diff --git a/agent-chat/adapters/transcript.ts b/agent-chat/adapters/transcript.ts
index 25fde57cb302..f972290bbd50 100644
--- a/agent-chat/adapters/transcript.ts
+++ b/agent-chat/adapters/transcript.ts
@@ -360,26 +360,35 @@ export const TRANSCRIPT_INITIAL_WINDOW_BYTES = 8 * 1024 * 1024;
 const TRANSCRIPT_POLL_MS = 500;
 const TRANSCRIPT_READ_CHUNK = 1024 * 1024;
 
-/** Follows an append-only JSONL file by offset, delivering complete lines. */
+/** Follows JSONL appends by offset, resetting when the file is replaced. */
 export class TranscriptTail {
   private offset = -1;
+  private identity: { dev: number; ino: number } | null = null;
   private pending = "";
   private timer: ReturnType<typeof setInterval> | null = null;
-  private inflight: Promise<void> | null = null;
+  // Filesystem calls can finish after stop/restart. Only the current lifetime
+  // may advance decoding state or deliver callbacks; old handles still close.
+  private generation = 0;
+  private stopped = false;
+  private inflight: { generation: number; promise: Promise<void> } | null = null;
   private decoder = new TextDecoder();
 
   constructor(
     readonly path: string,
     private readonly onLines: (lines: string[], mtimeMs: number) => void,
-    private readonly opts: { pollMs?: number; initialWindowBytes?: number } = {},
+    private readonly opts: { pollMs?: number; initialWindowBytes?: number; onReset?: () => void } = {},
   ) {}
 
   start() {
     if (this.timer) return;
+    this.stopped = false;
+    const generation = this.generation;
     // A transcript that stops being readable between stat and open (deleted,
     // or a root-owned file) must not reject out of the timer: an unhandled
     // rejection ends the whole sidecar. The next poll tries again.
-    const tick = () => void this.poll().catch(() => {});
+    const tick = () => {
+      if (this.ownsRead(generation)) void this.poll().catch(() => {});
+    };
     tick();
     this.timer = setInterval(tick, this.opts.pollMs ?? TRANSCRIPT_POLL_MS);
   }
@@ -387,39 +396,66 @@ export class TranscriptTail {
   stop() {
     if (this.timer) clearInterval(this.timer);
     this.timer = null;
+    this.stopped = true;
+    this.generation++;
   }
 
-  /** Reads everything appended since the last poll; joins a read in flight. */
+  /** Reads new appends, joining only a read from the current lifetime. */
   poll(): Promise<void> {
-    if (!this.inflight) {
-      this.inflight = this.read().finally(() => {
-        this.inflight = null;
+    if (this.stopped) return Promise.resolve();
+    const generation = this.generation;
+    if (!this.inflight || this.inflight.generation !== generation) {
+      const flight = { generation, promise: this.read(generation) };
+      this.inflight = flight;
+      flight.promise = flight.promise.finally(() => {
+        if (this.inflight === flight) this.inflight = null;
       });
     }
-    return this.inflight;
+    return this.inflight.promise;
   }
 
-  private async read(): Promise<void> {
-    const info = await stat(this.path).catch(() => null);
-    if (!info) return;
-    let skipPartialFirstLine = false;
-    if (this.offset < 0) {
-      const window = this.opts.initialWindowBytes ?? TRANSCRIPT_INITIAL_WINDOW_BYTES;
-      this.offset = Math.max(0, info.size - window);
-      skipPartialFirstLine = this.offset > 0;
-    } else if (info.size < this.offset) {
-      // Truncated or replaced: follow the new file from its start.
-      this.offset = 0;
-      this.pending = "";
-      this.decoder = new TextDecoder();
-    }
-    if (info.size === this.offset) return;
+  private ownsRead(generation: number): boolean {
+    return !this.stopped && this.generation === generation;
+  }
+
+  private async read(generation: number): Promise<void> {
+    const probe = await stat(this.path).catch(() => null);
+    if (!this.ownsRead(generation) || !probe) return;
+    if (this.identity?.dev === probe.dev && this.identity.ino === probe.ino && probe.size === this.offset) return;
     const handle = await open(this.path, "r");
     try {
+      if (!this.ownsRead(generation)) return;
+      // Use the opened file's identity and size: an atomic rename can replace
+      // the path between the probe and open, even with unchanged size/mtime.
