fix(gateway): return ISO 8601 timestamps from threads endpoints (#2599)

* fix(gateway): return ISO 8601 timestamps from threads endpoints (#2594)

ThreadResponse documents created_at / updated_at as ISO timestamps,
matching the LangGraph Platform schema (langgraph_sdk.schema.Thread
exposes them as datetime, JSON-encoded as ISO 8601). The gateway
threads router was instead emitting str(time.time()) — unix-second
floats — breaking frontend new Date() parsing and producing a mixed
ISO/unix wire format that also corrupted the search sort order.

Centralize timestamp generation in deerflow.utils.time:
- now_iso()       — datetime.now(UTC).isoformat()
- coerce_iso(x)   — heals legacy unix-timestamp strings on read so the
                    store converges to ISO without a one-shot migration

threads.py: replace 6 time.time() call sites with now_iso(); wrap all
read paths and Phase-2 checkpoint metadata with coerce_iso(); _store_upsert
opportunistically heals legacy created_at on update; drop unused time import.

thread_runs.py: reuse now_iso() instead of a private duplicate _now_iso(),
preventing future drift between the two timestamp call sites.

Tests: 9 unit tests for the helper; 5 integration tests pinning the ISO
contract for create/get/patch/search and the legacy-healing path on the
internal store upsert. Full suite: 2144 passed, 15 skipped, 0 failed.

Closes #2594

* fix(gateway): coerce checkpoint metadata timestamps to ISO on read

After the merge with main, three additional read paths in ``threads.py``
were still emitting raw ``str(metadata.get("created_at", ""))`` —
``get_thread_state``, ``update_thread_state``, and ``get_thread_history``.

Same root cause as #2594: when the checkpoint metadata's ``created_at``
is a unix-second float (legacy data, or a checkpoint written by an older
Gateway version), ``str(float)`` produces ``"1777252410.411327"`` and the
frontend's ``new Date(...)`` returns ``Invalid Date``. The fix on the
``/threads/{id}`` GET path was already in place; these three sibling
endpoints needed the same treatment.

All four call sites now flow through ``coerce_iso``, so:
- legacy float metadata heals to ISO on the way out,
- ISO metadata passes through unchanged,
- ``datetime`` instances (which the new ``coerce_iso`` branch handles
  explicitly) emit with the ``T`` separator instead of falling through
  to the space-separated ``str(datetime)`` form.

Coverage added for the two endpoints not already pinned by the merge:
- ``test_get_thread_state_returns_iso_for_legacy_checkpoint_metadata``
- ``test_get_thread_history_returns_iso_for_legacy_checkpoint_metadata``

Both pre-seed a checkpoint whose metadata carries the literal float
from the issue body and assert the wire format is ISO.

backend/app/gateway/routers/threads.py

@@ -13,7 +13,6 @@ matching the LangGraph Platform wire format expected by the
 from __future__ import annotations
 
 import logging
-import time
 import uuid
 from typing import Any
 
@@ -27,6 +26,7 @@ from app.gateway.utils import sanitize_log_param
 from deerflow.config.paths import Paths, get_paths
 from deerflow.runtime import serialize_channel_values
 from deerflow.runtime.user_context import get_effective_user_id
+from deerflow.utils.time import coerce_iso, now_iso
 
 logger = logging.getLogger(__name__)
 router = APIRouter(prefix="/api/threads", tags=["threads"])
@@ -234,7 +234,7 @@ async def create_thread(body: ThreadCreateRequest, request: Request) -> ThreadRe
     checkpointer = get_checkpointer(request)
     thread_store = get_thread_store(request)
     thread_id = body.thread_id or str(uuid.uuid4())
-    now = time.time()
+    now = now_iso()
     # ``body.metadata`` is already stripped of server-reserved keys by
     # ``ThreadCreateRequest._strip_reserved`` — see the model definition.
 
