diff --git a/apps/server/src/orchestration/Layers/ProjectionPipeline.test.ts b/apps/server/src/orchestration/Layers/ProjectionPipeline.test.ts
index 8ca77624cd8ce5caab104e5cd3b5035f347dc7da..abb411b724530f3d83fbb03d18afe9f4144fe973 100644
--- a/apps/server/src/orchestration/Layers/ProjectionPipeline.test.ts
+++ b/apps/server/src/orchestration/Layers/ProjectionPipeline.test.ts
@@ -480,6 +480,149 @@ projectionLayer("OrchestrationProjectionPipeline", (it) => {
       }),
   );
 
+  it.effect("overwrites attachment file bytes when a message updates the same attachment index", () =>
+    Effect.gen(function* () {
+      const projectionPipeline = yield* OrchestrationProjectionPipeline;
+      const eventStore = yield* OrchestrationEventStore;
+      const stateDir = fs.mkdtempSync(path.join(os.tmpdir(), "t3-projection-attachments-overwrite-"));
+      const now = new Date().toISOString();
+      const later = new Date(Date.now() + 1_000).toISOString();
+
+      const serverConfig = {
+        mode: "web",
+        port: 0,
+        host: undefined,
+        cwd: "/tmp/project-attachments",
+        keybindingsConfigPath: path.join(stateDir, "keybindings.json"),
+        stateDir,
+        staticDir: undefined,
+        devUrl: undefined,
+        noBrowser: true,
+        authToken: undefined,
+        autoBootstrapProjectFromCwd: false,
+        logWebSocketEvents: false,
+      } satisfies ServerConfigShape;
+
+      yield* eventStore.append({
+        type: "project.created",
+        eventId: EventId.makeUnsafe("evt-overwrite-1"),
+        aggregateKind: "project",
+        aggregateId: ProjectId.makeUnsafe("project-overwrite"),
+        occurredAt: now,
+        commandId: CommandId.makeUnsafe("cmd-overwrite-1"),
+        causationEventId: null,
+        correlationId: CommandId.makeUnsafe("cmd-overwrite-1"),
+        metadata: {},
+        payload: {
+          projectId: ProjectId.makeUnsafe("project-overwrite"),
+          title: "Project Overwrite",
+          workspaceRoot: "/tmp/project-overwrite",
+          defaultModel: null,
+          scripts: [],
+          createdAt: now,
+          updatedAt: now,
+        },
+      });
+
+      yield* eventStore.append({
+        type: "thread.created",
+        eventId: EventId.makeUnsafe("evt-overwrite-2"),
+        aggregateKind: "thread",
+        aggregateId: ThreadId.makeUnsafe("thread-overwrite"),
+        occurredAt: now,
+        commandId: CommandId.makeUnsafe("cmd-overwrite-2"),
+        causationEventId: null,
+        correlationId: CommandId.makeUnsafe("cmd-overwrite-2"),
+        metadata: {},
+        payload: {
+          threadId: ThreadId.makeUnsafe("thread-overwrite"),
+          projectId: ProjectId.makeUnsafe("project-overwrite"),
+          title: "Thread Overwrite",
+          model: "gpt-5-codex",
+          branch: null,
+          worktreePath: null,
+          createdAt: now,
+          updatedAt: now,
+        },
+      });
+
+      yield* eventStore.append({
+        type: "thread.message-sent",
+        eventId: EventId.makeUnsafe("evt-overwrite-3"),
+        aggregateKind: "thread",
+        aggregateId: ThreadId.makeUnsafe("thread-overwrite"),
+        occurredAt: now,
+        commandId: CommandId.makeUnsafe("cmd-overwrite-3"),
+        causationEventId: null,
+        correlationId: CommandId.makeUnsafe("cmd-overwrite-3"),
+        metadata: {},
+        payload: {
+          threadId: ThreadId.makeUnsafe("thread-overwrite"),
+          messageId: MessageId.makeUnsafe("message-overwrite"),
+          role: "user",
+          text: "first image",