+      const info = await handle.stat();
+      if (!this.ownsRead(generation)) return;
+      const reset = this.identity !== null && (
+        this.identity.dev !== info.dev || this.identity.ino !== info.ino || info.size < this.offset
+      );
+      if (this.identity?.dev !== info.dev || this.identity.ino !== info.ino) {
+        this.offset = -1;
+        this.pending = "";
+        this.decoder = new TextDecoder();
+      }
+      this.identity = { dev: info.dev, ino: info.ino };
+      let skipPartialFirstLine = false;
+      if (this.offset < 0) {
+        const window = this.opts.initialWindowBytes ?? TRANSCRIPT_INITIAL_WINDOW_BYTES;
+        this.offset = Math.max(0, info.size - window);
+        skipPartialFirstLine = this.offset > 0;
+      } else if (info.size < this.offset) {
+        // An in-place truncation keeps its inode but still resets decoding.
+        this.offset = 0;
+        this.pending = "";
+        this.decoder = new TextDecoder();
+      }
+      if (reset) this.opts.onReset?.();
+      if (!this.ownsRead(generation)) return;
+      if (info.size === this.offset) return;
       const buf = new Uint8Array(TRANSCRIPT_READ_CHUNK);
-      while (this.offset < info.size) {
+      while (this.ownsRead(generation) && this.offset < info.size) {
         const { bytesRead } = await handle.read(buf, 0, Math.min(buf.length, info.size - this.offset), this.offset);
-        if (bytesRead <= 0) break;
+        if (!this.ownsRead(generation) || bytesRead <= 0) break;
         this.offset += bytesRead;
         this.pending += this.decoder.decode(buf.subarray(0, bytesRead), { stream: true });
         if (skipPartialFirstLine) {
@@ -476,25 +512,43 @@ export function attachTranscript(
   onTitle?: (title: string) => void,
   opts: { pollMs?: number; initialWindowBytes?: number; onTick?: () => void } = {},
 ): TranscriptTail {
-  const parser = transcriptParser(agent);
+  let parser = transcriptParser(agent);
   const refreshStatus = () => {
     const st = transcriptState(sess);
-    if (!st) return;
+    if (!st || st.tail !== tail) return;
     sess.setStatus(transcriptLooksRunning(sess.events, st.lastWriteMs) ? "running" : "idle");
-    opts.onTick?.();
+    if (transcriptState(sess) === st) opts.onTick?.();
   };
   const tail = new TranscriptTail(path, (lines, mtimeMs) => {
     const st = transcriptState(sess);
+    if (!st || st.tail !== tail) return;
     // Activity comes from the file's own write time, so a transcript that
     // went idle long ago does not look busy when its history first loads.
