diff --git a/tests/fake-codex-server.ts b/tests/fake-codex-server.ts
new file mode 100644
index 000000000..dc71863a1
--- /dev/null
+++ b/tests/fake-codex-server.ts
@@ -0,0 +1,333 @@
+// FAB-03 deterministic fake `codex app-server` (0.146.0 wire protocol).
+// A child process that speaks the SAME newline-delimited JSON-RPC 2.0 protocol as
+// the real app-server, driven by the FAKE_CODEX_SCRIPT env var. Tests point the
+// adapter at this fake via its `command` option so process behavior (crash, hang,
+// late events, duplicate terminal, approval round-trips) is fully deterministic
+// with zero model spend.
+import { createInterface } from "node:readline";
+
+type ScriptTurn = {
+  inputMatch?: string;
+  notifications?: Array<{ method: string; params: Record<string, unknown> }>;
+  lateEvents?: Array<{ method: string; params: Record<string, unknown> }>;
+  duplicateTerminal?: boolean;
+  emitTerminalFirst?: boolean;
+  heldUntilInterrupt?: boolean;
+  approval?: { method: string; params: Record<string, unknown> };
+  status?: "completed" | "failed" | "interrupted" | "inProgress";
+  turnError?: string;
+};
+
+type Script = {
+  startupDelayMs?: number;
+  neverRespondToInit?: boolean;
+  exitAtStartup?: boolean;
+  exitCode?: number;
+  crashOnTurnStart?: boolean;
+  hangOnShutdown?: boolean;
+  rejectThreadStart?: string;
+  emitJunkBeforeInit?: boolean;
+  emitLateCompleteAfterInterrupt?: boolean;
+  turns?: ScriptTurn[];
+};
+
+const script: Script = JSON.parse(process.env.FAKE_CODEX_SCRIPT ?? "{}");
+
+const wait = (ms: number): Promise<void> =>
+  new Promise((r) => setTimeout(r, ms));
+
+function send(msg: unknown): void {
+  process.stdout.write(JSON.stringify(msg) + "\n");
+}
+
+function respond(id: string | number, result: unknown): void {
+  send({ jsonrpc: "2.0", id, result });
+}
+
+function respondError(id: string | number, message: string): void {
+  send({ jsonrpc: "2.0", id, error: { code: -32000, message } });
+}
+
+function notify(method: string, params: Record<string, unknown>): void {
+  send({ jsonrpc: "2.0", method, params, emittedAtMs: Date.now() });
+}
+
+let threadSeq = 0;
+let turnSeq = 0;
+let approvalSeq = 100;
+
+/** Responses we still owe after sending a server->client approval request. */
+const awaitingApprovalResponse = new Map<
+  string | number,
+  { resolve: (msg: Record<string, unknown>) => void }
+>();
+
+function threadOf(id: string): Record<string, unknown> {
+  return {
+    id,
+    sessionId: `session-${id}`,
+    forkedFromId: null,
+    parentThreadId: null,
+    preview: "",
+    ephemeral: true,
+    isPinned: false,
+    modelProvider: "openai",
+    createdAt: Date.now() / 1000,
+    updatedAt: Date.now() / 1000,
+    recencyAt: null,
+    status: { type: "idle" },
+    path: null,
+    cwd: process.cwd(),
+    cliVersion: "0.146.0",
+    source: { type: "codexAppServer" },
+    threadSource: null,
+    agentNickname: null,
+    agentRole: null,
+    gitInfo: null,
+    name: null,
+    turns: [],
+  };
+}
+
+function turnOf(id: string, status: string, error: Record<string, unknown> | null): Record<string, unknown> {
+  return {
+    id,
+    items: [],
+    itemsView: { itemIds: [] },
+    status,
+    error,
+    startedAt: Date.now() / 1000,
+    completedAt: status === "inProgress" ? null : Date.now() / 1000,
+    durationMs: null,
+  };
+}
+
+/** Terminal payload shared by every terminal-event mode so payloads stay identical. */
+function completedTurn(
+  turnId: string,
+  status: string | undefined,
+  turnError: string | undefined,
+): Record<string, unknown> {
+  const resolved = status ?? "completed";
+  return turnOf(
+    turnId,
+    resolved,
+    resolved === "failed"
+      ? { message: turnError ?? "fake failure", codexErrorInfo: null, additionalDetails: null }
+      : null,
+  );
+}
+
+const rl = createInterface({ input: process.stdin, crlfDelay: 100 });
+
+rl.on("line", (line) => {
+  let msg: Record<string, unknown>;
+  try {
+    msg = JSON.parse(line) as Record<string, unknown>;
+  } catch {
+    return; // malformed driver line: ignore
+  }
+  void handleMessage(msg);
+});
+
+async function handleMessage(msg: Record<string, unknown>): Promise<void> {
+  const id = msg["id"] as string | number | undefined;
+  const method = typeof msg["method"] === "string" ? msg["method"] : "";
+
+  // A response to one of OUR server->client approval requests.
