FirestoreSessionService compares event timestamps as text, reordering or skipping same-second events

Start in contrib/firestore-session-service/src/main/java/com/google/adk/sessions/FirestoreSessionService.java: the timestamp write near line 411, the orderBy calls near lines 234 and 611, and the afterTimestamp filter near lines 236-240. Run the two reproduction tests in FirestoreSessionServiceTest, starting with getSession_withAfterTimestamp_appliesFilterToQuery. Done means the stored strings sort in append order and the cursor filter is inclusive. Point 2 (tie-breaking) needs a maintainer decision, so leave it out of the first change.

🔴 Required Information

Describe the Bug:

FirestoreSessionService stores each event's time as text, Instant.ofEpochMilli(event.timestamp()).toString() (FirestoreSessionService.java:411), and then sorts and filters events on that text. getSession and listEvents order by it (:234, :611), numRecentEvents takes the last N of that order (:242-244), and afterTimestamp becomes whereGreaterThan("timestamp", afterTimestamp.toString()) (:236-240). Firestore orders strings by their UTF-8 bytes (data types). That causes three problems:

1. Events in the same second can come back out of order (variable-width text). Instant.toString() writes no fraction when the milliseconds are zero, so an event at 05:00:05.000 is stored as 2026-10-10T05:00:05Z and a later one at 05:00:05.400 as 2026-10-10T05:00:05.400Z. Because . (0x2E) sorts before Z (0x5A), the later event comes first.
2. Events in the same millisecond come back in arbitrary order (no tie-breaker). In Standard edition, Firestore orders equal values by document name (StructuredQuery.orderBy), and Enterprise edition does not guarantee a stable order. Event documents get random auto-IDs (document() with no argument, :709-715), so tied events come back in the order of those random IDs, not the order they were appended. On main (not in 1.11.0), a fixed InstantSource passed to Runner.Builder.instantSource (Runner.java:195-205), as in tests, gives every event the runner creates the same timestamp.
3. afterTimestamp returns the wrong events (text comparison, and > instead of >=). With a whole-second cursor such as 2026-10-10T05:00:05Z, every later event in that second (…05.001Z to …05.999Z) sorts below the cursor and is left out. With a millisecond cursor, an earlier whole-second event in the same second is included and an event at exactly the cursor is left out. A cursor with microseconds (…05.200300Z) includes an earlier …05.200Z event. InMemorySessionService keeps the events at or after the cursor (InMemorySessionService.java:223-229), and VertexAiSessionService sends an inclusive timestamp>= filter (VertexAiSessionService.java:263-272).

Runner.runAsync reads the session on every run (Runner.java:556-560), and Contents builds the history in list order (Contents.java:170-177), so points 1 and 2 reach the next model request. Once #1643 stores call IDs, function responses are moved back after their calls by ID (Contents.java:641-751); on main today, reloaded responses have no IDs and are dropped (#1640). Either way, other events, such as user and model text, keep the wrong order. The Runner passes Optional.empty() as the session config, so point 3 and numRecentEvents only affect code that calls getSession with a GetSessionConfig.

The stored times themselves are correct: eventFromMap reads both forms back with Instant.parse (:301).

Steps to Reproduce:

1. Use google-adk-firestore-session-service 1.11.0 or main at 189d463a.
2. Append three events whose timestamp is 2026-10-10T05:00:05.000Z, 05:00:05.400Z and 05:00:06.000Z (in epoch milliseconds), and capture the timestamp values appendEvent writes. A mocked Firestore as in FirestoreSessionServiceTest is enough (first test below).
3. Sort those values the way Firestore sorts strings. They are ASCII, so String order is the same as UTF-8 byte order.
4. For afterTimestamp, compare the stored values with the cursor that getSession passes, afterTimestamp.toString() (second test below). The existing test getSession_withAfterTimestamp_appliesFilterToQuery checks that call (FirestoreSessionServiceTest.java:273-295).

I did not run this against a real Firestore database or the emulator. The order and the result sets below come from the strings the code writes and Firestore's documented ordering rules.

Expected Behavior:

getSession and listEvents return events in the order they were appended, and afterTimestamp returns the events at or after the cursor, as it does in InMemorySessionService and VertexAiSessionService. ADK Python's Firestore session service stores timestamp as a Firestore timestamp (firestore_session_service.py:609-611), orders by it (:320) and filters with >= (:329).

Observed Behavior:

The first test below prints:

stored:  [2026-10-10T05:00:05Z, 2026-10-10T05:00:05.400Z, 2026-10-10T05:00:06Z]
ordered: [2026-10-10T05:00:05.400Z, 2026-10-10T05:00:05Z, 2026-10-10T05:00:06Z]

The second prints:

2026-10-10T05:00:05.001Z > 2026-10-10T05:00:05Z: false
2026-10-10T05:00:05.400Z > 2026-10-10T05:00:05Z: false
2026-10-10T05:00:05.999Z > 2026-10-10T05:00:05Z: false
2026-10-10T05:00:06Z > 2026-10-10T05:00:05Z: true

So with afterTimestamp at 05:00:05.000, Firestore would return only the events from 05:00:06 on, while InMemorySessionService returns all four for the same cursor (checked in a separate test).

