@@ -588,17 +588,17 @@ type HeardTextSubscribe = (listener: () => void) => () => void;
 const useHeardText: (params: UseHeardTextParams) => UseHeardTextResult;
 ```
 
-`heard(now)` returns how many characters of the text the user has heard by `now` (a `performance.now()` timestamp) and whether the speech for it has finished. `chars` counts Unicode code points (`Array.from(text).length`), not UTF-16 code units. Return `undefined` when the text is not paced, and `{ chars: 0, done: false }` for a spoken text whose audio has not started yet.
+`heard(now)` returns how many characters of the text the user has heard by `now` (a `performance.now()` timestamp) and whether the speech for it has finished. `chars` counts Unicode code points (`Array.from(text).length`), not UTF-16 code units; `Infinity` means the whole text. Return `undefined` only for a text that is not spoken at all. A text that will be spoken but whose audio has not started, or that the voice source does not know yet, must return `{ chars: 0, done: false }`: while `heard` returns `undefined` the full text is shown, and the reveal never moves back past what has been shown. `heard` is also called during render, so keep it free of side effects.
 
 #### Props
 
-| Name         | Type                 | Required | Description                                                                                                              |
-| ------------ | -------------------- | -------- | ------------------------------------------------------------------------------------------------------------------------ |
-| `text`       | `string`             | yes      | The full text to reveal.                                                                                                 |
-| `heard`      | `HeardTextFunction`  | yes      | The speech progress of `text`.                                                                                           |
-| `subscribe`  | `HeardTextSubscribe` | no       | Change notifications from the speech progress source; each one makes the hook check `heard` again.                       |
-| `generating` | `boolean`            | no       | `true` while the text is still being written. The full text is revealed once `heard` reports `done` and this is falsy.   |
-| `live`       | `boolean`            | no       | Defaults to `true`. `false` reveals the full text at once, for example for a message loaded from history. Read at mount. |
+| Name         | Type                 | Required | Description                                                                                                            |
+| ------------ | -------------------- | -------- | ---------------------------------------------------------------------------------------------------------------------- |
+| `text`       | `string`             | yes      | The full text to reveal.                                                                                               |
+| `heard`      | `HeardTextFunction`  | yes      | The speech progress of `text`.                                                                                         |
+| `subscribe`  | `HeardTextSubscribe` | no       | Change notifications from the speech progress source; each one makes the hook check `heard` again. See below.          |
+| `generating` | `boolean`            | no       | `true` while the text is still being written. The full text is revealed once `heard` reports `done` and this is falsy. |
+| `live`       | `boolean`            | no       | Defaults to `true`. `false` reveals the full text at once, for example for a message loaded from history. See below.   |
 
 #### Returns
 
@@ -610,10 +610,11 @@ const useHeardText: (params: UseHeardTextParams) => UseHeardTextResult;
 
 Behaviour:
 
-- It reveals the first `floor(chars)` code points of `text` and never moves backwards. Emoji and other astral characters are never split.
-- While the text is paced and its speech is not done, it samples `heard` on every animation frame, and stops when the speech is done or the component unmounts. `subscribe` notifications and changes of `text` or `generating` make it check again.
+- It reveals the first `floor(chars)` code points of `text` and never moves backwards while `text` grows or is truncated. Emoji and other astral characters are never split.
+- When `text` is replaced by a different text (the next reply rendered by the same component), the reveal starts over: the length shown and `skip()` are reset, and `live` is read again. `live` is otherwise read only at mount.
+- While the text is paced and its speech is not done, it samples `heard` on every animation frame, and stops when the speech is done or the component unmounts. `subscribe` notifications and changes of `text` or `generating` make it check again. Without `subscribe`, nothing else does: pass it when `heard` can change after the speech is done, or start returning a value after returning `undefined`.
 - If `heard` returns `undefined` at first and a value later, the reveal continues from the length already shown.
-- `heard` and `subscribe` are read through refs: passing new functions on every render (inline arrow functions) does not restart the pacing. `subscribe` is called once and unsubscribed on unmount.
+- `heard` and `subscribe` are read through refs: passing new functions on every render (inline arrow functions) does not restart the pacing. `subscribe` is called once and unsubscribed on unmount; to switch to another speech source, remount the component (for example with a `key`).