+  if (id !== undefined && !method) {
+    const pending = awaitingApprovalResponse.get(id);
+    if (pending) {
+      awaitingApprovalResponse.delete(id);
+      pending.resolve(msg);
+    }
+    return;
+  }
+
+  const params = (msg["params"] ?? {}) as Record<string, unknown>;
+  if (id === undefined) {
+    if (method === "shutdownRequested") {
+      process.stdout.write(JSON.stringify({ jsonrpc: "2.0", method: "shutdownAck", params: {} }) + "\n");
+      setTimeout(() => process.exit(0), 10);
+    }
+    return;
+  }
+
+  if (method === "initialize") {
+    if (script.startupDelayMs) {
+      await wait(script.startupDelayMs);
+    }
+    if (script.exitAtStartup) {
+      await wait(50);
+      process.exit(script.exitCode ?? 0);
+    }
+    if (script.neverRespondToInit) return; // startup timeout test
+    if (script.emitJunkBeforeInit) {
+      // Malformed + oversized lines must be dropped by the transport, not crash it.
+      process.stdout.write("this is not json\n");
+      process.stdout.write(JSON.stringify({ method: "junk", params: { x: "y".repeat(1000) } }) + "\n");
+    }
+    respond(id, {
+      userAgent: "fake-codex-app-server/0.146.0",
+      codexHome: process.env.CODEX_HOME ?? "",
+      platformFamily: "windows",
+      platformOs: "windows",
+    });
+    notify("configWarning", { summary: "fake", details: null });
+    return;
+  }
+  switch (method) {
+    case "thread/start": {
+      if (script.rejectThreadStart) {
+        respondError(id, script.rejectThreadStart);
+        return;
+      }
+      threadSeq++;
+      const threadId = `thread-${threadSeq}`;
+      respond(id, { thread: threadOf(threadId) });
+      notify("thread/started", { thread: threadOf(threadId) });
+      return;
+    }
+    case "thread/resume": {
+      const threadId = String(params["threadId"] ?? "thread-resumed");
+      respond(id, { thread: threadOf(threadId) });
+      return;
+    }
+    case "thread/fork": {
+      threadSeq++;
+      const threadId = `thread-${threadSeq}`;
+      respond(id, { thread: threadOf(threadId) });
+      return;
+    }
+    case "thread/list":
+      respond(id, { threads: [] });
+      return;
+    case "thread/delete":
+      respond(id, {});
+      return;
+    case "turn/start": {
+      const threadId = String(params["threadId"] ?? "thread-1");
+      if (script.crashOnTurnStart) {
+        await wait(30);
+        process.exit(7);
+      }
+      turnSeq++;
+      const turnId = `turn-${turnSeq}`;
+      const scripted = pickTurn(params);
+      respond(id, { turn: turnOf(turnId, "inProgress", null) });
+      notify("turn/started", {
+        threadId,
+        turn: { id: turnId, status: "inProgress", items: [], itemsView: { itemIds: [] } },
+      });
+      if (scripted?.emitTerminalFirst) {
+        // Out-of-order terminal: arrives BEFORE the scripted mid-turn notifications.
