioFileConflictError } from "../utils/studioSaveDiagnostics";
 
 (globalThis as unknown as { IS_REACT_ACT_ENVIRONMENT: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
 
+function useTestFileManager(projectId: string) {
+  return useFileManager({
+    projectId,
+    showToast: vi.fn(),
+    recordEdit: vi.fn(async () => {}),
+    domEditSaveTimestampRef: { current: 0 },
+    setRefreshKey: vi.fn(),
+  });
+}
+
+async function mountTestFileManager(projectId = "project-a") {
+  const captured: { manager: ReturnType<typeof useFileManager> | null } = { manager: null };
+  function Probe() {
+    captured.manager = useTestFileManager(projectId);
+    return null;
+  }
+  const root = createRoot(document.createElement("div"));
+  await act(async () => root.render(<Probe />));
+  const manager = captured.manager;
+  if (!manager) throw new Error("file manager did not render");
+  return { manager, root };
+}
+
+async function mountOverwriteRequest(response: Response) {
+  const fetchMock = vi.fn((_url: string, init?: RequestInit) => {
+    if (init?.method !== "PUT") {
+      throw new Error("overwrite unexpectedly performed a preflight read");
+    }
+    return Promise.resolve(response);
+  });
+  vi.stubGlobal("fetch", fetchMock);
+  return { ...(await mountTestFileManager()), fetchMock };
+}
+
+function createOverwriteConflict(currentVersion: string | null, currentContent: string | null) {
+  return new StudioFileConflictError({
+    filePath: "index.html",
+    currentVersion,
+    currentContent,
+    attemptedContent: "STUDIO",
+  });
+}
+
 describe("useFileManager project ownership", () => {
   afterEach(() => {
+    resetStudioWriteTokens();
+    vi.useRealTimers();
     vi.unstubAllGlobals();
   });
 
@@ -54,13 +104,7 @@ describe("useFileManager project ownership", () => {
 
     const captured: { manager: ReturnType<typeof useFileManager> | null } = { manager: null };
     function Probe({ projectId }: { projectId: string }) {
-      captured.manager = useFileManager({
-        projectId,
-        showToast: vi.fn(),
-        recordEdit: vi.fn(async () => {}),
-        domEditSaveTimestampRef: { current: 0 },
-        setRefreshKey: vi.fn(),
-      });
+      captured.manager = useTestFileManager(projectId);
       return null;
     }
 