@@ -244,8 +244,8 @@ async def create_thread(body: ThreadCreateRequest, request: Request) -> ThreadRe
         return ThreadResponse(
             thread_id=thread_id,
             status=existing_record.get("status", "idle"),
-            created_at=str(existing_record.get("created_at", "")),
-            updated_at=str(existing_record.get("updated_at", "")),
+            created_at=coerce_iso(existing_record.get("created_at", "")),
+            updated_at=coerce_iso(existing_record.get("updated_at", "")),
             metadata=existing_record.get("metadata", {}),
         )
 
@@ -280,8 +280,8 @@ async def create_thread(body: ThreadCreateRequest, request: Request) -> ThreadRe
     return ThreadResponse(
         thread_id=thread_id,
         status="idle",
-        created_at=str(now),
-        updated_at=str(now),
+        created_at=now,
+        updated_at=now,
         metadata=body.metadata,
     )
 
@@ -306,8 +306,11 @@ async def search_threads(body: ThreadSearchRequest, request: Request) -> list[Th
         ThreadResponse(
             thread_id=r["thread_id"],
             status=r.get("status", "idle"),
-            created_at=r.get("created_at", ""),
-            updated_at=r.get("updated_at", ""),
+            # ``coerce_iso`` heals legacy unix-second values that
+            # ``MemoryThreadMetaStore`` historically wrote with ``time.time()``;
+            # SQL-backed rows already arrive as ISO strings and pass through.
+            created_at=coerce_iso(r.get("created_at", "")),
+            updated_at=coerce_iso(r.get("updated_at", "")),
             metadata=r.get("metadata", {}),
             values={"title": r["display_name"]} if r.get("display_name") else {},
             interrupts={},
@@ -339,8 +342,8 @@ async def patch_thread(thread_id: str, body: ThreadPatchRequest, request: Reques
     return ThreadResponse(
         thread_id=thread_id,
         status=record.get("status", "idle"),
-        created_at=str(record.get("created_at", "")),
-        updated_at=str(record.get("updated_at", "")),
+        created_at=coerce_iso(record.get("created_at", "")),
+        updated_at=coerce_iso(record.get("updated_at", "")),
         metadata=record.get("metadata", {}),
     )
 
@@ -380,8 +383,8 @@ async def get_thread(thread_id: str, request: Request) -> ThreadResponse:
         record = {
             "thread_id": thread_id,
             "status": "idle",
-            "created_at": ckpt_meta.get("created_at", ""),
-            "updated_at": ckpt_meta.get("updated_at", ckpt_meta.get("created_at", "")),
+            "created_at": coerce_iso(ckpt_meta.get("created_at", "")),
+            "updated_at": coerce_iso(ckpt_meta.get("updated_at", ckpt_meta.get("created_at", ""))),
             "metadata": {k: v for k, v in ckpt_meta.items() if k not in ("created_at", "updated_at", "step", "source", "writes", "parents")},
         }
 
@@ -395,8 +398,8 @@ async def get_thread(thread_id: str, request: Request) -> ThreadResponse:
     return ThreadResponse(
         thread_id=thread_id,
         status=status,
-        created_at=str(record.get("created_at", "")),
-        updated_at=str(record.get("updated_at", "")),
+        created_at=coerce_iso(record.get("created_at", "")),
+        updated_at=coerce_iso(record.get("updated_at", "")),
         metadata=record.get("metadata", {}),
         values=serialize_channel_values(channel_values),
     )
@@ -447,10 +450,10 @@ async def get_thread_state(thread_id: str, request: Request) -> ThreadStateRespo
         values=values,
         next=next_tasks,
         metadata=metadata,
-        checkpoint={"id": checkpoint_id, "ts": str(metadata.get("created_at", ""))},
+        checkpoint={"id": checkpoint_id, "ts": coerce_iso(metadata.get("created_at", ""))},
         checkpoint_id=checkpoint_id,
         parent_checkpoint_id=parent_checkpoint_id,
-        created_at=str(metadata.get("created_at", "")),
+        created_at=coerce_iso(metadata.get("created_at", "")),
         tasks=tasks,
     )
 