+        await wait(5);
+        notify("turn/completed", {
+          threadId,
+          turn: completedTurn(turnId, scripted?.status, scripted?.turnError),
+        });
+      }
+      for (const n of scripted?.notifications ?? []) {
+        await wait(5);
+        notify(n.method, { threadId, turnId, ...n.params });
+      }
+      if (scripted?.approval) {
+        await wait(5);
+        const reqId = ++approvalSeq;
+        send({
+          jsonrpc: "2.0",
+          id: reqId,
+          method: scripted.approval.method,
+          params: {
+            threadId,
+            turnId,
+            itemId: `item-${turnId}`,
+            startedAtMs: Date.now(),
+            reason: null,
+            ...scripted.approval.params,
+          },
+        });
+        await new Promise<void>((resolve) => {
+          awaitingApprovalResponse.set(reqId, { resolve: () => resolve() });
+        });
+      }
+      if (scripted?.heldUntilInterrupt) {
+        // Stay in progress until a turn/interrupt lands (interrupt mid-turn test).
+        return;
+      }
+      if (scripted?.duplicateTerminal) {
+        await wait(5);
+        notify("turn/completed", {
+          threadId,
+          turn: completedTurn(turnId, scripted?.status, scripted?.turnError),
+        });
+      }
+      // emitTerminalFirst already delivered the terminal; the trailing terminal
+      // belongs to the normal/duplicate modes only (modes stay separate).
+      if (!scripted?.emitTerminalFirst) {
+        await wait(5);
+        notify("turn/completed", {
+          threadId,
+          turn: completedTurn(turnId, scripted?.status, scripted?.turnError),
+        });
+      }
+      for (const late of scripted?.lateEvents ?? []) {
+        await wait(5);
+        notify(late.method, { threadId, turnId, ...late.params });
+      }
+      return;
+    }
+    case "turn/interrupt": {
+      const threadId = String(params["threadId"] ?? "thread-1");
+      const turnId = String(params["turnId"] ?? "turn-1");
+      respond(id, {});
+      await wait(5);
+      notify("turn/completed", { threadId, turn: turnOf(turnId, "interrupted", null) });
+      // Optional adversarial late terminal: a completed arriving AFTER interrupt
+      // must not resurrect the turn (adapter terminal-once-per-turn invariant).
+      if (script.emitLateCompleteAfterInterrupt) {
+        await wait(10);
+        notify("turn/completed", { threadId, turn: turnOf(turnId, "completed", null) });
+      }
+      return;
+    }
+    default:
+      respondError(id, `fake: unsupported method ${method}`);
+  }
+}
+
+function pickTurn(params: Record<string, unknown>): ScriptTurn | undefined {
+  const input = (params["input"] ?? []) as Array<Record<string, unknown>>;
+  const text = String(input[0]?.["text"] ?? "");
+  const turns = script.turns ?? [];
+  if (turns.length === 0) return undefined;
+  return (
+    turns.find((t) => t.inputMatch !== undefined && text.includes(t.inputMatch)) ??
+    turns[0]
+  );
+}
+
+// The fake exits on stdin EOF unless the script asks it to hang (forced-shutdown test).
+process.stdin.on("end", () => {
+  if (script.hangOnShutdown) {
+    setInterval(() => {}, 1000); // stay alive despite EOF
+    return;
+  }
+  setTimeout(() => process.exit(0), 20);
+});
+
+// Parent-death watchdog: bun's --isolate runner can terminate a worker without
+// delivering stdin EOF on Windows, which leaves this child holding the worker's
+// stdout pipe. The runner then waits for pipe EOF forever on macOS (30-minute CI
+// job timeout) while Windows accumulates orphans that hold test ports across
+// runs. Exit as soon as the spawning process is gone, even mid-handshake or in
+// the hangOnShutdown script (the explicit force-kill test still wins first).
+const spawnerPid = process.ppid;
+const parentWatchdog = setInterval(() => {
+  try {
+    process.kill(spawnerPid, 0);
+  } catch {
+    process.exit(0);
+  }
+}, 500);
+parentWatchdog.unref?.();