@@ -106,4 +150,92 @@ describe("useFileManager project ownership", () => {
 
     await act(async () => root.unmount());
   });
+
+  it("uses a fresh write token when a lost response retries a committed save", async () => {
+    vi.useFakeTimers();
+    let putAttempt = 0;
+    const fetchMock = vi.fn((_url: string, init?: RequestInit) => {
+      if (!init?.method) {
+        return Promise.resolve({
+          ok: true,
+          json: async () => ({ content: "BEFORE", version: "v1" }),
+        } as Response);
+      }
+      putAttempt += 1;
+      if (putAttempt === 1) return Promise.reject(new TypeError("response lost"));
+      return Promise.resolve({ ok: true, json: async () => ({ version: "v2" }) } as Response);
+    });
+    vi.stubGlobal("fetch", fetchMock);
+
+    const { manager, root } = await mountTestFileManager();
+    await manager.readProjectFile("index.html");
+
+    const write = manager.writeProjectFile("index.html", "AFTER");
+    await vi.runAllTimersAsync();
+    await write;
+
+    const writeTokens = fetchMock.mock.calls
+      .filter(([, init]) => init?.method === "PUT")
+      .map(([, init]) => new Headers(init?.headers).get("X-Hyperframes-Write-Token"));
+    expect(writeTokens).toHaveLength(2);
+    expect(writeTokens[0]).toBeTruthy();
+    expect(writeTokens[1]).not.toBe(writeTokens[0]);
+
+    await act(async () => root.unmount());
+  });
+
+  it("overwrites the exact external content version with an If-Match precondition", async () => {
+    const { manager, root, fetchMock } = await mountOverwriteRequest({
+      ok: true,
+      json: async () => ({ version: "v3" }),
+    } as Response);
+    const conflict = createOverwriteConflict("v2", "EXTERNAL");
+
+    await manager.overwriteExternalConflict(conflict);
+
+    const [, init] = fetchMock.mock.calls[0] ?? [];
+    const headers = new Headers(init?.headers);
+    expect(init).toMatchObject({ method: "PUT", body: "STUDIO" });
+    expect(headers.get("If-Match")).toBe(await studioFileContentVersion("EXTERNAL"));
+    expect(headers.get("If-None-Match")).toBeNull();
+    await act(async () => root.unmount());
+  });
+
+  it("preserves a newer third-party edit when a content-less conflict version is stale", async () => {
+    const { manager, root, fetchMock } = await mountOverwriteRequest({
+      ok: false,
+      status: 409,
+      json: async () => ({ currentVersion: "v3", currentContent: "THIRD PARTY" }),
+    } as Response);
+    const conflict = createOverwriteConflict("v2", null);
+
+    await expect(manager.overwriteExternalConflict(conflict)).rejects.toMatchObject({
+      name: "StudioFileConflictError",
+      currentVersion: "v3",
+      currentContent: "THIRD PARTY",
+      attemptedContent: "STUDIO",
+    });
+
+    const [, init] = fetchMock.mock.calls[0] ?? [];
+    const headers = new Headers(init?.headers);
+    expect(headers.get("If-Match")).toBe("v2");
+    expect(headers.get("If-None-Match")).toBeNull();
+    await act(async () => root.unmount());
+  });
+
+  it("uses create-only semantics when the conflicted file was deleted", async () => {
+    const { manager, root, fetchMock } = await mountOverwriteRequest({
+      ok: true,
+      json: async () => ({ version: "v1" }),
+    } as Response);
+    const conflict = createOverwriteConflict(null, null);
+
+    await manager.overwriteExternalConflict(conflict);
+
+    const [, init] = fetchMock.mock.calls[0] ?? [];
+    const headers = new Headers(init?.headers);
+    expect(headers.get("If-Match")).toBeNull();
+    expect(headers.get("If-None-Match")).toBe("*");
+    await act(async () => root.unmount());
+  });
 });
@@ -10,7 +10,11 @@ import {
   StudioFileConflictError,
   StudioSaveNetworkError,
 } from "../utils/studioSaveDiagnostics";
-import { createStudioWriteToken, studioExpectedFileVersion } from "../utils/studioFileVersion";
+import {
+  createStudioWriteToken,
+  markStudioWriteToken,
+  studioExpectedFileVersion,
+} from "../utils/studioFileVersion";
 import { useFileTree } from "./useFileTree";
 import { useEditorSave } from "./useEditorSave";
 
@@ -117,8 +121,11 @@ export function useFileManager({
           throw await createStudioSaveHttpError(preflight, `Failed to read ${path} before save`);
         }
       }
-      const writeToken = createStudioWriteToken();
       await retryStudioSave(async () => {
+        // Each request gets its own receipt identity. If a committed request loses its response,
+        // the retry can produce a second filesystem receipt that must be suppressed independently.
+        const writeToken = createStudioWriteToken();
+        markStudioWriteToken(writeToken);
         let response: Response;
         try {
           response = await fetch(
@@ -191,7 +198,7 @@ export function useFileManager({
 
   // ── Editor save (debounced content change) ──
 
-  const { saveRafRef, handleContentChange } = useEditorSave({
+  const editorSave = useEditorSave({
     editingPathRef,
     projectIdRef,
     readProjectFile,
@@ -201,6 +208,24 @@ export function useFileManager({
     setRefreshKey,
     showToast,
   });
+  const { saveRafRef, handleContentChange } = editorSave;
+
+  const overwriteExternalConflict = useCallback(
+    async (conflict: StudioFileConflictError) => {
+      if (conflict.currentContent != null) {
+        await writeProjectFile(
+          conflict.filePath,
+          conflict.attemptedContent,
+          conflict.currentContent,
+        );
+      } else {
+        fileVersions.set(conflict.filePath, conflict.currentVersion);
+        await writeProjectFile(conflict.filePath, conflict.attemptedContent);
+      }
+      updateEditingFileContent(conflict.filePath, conflict.attemptedContent);
+    },
+    [fileVersions, updateEditingFileContent, writeProjectFile],
+  );
 
   // ── File select ──
 
@@ -491,11 +516,15 @@ export function useFileManager({
     editingPathRef,
     projectIdRef,
     saveRafRef,
+    flushPendingSourceSave: editorSave.flushPendingSave,
+    discardPendingSourceSave: editorSave.discardPendingSave,
+    getPendingSourceCandidate: editorSave.getPendingCandidate,
     importedFontAssetsRef,
 
     // Core I/O
     readProjectFile,
     writeProjectFile,
+    overwriteExternalConflict,
     readOptionalProjectFile,
     observeProjectFileVersion,
     updateEditingFileContent,