@@ -500,7 +503,7 @@ async def update_thread_state(thread_id: str, body: ThreadStateUpdateRequest, re
         channel_values.update(body.values)
 
     checkpoint["channel_values"] = channel_values
-    metadata["updated_at"] = time.time()
+    metadata["updated_at"] = now_iso()
 
     if body.as_node:
         metadata["source"] = "update"
@@ -541,7 +544,7 @@ async def update_thread_state(thread_id: str, body: ThreadStateUpdateRequest, re
         next=[],
         metadata=metadata,
         checkpoint_id=new_checkpoint_id,
-        created_at=str(metadata.get("created_at", "")),
+        created_at=coerce_iso(metadata.get("created_at", "")),
     )
 
 
@@ -608,7 +611,7 @@ async def get_thread_history(thread_id: str, body: ThreadHistoryRequest, request
                     parent_checkpoint_id=parent_id,
                     metadata=user_meta,
                     values=values,
-                    created_at=str(metadata.get("created_at", "")),
+                    created_at=coerce_iso(metadata.get("created_at", "")),
                     next=next_tasks,
                 )
             )

backend/packages/harness/deerflow/persistence/thread_meta/memory.py

@@ -7,13 +7,13 @@ router for thread records.
 
 from __future__ import annotations
 
-import time
 from typing import Any
 
 from langgraph.store.base import BaseStore
 
 from deerflow.persistence.thread_meta.base import ThreadMetaStore
 from deerflow.runtime.user_context import AUTO, _AutoSentinel, resolve_user_id
+from deerflow.utils.time import coerce_iso, now_iso
 
 THREADS_NS: tuple[str, ...] = ("threads",)
 
@@ -48,7 +48,7 @@ class MemoryThreadMetaStore(ThreadMetaStore):
         metadata: dict | None = None,
     ) -> dict:
         resolved_user_id = resolve_user_id(user_id, method_name="MemoryThreadMetaStore.create")
-        now = time.time()
+        now = now_iso()
         record: dict[str, Any] = {
             "thread_id": thread_id,
             "assistant_id": assistant_id,
@@ -106,7 +106,7 @@ class MemoryThreadMetaStore(ThreadMetaStore):
         if record is None:
             return
         record["display_name"] = display_name
-        record["updated_at"] = time.time()
+        record["updated_at"] = now_iso()
         await self._store.aput(THREADS_NS, thread_id, record)
 
     async def update_status(self, thread_id: str, status: str, *, user_id: str | None | _AutoSentinel = AUTO) -> None:
@@ -114,7 +114,7 @@ class MemoryThreadMetaStore(ThreadMetaStore):
         if record is None:
             return
         record["status"] = status
-        record["updated_at"] = time.time()
+        record["updated_at"] = now_iso()
         await self._store.aput(THREADS_NS, thread_id, record)
 
     async def update_metadata(self, thread_id: str, metadata: dict, *, user_id: str | None | _AutoSentinel = AUTO) -> None:
@@ -124,7 +124,7 @@ class MemoryThreadMetaStore(ThreadMetaStore):
         merged = dict(record.get("metadata") or {})
         merged.update(metadata)
         record["metadata"] = merged
-        record["updated_at"] = time.time()
+        record["updated_at"] = now_iso()
         await self._store.aput(THREADS_NS, thread_id, record)
 
     async def delete(self, thread_id: str, *, user_id: str | None | _AutoSentinel = AUTO) -> None:
@@ -144,6 +144,8 @@ class MemoryThreadMetaStore(ThreadMetaStore):
             "display_name": val.get("display_name"),
             "status": val.get("status", "idle"),
             "metadata": val.get("metadata", {}),
-            "created_at": str(val.get("created_at", "")),
-            "updated_at": str(val.get("updated_at", "")),
+            # ``coerce_iso`` heals legacy unix-second values written by
+            # earlier Gateway versions that called ``str(time.time())``.
+            "created_at": coerce_iso(val.get("created_at", "")),
+            "updated_at": coerce_iso(val.get("updated_at", "")),
         }

backend/packages/harness/deerflow/runtime/runs/manager.py

@@ -6,9 +6,10 @@ import asyncio
 import logging
 import uuid
 from dataclasses import dataclass, field
-from datetime import UTC, datetime
 from typing import TYPE_CHECKING
 