-    if (st) st.lastWriteMs = mtimeMs;
+    st.lastWriteMs = mtimeMs;
     const title = parser.title;
     for (const line of lines) {
-      for (const evt of parser.parse(line)) sess.emit(evt);
+      if (transcriptState(sess) !== st) return;
+      for (const evt of parser.parse(line)) {
+        if (transcriptState(sess) !== st) return;
+        sess.emit(evt);
+      }
     }
+    if (transcriptState(sess) !== st) return;
     if (parser.title && parser.title !== title) onTitle?.(parser.title);
     refreshStatus();
-  }, opts);
+  }, {
+    ...opts,
+    onReset: () => {
+      const st = transcriptState(sess);
+      if (!st || st.tail !== tail) return;
+      parser = transcriptParser(agent);
+      st.parser = parser;
+      st.lastWriteMs = 0;
+      if (sess.resetHistory) sess.resetHistory();
+      else sess.events.length = 0;
+      refreshStatus();
+    },
+  });
   const state: TranscriptState = {
     tail,
     parser,
diff --git a/agent-chat/server.ts b/agent-chat/server.ts
index a923b16efbb1..0493ecd310e9 100644
--- a/agent-chat/server.ts
+++ b/agent-chat/server.ts
@@ -395,6 +395,9 @@ function createSession(
       }
       emitSessionEvent(sess, evt);
     },
+    resetHistory() {
+      resetSessionHistory(sess);
+    },
     setStatus(status: SessionStatus) {
       const pendingDone = sess.internal.pendingDoneEmit as Promise<void> | undefined;
       if (status === "idle" && pendingDone) {
@@ -480,6 +483,12 @@ function broadcastSessionHistory(sess: Session) {
   for (const ws of sess.sockets) ws.send(payload);
 }
 
+export function resetSessionHistory(sess: Session) {
+  sess.events.length = 0;
+  delete sess.internal.eventGenerations;
+  broadcastSessionHistory(sess);
+}
+
 function activeAttributionGenerations(sess: Session): number[] {
   let generations = sess.internal.activeTurnGenerations as number[] | undefined;
   if (!generations) {
@@ -649,12 +658,10 @@ function ensureTranscriptSession(source: TranscriptSource): Session {
     // The agent's transcript moved (for example a resolved fallback path):
     // re-point the same session so open pages stay subscribed.
     existing.adapter.dispose(existing);
-    existing.events.length = 0;
-    delete existing.internal.eventGenerations;
     existing.transcript.path = source.path;
     existing.internal.transcriptTarget = { agentSessionId: source.sessionId, surfaceId: source.surfaceId };
     startTranscriptTail(existing, source);
-    broadcastSessionHistory(existing);
+    resetSessionHistory(existing);
     return existing;
   }
   const sess = createSession(source.agent, source.cwd ?? DEFAULT_CWD, false, transcriptTitle(source), {}, {}, {
diff --git a/agent-chat/test/transcript-disposal.test.ts b/agent-chat/test/transcript-disposal.test.ts
new file mode 100644
index 000000000000..f7b669b31c6b
--- /dev/null
+++ b/agent-chat/test/transcript-disposal.test.ts
@@ -0,0 +1,136 @@
+import assert from "node:assert/strict";
+import { mkdir, mkdtemp, rm, writeFile } from "node:fs/promises";
+import { join } from "node:path";
+import { attachTranscript, transcriptAdapter, TranscriptTail, type TranscriptAgent } from "../adapters/transcript";
+import type { AgentEvent, SessionCtx, SessionStatus } from "../types";
+
+const descriptors = new Map(["setInterval", "clearInterval"].map((key) => [key, Object.getOwnPropertyDescriptor(globalThis, key)]));
+const intervals = new Map<number, { callback: () => void; delay: number }>();
+let nextInterval = 1;
+Object.defineProperty(globalThis, "setInterval", { configurable: true, writable: true, value(callback: () => void, delay: number) {
+  const id = nextInterval++; intervals.set(id, { callback, delay }); return id;
+} });
+Object.defineProperty(globalThis, "clearInterval", { configurable: true, writable: true, value(id: number) { intervals.delete(id); } });
+
+const tails: TranscriptTail[] = [];
+const reads: Promise<void>[] = [];