 
 #### Example
 
 
@@ -72,14 +72,31 @@ describe('useHeardText', () => {
     expect(result.current.heardText).toBe(TEXT.slice(0, 4));
   });
 
-  it('treats non-finite and negative counts as nothing heard', () => {
+  it('treats NaN, negative and non-numeric counts as nothing heard', () => {
     let progress: HeardTextProgress = { chars: Number.NaN, done: false };
     const { result } = renderHeardText({ heard: () => progress, text: TEXT });
     expect(result.current.heardText).toBe('');
 
     progress = { chars: -3, done: false };
     advance(FRAME_MS);
     expect(result.current.heardText).toBe('');
+
+    progress = { chars: Number.NEGATIVE_INFINITY, done: false };
+    advance(FRAME_MS);
+    expect(result.current.heardText).toBe('');
+
+    // a JS caller returning no count at all
+    progress = { done: false } as unknown as HeardTextProgress;
+    advance(FRAME_MS);
+    expect(result.current.heardText).toBe('');
+  });
+
+  it('treats an infinite count as the whole text heard', () => {
+    const { result } = renderHeardText({
+      heard: () => ({ chars: Number.POSITIVE_INFINITY, done: false }),
+      text: TEXT,
+    });
+    expect(result.current).toMatchObject({ heardText: TEXT, isPaced: true });
   });
 
   it('never moves the reveal backwards', () => {
@@ -290,6 +307,73 @@ describe('useHeardText', () => {
     expect(result.current.heardText).toBe(TEXT);
   });
 
+  describe('when the text is replaced by a different text', () => {
+    const NEXT_TEXT = 'Sure, here is the forecast for tomorrow.';
+
+    it('starts a new reveal instead of carrying over the length already shown', () => {
+      let progress: HeardTextProgress = { chars: 100, done: false };
+      const heard: HeardTextFunction = () => progress;
+      const { rerender, result } = renderHeardText({ heard, text: TEXT });
+      expect(result.current.heardText).toBe(TEXT);
+
+      // the next reply, whose audio has not started yet
+      progress = { chars: 0, done: false };
+      rerender({ heard, text: NEXT_TEXT });
+      expect(result.current).toMatchObject({ heardText: '', isPaced: true });
+
+      progress = { chars: 4, done: false };
+      advance(FRAME_MS);
+      expect(result.current.heardText).toBe('Sure');
+    });
+
+    it('starts a new reveal after the previous text was shown unpaced', () => {
+      let progress: HeardTextProgress | undefined = undefined;
+      const heard: HeardTextFunction = () => progress;
+      const { rerender, result } = renderHeardText({ heard, text: TEXT });
+      expect(result.current).toMatchObject({ heardText: TEXT, isPaced: false });
+
+      progress = { chars: 2, done: false };
+      rerender({ heard, text: NEXT_TEXT });
+      expect(result.current).toMatchObject({ heardText: 'Su', isPaced: true });
+    });
+
+    it('clears a previous skip', () => {
+      const heard: HeardTextFunction = () => ({ chars: 3, done: false });
+      const { rerender, result } = renderHeardText({ heard, text: TEXT });
+      act(() => result.current.skip());
+      expect(result.current.heardText).toBe(TEXT);
+
+      rerender({ heard, text: NEXT_TEXT });
+      expect(result.current.heardText).toBe('Sur');
+    });
+
+    it('reads live again for the new text', () => {
+      const heard: HeardTextFunction = () => ({ chars: 3, done: false });
+      const { rerender, result } = renderHeardText({ heard, live: false, text: TEXT });
+      expect(result.current.heardText).toBe(TEXT);
+
+      rerender({ heard, live: true, text: NEXT_TEXT });
+      expect(result.current.heardText).toBe('Sur');
+    });
+
+    it('keeps the reveal when the text is truncated, and does not reuse the cut length', () => {
+      let progress: HeardTextProgress = { chars: 12, done: false };
+      const heard: HeardTextFunction = () => progress;
+      const { rerender, result } = renderHeardText({ heard, text: TEXT });
+      expect(result.current.heardText).toBe(TEXT.slice(0, 12));
+
+      // e.g. the reply was cut short when the user stopped it
+      rerender({ heard, text: TEXT.slice(0, 8) });
+      expect(result.current.heardText).toBe(TEXT.slice(0, 8));
+
+      // growing again from the truncated text continues from 8, not from 12
+      progress = { chars: 3, done: false };
+      rerender({ heard, text: `${TEXT.slice(0, 8)} again and again` });
+      advance(FRAME_MS);
+      expect(result.current.heardText).toBe(TEXT.slice(0, 8));
+    });
+  });
+
   describe('with emoji and other astral characters', () => {
     // waving hand + skin tone modifier, mathematical script X, grinning face, the
     // US flag (two regional indicators) and a BMP character: all but the last are
 
@@ -15,9 +15,14 @@ export type HeardTextProgress = {
 
 /**
  * Returns how much of the text the user has heard by `now` (a `performance.now()`
- * timestamp). Return `undefined` when the text is not paced (not spoken, or not known
- * to the voice source yet); return `{ chars: 0, done: false }` for a spoken text whose
- * audio has not started yet.