Environment Details:

• ADK Library Version (see maven dependency): google-adk-firestore-session-service 1.11.0 and main at 189d463a
• OS: Windows 11 (not OS-specific)
• TS Version (tsc --version): N/A (Java: Microsoft OpenJDK 17.0.19)

Model Information:

• Which model is being used: N/A (session service bug, model independent)

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🟡 Optional Information

Regression:

No. The text timestamp, the orderBy, the whereGreaterThan filter and the auto-ID event documents have been there since the service was added in 0.4.0 (b75608ff).

Additional Context:

• Proposed fix for points 1 and 3, keeping the field a string: write the timestamp with a fixed three-digit fraction (new DateTimeFormatterBuilder().appendInstant(3).toFormatter() gives 2026-10-10T05:00:05.000Z, which Instant.parse still reads). Format the afterTimestamp cursor the same way, rounding it up to the millisecond first because appendInstant(3) truncates extra digits, and use whereGreaterThanOrEqualTo. FirestoreMemoryService reads the field as a string (FirestoreMemoryService.java:154) and keeps working. Events stored before the fix keep their old strings, so an old pair like …05Z and …05.400Z stays misordered, and a later cursor in the same second still includes the old …05Z event, unless the old documents are rewritten.
• Storing a Firestore timestamp, as ADK Python does, would also fix new data, but sessions with older events would mix types: Firestore orders timestamps before strings, so in such a session every new event would sort before every old one, and afterTimestamp would not compare across the two types. eventFromMap and FirestoreMemoryService read the field as a string and would need to change too.
• Point 2 needs a tie-breaker either way, for example a per-session sequence number or document IDs that sort in append order, named explicitly in orderBy because Enterprise edition does not guarantee a stable order otherwise. A new order field would need old documents to be backfilled (orderBy skips documents without the field) and a composite index if it follows timestamp. Re-sorting on the client by Event.timestamp(), as VertexAiSessionService does (VertexAiSessionService.java:281-284), fixes the list order for point 1 but not ties, and limitToLast and afterTimestamp would still select documents by the stored text. ADK Python has the same gap: ties fall back to the document ID, a random event.id, but its microsecond timestamps make ties much rarer.
• #1643 changes other parts of FirestoreSessionService.java (constants, eventFromMap, and eventToMap from line 443) but none of the lines above. I'm happy to send a PR for points 1 and 3 if this direction looks right, and to follow your choice for point 2.

Minimal Reproduction Code:

Both tests go into FirestoreSessionServiceTest and use its mocks, constants and imports:

  @Test
  @SuppressWarnings("unchecked")
  void eventTimestampsSortAsText() {
    Session session =
        Session.builder(SESSION_ID)
            .appName(APP_NAME)
            .userId(USER_ID)
            .state(new ConcurrentHashMap<>())
            .build();
    when(mockSessionsCollection.document(SESSION_ID)).thenReturn(mockSessionDocRef);
    when(mockEventsCollection.document()).thenReturn(mockEventDocRef);
    when(mockEventDocRef.getId()).thenReturn(EVENT_ID);
    when(mockEventsCollection.document(EVENT_ID)).thenReturn(mockEventDocRef);

    long t = Instant.parse("2026-10-10T05:00:05Z").toEpochMilli();
    for (long timestamp : new long[] {t, t + 400, t + 1_000}) {
      Event event =
          Event.builder()
              .author("model")
              .timestamp(timestamp)
              .content(Content.fromParts(Part.fromText("hi")))
              .build();
      sessionService.appendEvent(session, event).blockingGet();
    }

    ArgumentCaptor<Map<String, Object>> saved = ArgumentCaptor.forClass(Map.class);
    verify(mockEventDocRef, times(3)).set(saved.capture());
    List<String> stored =
        saved.getAllValues().stream()
            .map(data -> (String) data.get(Constants.KEY_TIMESTAMP))
            .toList();
    System.out.println("stored:  " + stored);
    // Firestore orders strings by UTF-8 bytes; for these ASCII strings that is String order.
    System.out.println("ordered: " + stored.stream().sorted().toList());
  }

  @Test
  void afterTimestampCursorComparesAsText() {
    // getSession passes afterTimestamp.toString() to whereGreaterThan.
    String cursor = Instant.parse("2026-10-10T05:00:05Z").toString();
    for (String stored :
        List.of(
            "2026-10-10T05:00:05.001Z",
            "2026-10-10T05:00:05.400Z",
            "2026-10-10T05:00:05.999Z",
            "2026-10-10T05:00:06Z")) {
      System.out.println(stored + " > " + cursor + ": " + (stored.compareTo(cursor) > 0));
    }
  }

How often has this issue occurred?:

• Intermittently (<50%). Point 1 needs an event whose timestamp is a whole second (about 1 in 1000 events, if milliseconds are spread evenly) followed by another event in the same second. Point 2 needs two events in the same millisecond. Point 3 needs a caller that passes afterTimestamp: with a whole-second cursor, every later event in that second is affected; with any other cursor, only an event in the cursor's millisecond or one at the start of that second. I have not measured how often this happens in a deployment.