+let root: string | undefined;
+const poll = (tail: TranscriptTail) => { const read = tail.poll(); reads.push(read); return read; };
+const remember = (tail: TranscriptTail) => { tails.push(tail); return tail; };
+const user = (agent: TranscriptAgent, id: string, text: string) => agent === "claude"
+  ? { type: "user", uuid: id, message: { role: "user", content: text } }
+  : { type: "event_msg", payload: { type: "user_message", message: text } };
+const title = (agent: TranscriptAgent, text: string) => agent === "claude"
+  ? { type: "ai-title", aiTitle: text }
+  : { type: "event_msg", payload: { type: "thread_name_updated", thread_name: text } };
+const jsonl = (...records: unknown[]) => records.map((record) => JSON.stringify(record)).join("\n") + "\n";
+function session() {
+  const statuses: SessionStatus[] = [];
+  const sess = {
+    events: [] as AgentEvent[], internal: {} as Record<string, unknown>,
+    emit(event: AgentEvent) { this.events.push(event); },
+    setStatus(status: SessionStatus) { statuses.push(status); },
+  } as unknown as SessionCtx;
+  return { sess, statuses };
+}
+
+try {
+  const scratch = join(import.meta.dir, "../scratch");
+  await mkdir(scratch, { recursive: true });
+  root = await mkdtemp(join(scratch, "transcript-disposal-"));
+  for (const agent of ["claude", "codex"] as const) {
+    const path = join(root, `${agent}-disposed.jsonl`);
+    await writeFile(path, jsonl(user(agent, "old", "disposed content"), title(agent, "disposed title")));
+    const { sess, statuses } = session();
+    const titles: string[] = [];
+    let ticks = 0;
+    const tail = remember(attachTranscript(sess, agent, path, (next) => titles.push(next), { pollMs: 10_000, onTick: () => ticks++ }));
+    const activeRead = poll(tail);
+    transcriptAdapter.dispose(sess);
+    await activeRead;
+    assert.deepEqual(sess.events, [], "a disposed transcript read must not emit late events");
+    assert.deepEqual(titles, [], "a disposed transcript read must not rename the view");
+    assert.deepEqual(statuses, []);
+    assert.equal(ticks, 0);
+    assert.equal(intervals.size, 0);
+  }
+
+  for (const agent of ["claude", "codex"] as const) {
+    const oldPath = join(root, `${agent}-old.jsonl`);
+    const newPath = join(root, `${agent}-new.jsonl`);
+    await writeFile(oldPath, jsonl(user(agent, "old", "old content"), title(agent, "old title")));
+    await writeFile(newPath, jsonl(user(agent, "new", "new content"), title(agent, "new title")));
+    const { sess } = session();
+    const titles: string[] = [];
+    let oldTicks = 0;
+    let newTicks = 0;
+    const oldTail = remember(attachTranscript(sess, agent, oldPath, (next) => titles.push(next), { pollMs: 10_000, onTick: () => oldTicks++ }));
+    const oldStatus = [...intervals.values()].find((interval) => interval.delay === 2_000)!.callback;
+    const oldRead = poll(oldTail);
+    transcriptAdapter.dispose(sess);
+    const newTail = remember(attachTranscript(sess, agent, newPath, (next) => titles.push(next), { pollMs: 10_000, onTick: () => newTicks++ }));
+    await Promise.all([oldRead, poll(newTail)]);
+    assert.deepEqual(sess.events, [{ kind: "user", text: "new content" }], "old reads must not append to a redirected session");
+    assert.deepEqual(titles, ["new title"]);
+    assert.equal(oldTicks, 0);
+    assert.ok(newTicks > 0);
+    oldStatus();
+    assert.equal(oldTicks, 0, "a retained old status callback must not inspect the new attachment");
+    transcriptAdapter.dispose(sess);
+    assert.equal(intervals.size, 0);
+  }
+
+  const restartPath = join(root, "restart.jsonl");
+  await writeFile(restartPath, '{"line":1}\n');
+  const restartedLines: string[] = [];
+  const restart = remember(new TranscriptTail(restartPath, (lines) => restartedLines.push(...lines), { pollMs: 10_000 }));
+  const oldRead = poll(restart);
+  restart.stop();
+  restart.start();
+  const newRead = poll(restart);