+from deerflow.utils.time import now_iso as _now_iso
+
 from .schemas import DisconnectMode, RunStatus
 
 if TYPE_CHECKING:
@@ -17,10 +18,6 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
-def _now_iso() -> str:
-    return datetime.now(UTC).isoformat()
-
-
 @dataclass
 class RunRecord:
     """Mutable record for a single run."""

backend/packages/harness/deerflow/utils/time.py

@@ -0,0 +1,75 @@
+"""ISO 8601 timestamp helpers for the Gateway and embedded runtime.
+
+DeerFlow stores and serializes thread/run timestamps as ISO 8601 UTC
+strings to match the LangGraph Platform schema (see
+``langgraph_sdk.schema.Thread``, where ``created_at`` / ``updated_at``
+are ``datetime`` and JSON-encode to ISO 8601). All timestamp generation
+should funnel through :func:`now_iso` so the wire format stays
+consistent across endpoints, the embedded ``RunManager``, and the
+checkpoint metadata written by the Gateway.
+
+:func:`coerce_iso` provides a forward-compatible read path for legacy
+records that historically stored ``str(time.time())`` floats.
+"""
+
+from __future__ import annotations
+
+import re
+from datetime import UTC, datetime
+
+__all__ = ["coerce_iso", "now_iso"]
+
+_UNIX_TIMESTAMP_PATTERN = re.compile(r"^\d{10}(?:\.\d+)?$")
+"""Matches the unix-timestamp string shape historically written by
+``str(time.time())`` (10-digit seconds with optional fractional part).
+The 10-digit anchor avoids accidentally rewriting ISO years like
+``"2026"`` and stays valid until the year 2286.
+"""
+
+
+def now_iso() -> str:
+    """Return the current UTC time as an ISO 8601 string.
+
+    Example: ``"2026-04-27T03:19:46.511479+00:00"``.
+    """
+    return datetime.now(UTC).isoformat()
+
+
+def coerce_iso(value: object) -> str:
+    """Best-effort coerce a stored timestamp to an ISO 8601 string.
+
+    Translates legacy unix-timestamp floats / strings written by older
+    DeerFlow versions into ISO without a one-shot migration. ISO strings
+    pass through unchanged; ``datetime`` instances are normalised to UTC
+    (tz-naive values are assumed to be UTC) and emitted via
+    ``isoformat()`` so the wire format always uses the ``T`` separator;
+    empty values become ``""``; unrecognised values are stringified as a
+    last resort.
+    """
+    if value is None or value == "":
+        return ""
+    if isinstance(value, bool):
+        # ``bool`` is a subclass of ``int`` — treat as garbage, not 0/1.
+        return str(value)
+    if isinstance(value, datetime):
+        # ``datetime`` must be handled before the ``int``/``float`` check;
+        # str(datetime) would produce ``"YYYY-MM-DD HH:MM:SS+00:00"``
+        # (space separator), which breaks strict ISO 8601 consumers.
+        if value.tzinfo is None:
+            value = value.replace(tzinfo=UTC)
+        else:
+            value = value.astimezone(UTC)
+        return value.isoformat()
+    if isinstance(value, (int, float)):
+        try:
+            return datetime.fromtimestamp(float(value), UTC).isoformat()
+        except (ValueError, OverflowError, OSError):
+            return str(value)
+    if isinstance(value, str):
+        if _UNIX_TIMESTAMP_PATTERN.match(value):
+            try:
+                return datetime.fromtimestamp(float(value), UTC).isoformat()
+            except (ValueError, OverflowError, OSError):
+                return value
+        return value
+    return str(value)

backend/tests/test_threads_router.py

@@ -1,12 +1,66 @@
+import re
 from unittest.mock import patch
 
 import pytest
 from _router_auth_helpers import make_authed_test_app
-from fastapi import HTTPException
+from fastapi import FastAPI, HTTPException
 from fastapi.testclient import TestClient
+from langgraph.checkpoint.memory import InMemorySaver
+from langgraph.store.memory import InMemoryStore
 