+          attachments: [
+            {
+              type: "image",
+              name: "file.png",
+              mimeType: "image/png",
+              sizeBytes: 5,
+              dataUrl: "data:image/png;base64,SGVsbG8=",
+            },
+          ],
+          turnId: null,
+          streaming: false,
+          createdAt: now,
+          updatedAt: now,
+        },
+      });
+
+      yield* eventStore.append({
+        type: "thread.message-sent",
+        eventId: EventId.makeUnsafe("evt-overwrite-4"),
+        aggregateKind: "thread",
+        aggregateId: ThreadId.makeUnsafe("thread-overwrite"),
+        occurredAt: later,
+        commandId: CommandId.makeUnsafe("cmd-overwrite-4"),
+        causationEventId: null,
+        correlationId: CommandId.makeUnsafe("cmd-overwrite-4"),
+        metadata: {},
+        payload: {
+          threadId: ThreadId.makeUnsafe("thread-overwrite"),
+          messageId: MessageId.makeUnsafe("message-overwrite"),
+          role: "user",
+          text: "",
+          attachments: [
+            {
+              type: "image",
+              name: "file.png",
+              mimeType: "image/png",
+              sizeBytes: 5,
+              dataUrl: "data:image/png;base64,V29ybGQ=",
+            },
+          ],
+          turnId: null,
+          streaming: false,
+          createdAt: now,
+          updatedAt: later,
+        },
+      });
+
+      yield* projectionPipeline.bootstrap.pipe(Effect.provideService(ServerConfig, serverConfig));
+
+      const attachmentPath = path.join(
+        stateDir,
+        "attachments",
+        "thread-overwrite",
+        "message-overwrite",
+        "0.png",
+      );
+      assert.equal(fs.existsSync(attachmentPath), true);
+      assert.deepEqual(fs.readFileSync(attachmentPath), Buffer.from("V29ybGQ=", "base64"));
+      fs.rmSync(stateDir, { recursive: true, force: true });
+    }),
+  );
+
   it.effect("does not persist attachment files when projector transaction rolls back", () =>
     Effect.gen(function* () {
       const projectionPipeline = yield* OrchestrationProjectionPipeline;
diff --git a/apps/server/src/orchestration/Layers/ProjectionPipeline.ts b/apps/server/src/orchestration/Layers/ProjectionPipeline.ts
index 162468b18b0947c4a517aecfb63b46be4a8e8590..5474c602bb70e40ff378950226e89d60c8ccd1ec 100644
--- a/apps/server/src/orchestration/Layers/ProjectionPipeline.ts
+++ b/apps/server/src/orchestration/Layers/ProjectionPipeline.ts
@@ -364,9 +364,8 @@ const runAttachmentSideEffects = Effect.fn(function* (input: {
 
   const writesByPath = new Map<string, Uint8Array>();
   for (const pendingWrite of input.sideEffects.writes) {
-    if (!writesByPath.has(pendingWrite.absolutePath)) {
-      writesByPath.set(pendingWrite.absolutePath, pendingWrite.bytes);
-    }
+    // Last write wins for a path so updated attachment contents replace stale bytes.
+    writesByPath.set(pendingWrite.absolutePath, pendingWrite.bytes);
   }
   yield* Effect.forEach(
     writesByPath.entries(),
@@ -375,12 +374,7 @@ const runAttachmentSideEffects = Effect.fn(function* (input: {
         yield* input.fileSystem.makeDirectory(input.path.dirname(absolutePath), {
           recursive: true,
         });
-        const alreadyExists = yield* input.fileSystem
-          .exists(absolutePath)
-          .pipe(Effect.catch(() => Effect.succeed(false)));
-        if (!alreadyExists) {
-          yield* input.fileSystem.writeFile(absolutePath, bytes);
-        }
+        yield* input.fileSystem.writeFile(absolutePath, bytes);
       }),
     { concurrency: 1 },
   );