+  assert.notEqual(newRead, oldRead, "restart must not join the canceled read from the previous lifetime");
+  await Promise.all([oldRead, newRead]);
+  assert.deepEqual(restartedLines, ['{"line":1}']);
+  restart.stop();
+  await poll(restart);
+  assert.deepEqual(restartedLines, ['{"line":1}']);
+  assert.equal(intervals.size, 0);
+
+  const burstPath = join(root, "burst.jsonl");
+  const rows = Array.from({ length: 256 }, (_, index) => JSON.stringify({ index, text: "x".repeat(16_384) }));
+  await writeFile(burstPath, rows.join("\n") + "\n");
+  const received: string[] = [];
+  let batches = 0;
+  const burst = remember(new TranscriptTail(burstPath, (lines) => {
+    batches++; received.push(...lines);
+    if (batches === 1) burst.stop();
+  }, { pollMs: 10_000 }));
+  await poll(burst);
+  assert.equal(batches, 1, "stop during delivery must prevent reading more chunks");
+  assert.ok(received.length > 0 && received.length < rows.length);
+  burst.start();
+  await poll(burst);
+  assert.deepEqual(received, rows, "restart resumes at the last delivered byte offset without duplicating rows");
+  burst.stop();
+
+  const batchPath = join(root, "dispose-in-batch.jsonl");
+  await writeFile(batchPath, jsonl(user("claude", "first", "first"), user("claude", "second", "second"), title("claude", "stale title")));
+  const { sess, statuses } = session();
+  sess.emit = (event) => { sess.events.push(event); transcriptAdapter.dispose(sess); };
+  const titles: string[] = [];
+  const batch = remember(attachTranscript(sess, "claude", batchPath, (next) => titles.push(next), { pollMs: 10_000 }));
+  await poll(batch);
+  assert.deepEqual(sess.events, [{ kind: "user", text: "first" }], "disposal during a batch must stop remaining event delivery");
+  assert.deepEqual(titles, []);
+  assert.deepEqual(statuses, []);
+  assert.equal(intervals.size, 0, "all owned timers are released");
+  console.log("Transcript disposal, redirected views, retained status callbacks, chunk cancellation, and immediate restart: OK");
+} finally {
+  for (const tail of tails) tail.stop();
+  await Promise.allSettled(reads);
+  if (root) await rm(root, { recursive: true, force: true });
+  for (const [key, descriptor] of descriptors) {
+    if (descriptor) Object.defineProperty(globalThis, key, descriptor);
+    else delete (globalThis as any)[key];
+  }
+}
diff --git a/agent-chat/test/transcript-replacement.test.ts b/agent-chat/test/transcript-replacement.test.ts
new file mode 100644
index 000000000000..829c232b091c
--- /dev/null
+++ b/agent-chat/test/transcript-replacement.test.ts
@@ -0,0 +1,189 @@
+import { afterEach, describe, expect, test } from "bun:test";
+import { appendFileSync, mkdirSync, mkdtempSync, renameSync, rmSync, statSync, utimesSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+import { attachTranscript, transcriptAdapter, TranscriptTail } from "../adapters/transcript";
+import type { AgentEvent, SessionCtx } from "../types";
+
+const roots: string[] = [];
+function fixture() {
+  const scratch = join(import.meta.dir, "../scratch");
+  mkdirSync(scratch, { recursive: true });
+  const root = mkdtempSync(join(scratch, "transcript-replacement-"));
+  roots.push(root);
+  return join(root, "transcript.jsonl");
+}
+function replace(path: string, bytes: string | Uint8Array, preserveTimes = false) {
+  const previous = statSync(path);
+  writeFileSync(`${path}.next`, bytes);
+  if (preserveTimes) utimesSync(`${path}.next`, previous.atime, previous.mtime);
+  renameSync(`${path}.next`, path);
+  expect(statSync(path).ino).not.toBe(previous.ino);
+}
+afterEach(() => {
+  for (const root of roots.splice(0)) rmSync(root, { recursive: true, force: true });
+});
+
+describe("transcript file replacement", () => {
+  test("follows a replacement with the same size and timestamp", async () => {
+    const path = fixture();
+    writeFileSync(path, '{"old":1}\n');