 from app.gateway.routers import threads
 from deerflow.config.paths import Paths
+from deerflow.persistence.thread_meta.memory import THREADS_NS, MemoryThreadMetaStore
+
+_ISO_TIMESTAMP_RE = re.compile(r"^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}")
+
+
+class _PermissiveThreadMetaStore(MemoryThreadMetaStore):
+    """Memory store that skips user-id filtering for router tests.
+
+    Owner isolation is exercised separately in
+    ``test_memory_thread_meta_isolation.py``. Router tests need to drive
+    the FastAPI surface end-to-end with a single fixed app user, but the
+    stub auth middleware in ``_router_auth_helpers`` stamps a fresh UUID
+    on every request, so the production filtering would reject every
+    pre-seeded record. Bypass that filter so the test can focus on the
+    timestamp wire format.
+    """
+
+    async def _get_owned_record(self, thread_id, user_id, method_name):  # type: ignore[override]
+        item = await self._store.aget(THREADS_NS, thread_id)
+        return dict(item.value) if item is not None else None
+
+    async def check_access(self, thread_id, user_id, *, require_existing=False):  # type: ignore[override]
+        item = await self._store.aget(THREADS_NS, thread_id)
+        if item is None:
+            return not require_existing
+        return True
+
+    async def create(self, thread_id, *, assistant_id=None, user_id=None, display_name=None, metadata=None):  # type: ignore[override]
+        return await super().create(thread_id, assistant_id=assistant_id, user_id=None, display_name=display_name, metadata=metadata)
+
+    async def search(self, *, metadata=None, status=None, limit=100, offset=0, user_id=None):  # type: ignore[override]
+        return await super().search(metadata=metadata, status=status, limit=limit, offset=offset, user_id=None)
+
+
+def _build_thread_app() -> tuple[FastAPI, InMemoryStore, InMemorySaver]:
+    """Build a stub-authed FastAPI app wired with an in-memory ThreadMetaStore.
+
+    The thread_store on ``app.state`` is a permissive subclass of
+    ``MemoryThreadMetaStore`` so tests can drive ``/api/threads``
+    end-to-end and pre-seed legacy records via the underlying BaseStore.
+
+    Returns ``(app, store, checkpointer)`` for direct seeding/inspection.
+    """
+    app = make_authed_test_app()
+    store = InMemoryStore()
+    checkpointer = InMemorySaver()
+    app.state.store = store
+    app.state.checkpointer = checkpointer
+    app.state.thread_store = _PermissiveThreadMetaStore(store)
+    app.include_router(threads.router)
+    return app, store, checkpointer
 
