fix(server): truncated Claude task output no longer starts with a replacement character

apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.test.ts

@@ -20,7 +20,7 @@ const makeTaskFile = Effect.gen(function* () {
   return { root, tasks, file: path.join(tasks, `${TASK_ID}.output`) };
 });
 
-// TestClock cannot interleave with real filesystem I/O; use the live clock.
+// TestClock cannot interleave with real filesystem I/O; tests that wait on a poll use the live clock.
 describe("tailClaudeTaskOutput", () => {
   it.live("follows a file that appears after the task starts, across split characters", () =>
     Effect.scoped(
@@ -48,6 +48,32 @@ describe("tailClaudeTaskOutput", () => {
     ).pipe(Effect.provide(NodeServices.layer)),
   );
 
+  it.effect("drops a split character when the file is truncated and rewritten", () =>
+    Effect.scoped(
+      Effect.gen(function* () {
+        const fileSystem = yield* FileSystem.FileSystem;
+        const { root, tasks, file } = yield* makeTaskFile;
+        yield* fileSystem.makeDirectory(tasks, { recursive: true });
+        // End the first read inside the two-byte é.
+        yield* fileSystem.writeFile(file, new TextEncoder().encode("café").subarray(0, 4));
+        const chunks = yield* Queue.unbounded<string>();
+        const stop = yield* Deferred.make<void>();
+        yield* tailClaudeTaskOutput({
+          root,
+          sessionId: SESSION_ID,
+          taskId: TASK_ID,
+          stop,
+          onChunk: (chunk) => Queue.offer(chunks, chunk),
+        }).pipe(Effect.forkScoped);
+        assert.equal(yield* Queue.take(chunks), "caf");
+
+        yield* fileSystem.writeFileString(file, "ok\n");
+        yield* Deferred.succeed(stop, undefined);
+        assert.equal(yield* Queue.take(chunks), "ok\n");
+      }),
+    ).pipe(Effect.provide(NodeServices.layer)),
+  );
+
   it.live("skips ahead when the command writes faster than it is read", () =>
     Effect.scoped(
       Effect.gen(function* () {

apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.ts

@@ -88,7 +88,10 @@ export const tailClaudeTaskOutput = Effect.fn("ClaudeTaskOutputTail.tail")(funct
       Effect.map((info) => Number(info.size)),
       Effect.orElseSucceed(() => offset),
     );
-    if (size < offset) offset = 0;
+    if (size < offset) {
+      offset = 0;
+      decoder = new TextDecoder("utf-8", { fatal: false });
+    }
     if (size > offset) {
       const start = size - offset > MAX_READ_BYTES ? size - MAX_READ_BYTES : offset;
       const bytes = yield* Effect.scoped(