+    utimesSync(path, 1_700_000_000, 1_700_000_000);
+    const seen: string[] = [];
+    const tail = new TranscriptTail(path, (lines) => seen.push(...lines));
+    await tail.poll();
+    const before = statSync(path);
+    replace(path, '{"new":2}\n', true);
+    expect(statSync(path).size).toBe(before.size);
+    expect(statSync(path).mtimeMs).toBe(before.mtimeMs);
+    await tail.poll();
+    expect(seen).toEqual(['{"old":1}', '{"new":2}']);
+    await tail.poll();
+    expect(seen).toHaveLength(2);
+  });
+
+  test("follows a larger replacement from its own start", async () => {
+    const path = fixture();
+    writeFileSync(path, '{"old":1}\n');
+    const seen: string[] = [];
+    const tail = new TranscriptTail(path, (lines) => seen.push(...lines));
+    await tail.poll();
+    replace(path, '{"new":2}\n{"next":3}\n');
+    await tail.poll();
+    expect(seen).toEqual(['{"old":1}', '{"new":2}', '{"next":3}']);
+    appendFileSync(path, '{"appended":4}\n');
+    await tail.poll();
+    expect(seen.at(-1)).toBe('{"appended":4}');
+    expect(seen).toHaveLength(4);
+  });
+
+  test("does not join an old partial UTF-8 line to a replacement", async () => {
+    const path = fixture();
+    writeFileSync(path, Buffer.concat([Buffer.from('{"old":"'), Buffer.from([0xe2, 0x82])]));
+    const seen: string[] = [];
+    const tail = new TranscriptTail(path, (lines) => seen.push(...lines));
+    await tail.poll();
+    expect(seen).toEqual([]);
+    replace(path, '{"new":"🙂"}\n');
+    await tail.poll();
+    expect(seen).toEqual(['{"new":"🙂"}']);
+  });
+
+  test("preserves partial UTF-8 decoding for ordinary appends", async () => {
+    const path = fixture();
+    writeFileSync(path, Buffer.concat([Buffer.from('{"text":"'), Buffer.from([0xe2, 0x82])]));
+    const seen: string[] = [];
+    const tail = new TranscriptTail(path, (lines) => seen.push(...lines));
+    await tail.poll();
+    appendFileSync(path, Buffer.concat([Buffer.from([0xac]), Buffer.from('"}\n')]));
+    await tail.poll();
+    expect(seen).toEqual(['{"text":"€"}']);
+  });
+
+  test("applies the bounded initial window to a replacement", async () => {
+    const path = fixture();
+    writeFileSync(path, '{"old":1}\n');
+    const seen: string[] = [];
+    const tail = new TranscriptTail(path, (lines) => seen.push(...lines), { initialWindowBytes: 40 });
+    await tail.poll();
+    replace(path, `${"x".repeat(200)}\n{"keep":1}\n`);
+    await tail.poll();
+    expect(seen).toEqual(['{"old":1}', '{"keep":1}']);
+  });
+
+  test("remembers an empty replacement before its first append", async () => {
+    const path = fixture();
+    writeFileSync(path, '{"old":"unfinished');
+    const seen: string[] = [];
+    const tail = new TranscriptTail(path, (lines) => seen.push(...lines));
+    await tail.poll();
+    replace(path, "");
+    await tail.poll();
+    appendFileSync(path, '{"new":1}\n');
+    await tail.poll();
+    expect(seen).toEqual(['{"new":1}']);
+  });
+
+  test.each(["claude", "codex"] as const)("attached %s view receives events after replacement", async (agent) => {
+    const path = fixture();
+    const record = (id: string, text: string) => agent === "claude"
+      ? { type: "user", uuid: id, message: { role: "user", content: text } }
+      : { type: "event_msg", payload: { type: "user_message", message: text } };
+    writeFileSync(path, JSON.stringify(record("old", "old")) + "\n");
+    const sess = {
+      events: [] as AgentEvent[], internal: {} as Record<string, unknown>,
+      emit(event: AgentEvent) { this.events.push(event); }, setStatus() {},
+    } as unknown as SessionCtx;
+    const tail = attachTranscript(sess, agent, path, undefined, { pollMs: 10_000 });
+    try {
+      await tail.poll();
+      replace(path, JSON.stringify(record("new", "new content after replacement")) + "\n");
+      await tail.poll();
+      expect(sess.events).toEqual([