+ * timestamp).
+ *
+ * Return `undefined` only for a text that is not spoken at all: the full text is then
+ * shown, and the reveal never moves back past what has been shown. A text that will be
+ * spoken but whose audio has not started, or that the voice source does not know yet,
+ * must return `{ chars: 0, done: false }`, or it is shown in full before it is heard.
+ *
+ * Also called during render, so keep it free of side effects.
  */
 export type HeardTextFunction = (now: number) => HeardTextProgress | undefined;
 
@@ -39,10 +44,16 @@ export type UseHeardTextParams = {
   generating?: boolean;
   /**
    * Defaults to `true`. `false` reveals the full text at once, for example for a message
-   * loaded from history. Read once, when the hook mounts.
+   * loaded from history. Read when the hook mounts and when `text` is replaced by a
+   * different text, not on every change.
    */
   live?: boolean;
-  /** Optional change notifications from the speech progress source. */
+  /**
+   * Optional change notifications from the speech progress source. Without it, `heard`
+   * is only checked again while the text is paced and its speech is not done, and when
+   * `text` or `generating` change. Subscribed once; to switch to another source, remount
+   * the component (for example with a `key`).
+   */
   subscribe?: HeardTextSubscribe;
 };
 
@@ -56,13 +67,22 @@ export type UseHeardTextResult = {
 };
 
 type PacingState = {
-  /** Number of code points revealed. Never decreases. */
+  /** Number of code points revealed. Never decreases while the text continues. */
   count: number;
   isPaced: boolean;
   /** No more ticking is needed until the inputs change or the source notifies. */
   settled: boolean;
 };
 
+/** The reveal of one text. Starts over when the text is replaced by a different text. */
+type RevealState = PacingState & {
+  /** `live` as read when the reveal started. */
+  live: boolean;
+  skipped: boolean;
+  /** The text `count` refers to. */
+  text: string;
+};
+
 const now = () =>
   typeof performance !== 'undefined' && typeof performance.now === 'function'
     ? performance.now()
@@ -121,9 +141,10 @@ const sample = ({
     return { count: Math.max(previousCount, textLength), isPaced: true, settled: true };
   }
 
-  const heardChars = Number.isFinite(progress.chars)
-    ? Math.max(0, Math.floor(progress.chars))
-    : 0;
+  // `+Infinity` means the whole text has been heard; NaN, negative and non-numeric
+  // counts mean nothing has
+  const flooredChars = Math.floor(progress.chars);
+  const heardChars = flooredChars > 0 ? flooredChars : 0;
 
   return {
     count: Math.max(previousCount, Math.min(heardChars, textLength)),
@@ -132,6 +153,39 @@ const sample = ({
   };
 };
 
+const startReveal = ({
+  generating,
+  heard,
+  live,
+  text,
+  textLength,
+}: {
+  generating: boolean;
+  heard: HeardTextFunction;
+  live: boolean;
+  text: string;
+  textLength: number;
+}): RevealState => ({
+  ...sample({ generating, heard, live, previousCount: 0, skipped: false, textLength }),
+  live,
+  skipped: false,
+  text,
+});
+
+/**
+ * Whether `text` continues the reveal of `previous.text`: it extends or truncates the part
+ * already shown. Anything else (the next reply rendered by the same component) is a new
+ * text, whose reveal must not start from the length shown for the previous one.
+ */
+const continuesReveal = (previous: RevealState, text: string) => {
+  const offsets = getCodePointOffsets(previous.text);
+  const shown = previous.text.slice(
+    0,
+    offsets[Math.min(previous.count, offsets.length - 1)],
+  );
+  return text.startsWith(shown) || shown.startsWith(text);
+};
+
 const noop = () => undefined;
 
 /**
@@ -141,20 +195,24 @@ const noop = () => undefined;
  * backend and has no dependency on the chat SDK.
