diff --git a/web/standalone/src/components/WasmTool.tsx b/web/standalone/src/components/WasmTool.tsx
index 0ce6718..8b729cf 100644
--- a/web/standalone/src/components/WasmTool.tsx
+++ b/web/standalone/src/components/WasmTool.tsx
@@ -973,6 +973,7 @@ export function WasmTool({
   files,
   targetPath,
   fetchBytes,
+  onStagedRevision,
   saveBytes,
   createFile,
   docSource,
@@ -1008,6 +1009,12 @@ export function WasmTool({
   boot?: import("@/lib/boot-payload").BootPayload | null;
   /** Fetch one project-relative file's bytes (contract loader or local folder). */
   fetchBytes: (relPath: string) => Promise<Uint8Array>;
+  /**
+   * Files staged from the project sync namespace bundle never pass through
+   * `fetchBytes`; this reports their listing revision so the source can record
+   * the CAS ancestry `saveBytes` must publish against (see DriveOptions).
+   */
+  onStagedRevision?: (relPath: string, revision: number) => void;
   /**
    * Persist one file the user saved in the editor (File→Save writes MEMFS, then
    * the wasm fires window.kicadCollab.onSave → this). API upload for backend
@@ -2055,6 +2062,7 @@ export function WasmTool({
                   digest: boot?.projectSync.digest,
                 }
               : null,
+          onStagedRevision,
           log: append,
           onStatus: setStatus,
           onFileProgress: (done, total) =>
diff --git a/web/standalone/src/lib/api.ts b/web/standalone/src/lib/api.ts
index 8976705..591734a 100644
--- a/web/standalone/src/lib/api.ts
+++ b/web/standalone/src/lib/api.ts
@@ -152,6 +152,19 @@ export async function uploadFileBytes(
   }
 }
 
+/**
+ * Record a file's CAS ancestry when its bytes were staged WITHOUT going through
+ * `fetchFileBytes` (project sync namespace bundle). See
+ * ProjectSource.rememberBaseRevision.
+ */
+export function rememberFileBaseRevision(
+  slug: string,
+  relPath: string,
+  revision: number,
+): void {
+  projectSource().rememberBaseRevision?.(slug, relPath, revision);
+}
+
 /** Observe the server revision after an ambiguous save; never rebases this model. */
 export async function refreshFileRevision(
   slug: string,
diff --git a/web/standalone/src/lib/project-source-cache.test.ts b/web/standalone/src/lib/project-source-cache.test.ts
index 57cba93..e3a7504 100644
--- a/web/standalone/src/lib/project-source-cache.test.ts
+++ b/web/standalone/src/lib/project-source-cache.test.ts
@@ -249,3 +249,33 @@ describe("remote source file-body cache", () => {
     );
   });
 });
+
+describe("remote source CAS ancestry for bundle-staged files", () => {
+  it("rememberBaseRevision sets the expected revision of the next PUT", async () => {
+    // A file staged from the project sync namespace never passes through
+    // fetchFileBytes; without rememberBaseRevision its first save would carry
+    // expected revision 0 and 409 against the real row (assign-footprints
+    // .kicad_pro conflict: "local base 0, server 1").
+    const source = await loadRemote(cacheMock());
+    const fetchMock = vi.fn(async () => ({
+      ok: true,
+      status: 200,
+      headers: { get: (h: string) => (h === "x-pcbjam-file-revision" ? "2" : null) },
+      json: async () => ({ revision: 2 }),
+    }));
+    vi.stubGlobal("fetch", fetchMock);
+    const relPath = "Leonardo/Arduino Leonardo.kicad_pro";
+    source().rememberBaseRevision?.("leo", relPath, 1);
+    await source().uploadFileBytes!("leo", relPath, new Uint8Array([1]));
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+    const init = (fetchMock.mock.calls[0] as unknown[])[1] as {
+      method: string;
+      headers: Record<string, string>;
+    };
+    expect(init.method).toBe("PUT");
+    const revisionHeader = Object.entries(init.headers).find(
+      ([k]) => k.toLowerCase().includes("revision"),
+    );
+    expect(revisionHeader?.[1]).toBe("1");
+  });
+});
diff --git a/web/standalone/src/lib/project-source.ts b/web/standalone/src/lib/project-source.ts
index 9484a9c..e82a9ed 100644
--- a/web/standalone/src/lib/project-source.ts
+++ b/web/standalone/src/lib/project-source.ts
@@ -80,6 +80,17 @@ export interface ProjectSource {
    * becomes a legal write precondition. Optional for non-CAS sources.
    */
   refreshFileRevision?(slug: string, relPath: string): Promise<void>;
+  /**
+   * Record that the in-memory model of `relPath` was built from `revision`
+   * WITHOUT this source having served the bytes — the project sync namespace
+   * stages plain files from one bundle (kicad-runner stageViaProjectSync), so
+   * `fetchFileBytes` never sees them. Without this the first CAS PUT of such
+   * a file goes out with expected revision 0 and 409s against any row that
+   * was ever re-saved (e.g. `.kicad_pro` at revision 1 → "local base 0,
+   * server 1"). The revision comes from the listing the namespace snapshot
+   * was vouched against, so it IS the model's ancestry, not merely observed.
+   */
+  rememberBaseRevision?(slug: string, relPath: string, revision: number): void;
 }
 