+        { kind: "user", text: "new content after replacement" },
+      ]);
+    } finally {
+      transcriptAdapter.dispose(sess);
+    }
+  });
+
+  test.each(["claude", "codex"] as const)("%s replaces copied history without duplicating the open view", async (agent) => {
+    const path = fixture();
+    const row = (id: string, text: string) => agent === "claude"
+      ? { type: "user", uuid: id, message: { role: "user", content: text } }
+      : { type: "event_msg", payload: { type: "user_message", message: text } };
+    const original = [row("old-1", "first"), row("old-2", "second")].map((entry) => JSON.stringify(entry)).join("\n") + "\n";
+    writeFileSync(path, original);
+    const sess = {
+      events: [] as AgentEvent[], internal: {} as Record<string, unknown>,
+      emit(event: AgentEvent) { this.events.push(event); }, setStatus() {},
+    } as unknown as SessionCtx;
+    const tail = attachTranscript(sess, agent, path, undefined, { pollMs: 10_000 });
+    try {
+      await tail.poll();
+      replace(path, original + JSON.stringify(row("new", "third")) + "\n");
+      await tail.poll();
+      expect(sess.events).toEqual([
+        { kind: "user", text: "first" }, { kind: "user", text: "second" }, { kind: "user", text: "third" },
+      ]);
+    } finally {
+      transcriptAdapter.dispose(sess);
+    }
+  });
+
+  test("notifies subscribed views of the reset before replaying replacement events", async () => {
+    const { emitSessionEventForTest, resetSessionHistory } = await import("../server");
+    const path = fixture();
+    const row = (text: string) => JSON.stringify({ type: "event_msg", payload: { type: "user_message", message: text } }) + "\n";
+    const original = row("first") + row("second");
+    writeFileSync(path, original);
+    const messages: any[] = [];
+    const sess = {
+      id: "t-replacement", provider: "codex", cwd: join(path, ".."), title: "fixture", status: "idle",
+      autoApprove: false, startOptions: {}, events: [], internal: {}, createdAt: 0,
+      adapter: transcriptAdapter, transcript: { agent: "codex", path },
+      sockets: new Set([{ send(data: any) { messages.push(JSON.parse(String(data))); return 1; } }]),
+      resetHistory() { resetSessionHistory(sess); },
+      emit(event: AgentEvent) { emitSessionEventForTest(sess, event); },
+      setStatus(status: SessionCtx["status"]) { sess.status = status; },
+    } as unknown as Parameters<typeof resetSessionHistory>[0];
+    const tail = attachTranscript(sess, "codex", path, undefined, { pollMs: 10_000 });
+    try {
+      await tail.poll();
+      messages.length = 0;
+      replace(path, original + row("third"));
+      await tail.poll();
+      expect(messages[0]).toMatchObject({ kind: "history", sessionId: sess.id, events: [] });
+      expect(messages.filter((message) => message.kind === "history")).toHaveLength(1);
+      expect(messages.filter((message) => message.kind === "event").map((message) => message.evt.text)).toEqual(["first", "second", "third"]);
+      expect(sess.internal.eventGenerations).toEqual([0, 0, 0]);
+      messages.length = 0;
+      replace(path, "");
+      await tail.poll();
+      expect(sess.events).toEqual([]);
+      expect(messages).toHaveLength(1);
+      expect(messages[0]).toMatchObject({ kind: "history", sessionId: sess.id, events: [] });
+    } finally {
+      transcriptAdapter.dispose(sess);
+    }
+  });
+});
diff --git a/agent-chat/types.ts b/agent-chat/types.ts
index 8a1518aa91bf..c88c34645ded 100644
--- a/agent-chat/types.ts
+++ b/agent-chat/types.ts
@@ -108,6 +108,8 @@ export interface SessionCtx {
   // Adapter-private state (child proc, provider session/thread ids, rpc counters).
   internal: Record<string, unknown>;
   emit(evt: AgentEvent): void;
+  /** Replace replayed transcript history when its source file resets. */
+  resetHistory?(): void;
   setStatus(status: SessionStatus): void;
 }