 
 def test_delete_thread_data_removes_thread_directory(tmp_path):
@@ -136,3 +190,244 @@ def test_strip_reserved_metadata_empty_input():
 def test_strip_reserved_metadata_strips_all_reserved_keys():
     out = threads._strip_reserved_metadata({"user_id": "x", "keep": "me"})
     assert out == {"keep": "me"}
+
+
+# ---------------------------------------------------------------------------
+# ISO 8601 timestamp contract (issue #2594)
+# ---------------------------------------------------------------------------
+#
+# Threads endpoints document ``created_at`` / ``updated_at`` as ISO
+# timestamps and that is the format LangGraph Platform uses
+# (``langgraph_sdk.schema.Thread.created_at: datetime`` JSON-encodes to
+# ISO 8601). The tests below pin that contract end-to-end and also
+# exercise the ``coerce_iso`` healing path for legacy unix-timestamp
+# records written by older Gateway versions.
+
+
+def test_create_thread_returns_iso_timestamps() -> None:
+    app, _store, _checkpointer = _build_thread_app()
+
+    with TestClient(app) as client:
+        response = client.post("/api/threads", json={"metadata": {}})
+
+    assert response.status_code == 200, response.text
+    body = response.json()
+    assert _ISO_TIMESTAMP_RE.match(body["created_at"]), body["created_at"]
+    assert _ISO_TIMESTAMP_RE.match(body["updated_at"]), body["updated_at"]
+    assert body["created_at"] == body["updated_at"]
+
+
+def test_get_thread_returns_iso_for_legacy_unix_record() -> None:
+    """A thread record written by older versions stores ``time.time()``
+    floats. ``get_thread`` must transparently surface them as ISO so the
+    frontend's ``new Date(...)`` parser does not break.
+    """
+    app, store, checkpointer = _build_thread_app()
+
+    legacy_thread_id = "legacy-thread"
+    legacy_ts = "1777252410.411327"
+
+    async def _seed() -> None:
+        await store.aput(
+            THREADS_NS,
+            legacy_thread_id,
+            {
+                "thread_id": legacy_thread_id,
+                "status": "idle",
+                "created_at": legacy_ts,
+                "updated_at": legacy_ts,
+                "metadata": {},
+            },
+        )
+        from langgraph.checkpoint.base import empty_checkpoint
+
+        await checkpointer.aput(
+            {"configurable": {"thread_id": legacy_thread_id, "checkpoint_ns": ""}},
+            empty_checkpoint(),
+            {"step": -1, "source": "input", "writes": None, "parents": {}},
+            {},
+        )
+
+    import asyncio
+
+    asyncio.run(_seed())
+
+    with TestClient(app) as client:
+        response = client.get(f"/api/threads/{legacy_thread_id}")
+
+    assert response.status_code == 200, response.text
+    body = response.json()
+    assert _ISO_TIMESTAMP_RE.match(body["created_at"]), body["created_at"]
+    assert _ISO_TIMESTAMP_RE.match(body["updated_at"]), body["updated_at"]
+
+
+def test_patch_thread_returns_iso_and_advances_updated_at() -> None:
+    app, store, _checkpointer = _build_thread_app()
+    thread_id = "patch-target"
+
+    legacy_created = "1777000000.000000"
+    legacy_updated = "1777000000.000000"
+
+    async def _seed() -> None:
+        await store.aput(
+            THREADS_NS,
+            thread_id,
+            {
+                "thread_id": thread_id,
+                "status": "idle",
+                "created_at": legacy_created,
+                "updated_at": legacy_updated,
+                "metadata": {"k": "v0"},
+            },
+        )
+
+    import asyncio
+
+    asyncio.run(_seed())
+
+    with TestClient(app) as client:
+        response = client.patch(f"/api/threads/{thread_id}", json={"metadata": {"k": "v1"}})
+
+    assert response.status_code == 200, response.text
+    body = response.json()
+    assert _ISO_TIMESTAMP_RE.match(body["created_at"]), body["created_at"]
+    assert _ISO_TIMESTAMP_RE.match(body["updated_at"]), body["updated_at"]
+    # Patch issues a fresh ``updated_at`` via ``MemoryThreadMetaStore.update_metadata``,
+    # so it must be > the migrated legacy ``created_at`` (both ISO strings
+    # sort lexicographically by time when the format is consistent).
+    assert body["updated_at"] > body["created_at"]
+    assert body["metadata"] == {"k": "v1"}
+
+
+def test_search_threads_normalizes_legacy_unix_seconds_to_iso() -> None:
+    """``MemoryThreadMetaStore`` may hold legacy ``time.time()`` floats
+    written by older Gateway versions. ``/search`` must surface them as
+    ISO via ``coerce_iso`` so the frontend's ``new Date(...)`` parser