  *
  * - Reveals the first `floor(chars)` Unicode code points of `text` (emoji and other
- *   astral characters are never split) and never moves backwards.
+ *   astral characters are never split) and never moves backwards while `text` grows or
+ *   is truncated. When `text` is replaced by a different text (the next reply rendered
+ *   by the same component), the reveal starts over: the length shown, `skip()` and
+ *   `live` are reset.
  * - While the text is paced and its speech is not done, samples `heard` on every
  *   animation frame (a 16 ms timeout without `requestAnimationFrame`). It stops ticking
  *   when the speech is done or the component unmounts; `subscribe` notifications, and
  *   changes of `text` or `generating`, make it check again.
  * - Reveals the full text once `heard` reports `done` and `generating` is falsy, when
- *   `live` is `false` at mount, and after `skip()`.
+ *   `live` is `false`, and after `skip()`.
  * - When `heard` returns `undefined`, `isPaced` is `false` and `heardText` is the full
  *   text. If it becomes paced later, the reveal continues from the length already shown.
  *
  * `heard` and `subscribe` are read through refs, so passing new function identities on
  * every render (inline arrow functions) does not restart the pacing. `subscribe` is
  * called once on mount (and again only if it goes from absent to present) and its
- * unsubscribe function runs on unmount.
+ * unsubscribe function runs on unmount; to switch to another speech source, remount the
+ * component. `heard` is also called during render, so keep it free of side effects.
  *
  * @example
  * ```tsx
@@ -186,21 +244,23 @@ export const useHeardText = ({
   const subscribeRef = useRef(subscribe);
   subscribeRef.current = subscribe;
 
-  const [isLive] = useState(live);
-  const [skipped, setSkipped] = useState(false);
-  const [pacingState, setPacingState] = useState<PacingState>(() =>
-    sample({
-      generating,
-      heard,
-      live: isLive,
-      previousCount: 0,
-      skipped,
-      textLength,
-    }),
+  const [revealState, setRevealState] = useState<RevealState>(() =>
+    startReveal({ generating, heard, live, text, textLength }),
   );
-  const pacingStateRef = useRef(pacingState);
+  let reveal = revealState;
+  if (revealState.text !== text) {
+    // Updated during render so that the new text never renders with the previous reveal.
+    reveal = continuesReveal(revealState, text)
+      ? { ...revealState, count: Math.min(revealState.count, textLength), text }
+      : startReveal({ generating, heard, live, text, textLength });
+    setRevealState(reveal);
+  }
+  const revealRef = useRef(reveal);
+  revealRef.current = reveal;
   const wakeRef = useRef<() => void>(noop);
 
+  const { live: isLive, skipped } = reveal;
+
   useEffect(() => {
     let cancelFrame: (() => void) | undefined;
     let disposed = false;
@@ -209,7 +269,7 @@ export const useHeardText = ({
       cancelFrame = undefined;
       if (disposed) return;
 
-      const previous = pacingStateRef.current;
+      const previous = revealRef.current;
       const next = sample({
         generating,
         heard: heardRef.current,
@@ -224,8 +284,9 @@ export const useHeardText = ({
         next.isPaced !== previous.isPaced ||
         next.settled !== previous.settled
       ) {
-        pacingStateRef.current = next;
-        setPacingState(next);
+        const updated = { ...previous, ...next };
+        revealRef.current = updated;
+        setRevealState(updated);
       }
 
       if (!next.settled) cancelFrame = scheduleFrame(tick);
@@ -249,13 +310,16 @@ export const useHeardText = ({
     return subscribeRef.current?.(() => wakeRef.current());
   }, [hasSubscribe]);
 
-  const skip = useCallback(() => setSkipped(true), []);
+  const skip = useCallback(
+    () => setRevealState((previous) => ({ ...previous, skipped: true })),
+    [],
+  );
 
   return {
-    heardText: pacingState.isPaced
-      ? text.slice(0, codePointOffsets[Math.min(pacingState.count, textLength)])
+    heardText: reveal.isPaced
+      ? text.slice(0, codePointOffsets[Math.min(reveal.count, textLength)])
       : text,
-    isPaced: pacingState.isPaced,
+    isPaced: reveal.isPaced,
     skip,
   };
 };