 // --- remote (REST backend over the shared contract) ---------------------------
@@ -254,6 +265,11 @@ function remoteProjectSource(): ProjectSource {
       const file = res.body.find((candidate) => candidate.path === relPath);
       rememberObservedRevision(slug, relPath, file?.revision);
     },
+    rememberBaseRevision(slug, relPath, revision) {
+      if (!Number.isSafeInteger(revision) || revision < 0) return;
+      rememberObservedRevision(slug, relPath, revision);
+      baseRevisions.set(revisionKey(slug, relPath), revision);
+    },
     // Editor saves publish through the CAS PUT (findings D-3 client half): the
     // expected revision is the model's ANCESTRY (baseRevisions), never the
     // latest observed metadata — so a save issued after a conflict cannot
@@ -490,6 +506,9 @@ function compositeProjectSource(
       const s = await route(slug);
       await s.refreshFileRevision?.(slug, p);
     },
+    rememberBaseRevision: (slug, p, revision) => {
+      void route(slug).then((s) => s.rememberBaseRevision?.(slug, p, revision));
+    },
   };
 }
 
diff --git a/web/standalone/src/pages/ToolPage.tsx b/web/standalone/src/pages/ToolPage.tsx
index ca4730f..d7fcf0d 100644
--- a/web/standalone/src/pages/ToolPage.tsx
+++ b/web/standalone/src/pages/ToolPage.tsx
@@ -5,6 +5,7 @@ import { Loader2 } from "lucide-react";
 import {
   createProjectFileIfMissing,
   fetchFileBytes,
+  rememberFileBaseRevision,
   uploadFileBytes,
   useProjectBoot,
   useSourceDescriptor,
@@ -100,6 +101,9 @@ export function ToolPage() {
         fetchBytes={(relPath) =>
           fetchFileBytes(slug, relPath, filesByPath.get(relPath))
         }
+        onStagedRevision={(relPath, revision) =>
+          rememberFileBaseRevision(slug, relPath, revision)
+        }
         saveBytes={
           readOnly
             ? undefined
diff --git a/web/standalone/src/wasm/kicad-runner.ts b/web/standalone/src/wasm/kicad-runner.ts
index 205c91e..4904d5c 100644
--- a/web/standalone/src/wasm/kicad-runner.ts
+++ b/web/standalone/src/wasm/kicad-runner.ts
@@ -52,6 +52,13 @@ export interface DriveOptions {
   targetPath?: string;
   fetchBytes: (relPath: string) => Promise<Uint8Array>;
   projectSync?: ProjectSyncConfig | null;
+  /**
+   * A file staged from the sync namespace bundle bypassed `fetchBytes`, so the
+   * project source never learned which revision the model was built from.
+   * Called with the listing's revision for every such file so the CAS save
+   * path (uploadFileBytes) publishes against the right ancestry instead of 0.
+   */
+  onStagedRevision?: (relPath: string, revision: number) => void;
   log: (msg: string) => void;
   onStatus: (text: string) => void;
   /** Per-file staging progress (files fetched+written so far, total) — drives
@@ -200,7 +207,10 @@ async function syncProjectToMemfs(win: ToolWindow, opts: DriveOptions): Promise<
  * the files that still need the per-file path. Exported for tests.
  */
 export async function stageViaProjectSync(
-  opts: Pick<DriveOptions, "slug" | "files" | "targetPath" | "log"> & {
+  opts: Pick<
+    DriveOptions,
+    "slug" | "files" | "targetPath" | "log" | "onStagedRevision"
+  > & {
     projectSync?: ProjectSyncConfig | null;
   },
   stageOne: (path: string, bytes: Uint8Array) => void,
@@ -242,8 +252,13 @@ export async function stageViaProjectSync(
       // The namespace manifest raced the project listing (file changed to
       // ydoc-backed, or was just deleted/renamed): let the per-file path
       // answer authoritatively.
-      if (!bytes) missed.push(f);
-      else stageOne(f.path, bytes);
+      if (!bytes) {
+        missed.push(f);
+        continue;
+      }
+      stageOne(f.path, bytes);
+      // `eligible` requires revision > 0, so this is always a real row.
+      if (f.revision !== undefined) opts.onStagedRevision?.(f.path, f.revision);
     }
     if (missed.length) {
       opts.log(
diff --git a/web/standalone/src/wasm/project-stage-sync.test.ts b/web/standalone/src/wasm/project-stage-sync.test.ts
index f504ba5..8e7963b 100644
--- a/web/standalone/src/wasm/project-stage-sync.test.ts
+++ b/web/standalone/src/wasm/project-stage-sync.test.ts
@@ -168,3 +168,32 @@ describe("stageViaProjectSync", () => {
     expect(rest).toEqual(files);
   });
 });
+
+describe("stageViaProjectSync CAS ancestry", () => {
+  it("reports the listing revision for every namespace-staged file", async () => {
+    // A bundle-staged file bypasses fetchFileBytes, so the source would
+    // otherwise publish its first save against revision 0 → 409 "local base 0,
+    // server N" (the .kicad_pro assign-footprints conflict).
+    const server = fakeSyncServer({ "a.kicad_pro": "(pro)", "b.txt": "B" });
+    const files: ToolFile[] = [
+      { path: "a.kicad_pro", revision: 1 },
+      { path: "b.txt", revision: 4 },
+      { path: "missing.txt", revision: 2 },
+    ];
+    const seen: Array<[string, number]> = [];
+    await stageViaProjectSync(
+      {
+        slug: "p",
+        files,
+        projectSync: syncConfig(server.fetchImpl),
+        log: () => {},
+        onStagedRevision: (path, revision) => seen.push([path, revision]),
+      },
+      () => {},
+    );
+    expect(seen.sort()).toEqual([
+      ["a.kicad_pro", 1],
+      ["b.txt", 4],
+    ]);
+  });
+});