+    does not break.
+    """
+    app, store, _checkpointer = _build_thread_app()
+
+    async def _seed() -> None:
+        # Legacy unix-second float (the literal value from issue #2594).
+        await store.aput(
+            THREADS_NS,
+            "legacy",
+            {
+                "thread_id": "legacy",
+                "status": "idle",
+                "created_at": 1777000000.0,
+                "updated_at": 1777000000.0,
+                "metadata": {},
+            },
+        )
+        # Modern ISO string, slightly later.
+        await store.aput(
+            THREADS_NS,
+            "modern",
+            {
+                "thread_id": "modern",
+                "status": "idle",
+                "created_at": "2026-04-27T00:00:00+00:00",
+                "updated_at": "2026-04-27T00:00:00+00:00",
+                "metadata": {},
+            },
+        )
+
+    import asyncio
+
+    asyncio.run(_seed())
+
+    with TestClient(app) as client:
+        response = client.post("/api/threads/search", json={"limit": 10})
+
+    assert response.status_code == 200, response.text
+    items = response.json()
+    assert {item["thread_id"] for item in items} == {"legacy", "modern"}
+    for item in items:
+        assert _ISO_TIMESTAMP_RE.match(item["created_at"]), item
+        assert _ISO_TIMESTAMP_RE.match(item["updated_at"]), item
+
+
+def test_memory_thread_meta_store_writes_iso_on_create() -> None:
+    """``MemoryThreadMetaStore.create`` must emit ISO so newly created
+    threads serialize correctly without depending on the router's
+    ``coerce_iso`` heal path.
+    """
+    import asyncio
+
+    store = InMemoryStore()
+    repo = MemoryThreadMetaStore(store)
+
+    async def _scenario() -> dict:
+        await repo.create("fresh", user_id=None, metadata={"a": 1})
+        record = (await store.aget(THREADS_NS, "fresh")).value
+        return record
+
+    record = asyncio.run(_scenario())
+    assert _ISO_TIMESTAMP_RE.match(record["created_at"]), record
+    assert _ISO_TIMESTAMP_RE.match(record["updated_at"]), record
+
+
+def test_get_thread_state_returns_iso_for_legacy_checkpoint_metadata() -> None:
+    """Checkpoints written by older Gateway versions stored
+    ``created_at`` as a unix-second float in their metadata. The
+    ``/state`` endpoint must surface that value as ISO so the frontend's
+    ``new Date(...)`` parser does not break — same root cause as the
+    thread-record bug fixed in #2594, but on the checkpoint side.
+    """
+    app, _store, checkpointer = _build_thread_app()
+    thread_id = "legacy-state"
+
+    async def _seed() -> None:
+        from langgraph.checkpoint.base import empty_checkpoint
+
+        await checkpointer.aput(
+            {"configurable": {"thread_id": thread_id, "checkpoint_ns": ""}},
+            empty_checkpoint(),
+            {"step": -1, "source": "input", "writes": None, "parents": {}, "created_at": 1777252410.411327},
+            {},
+        )
+
+    import asyncio
+
+    asyncio.run(_seed())
+
+    with TestClient(app) as client:
+        response = client.get(f"/api/threads/{thread_id}/state")
+
+    assert response.status_code == 200, response.text
+    body = response.json()
+    assert _ISO_TIMESTAMP_RE.match(body["created_at"]), body["created_at"]
+    assert _ISO_TIMESTAMP_RE.match(body["checkpoint"]["ts"]), body["checkpoint"]
+
+
+def test_get_thread_history_returns_iso_for_legacy_checkpoint_metadata() -> None:
+    """``/history`` walks ``checkpointer.alist`` and emits one entry per
+    checkpoint. Each entry's ``created_at`` must come out as ISO even if
+    older checkpoints stored a unix-second float in their metadata.
+    """
+    app, _store, checkpointer = _build_thread_app()
+    thread_id = "legacy-history"
+
+    async def _seed() -> None:
+        from langgraph.checkpoint.base import empty_checkpoint
+
+        await checkpointer.aput(
+            {"configurable": {"thread_id": thread_id, "checkpoint_ns": ""}},
+            empty_checkpoint(),
+            {"step": -1, "source": "input", "writes": None, "parents": {}, "created_at": 1777252410.411327},
+            {},
+        )
+
+    import asyncio
+
+    asyncio.run(_seed())
+
+    with TestClient(app) as client:
+        response = client.post(f"/api/threads/{thread_id}/history", json={"limit": 10})
+
+    assert response.status_code == 200, response.text
+    entries = response.json()
+    assert entries, "expected at least one history entry"
+    for entry in entries:
+        assert _ISO_TIMESTAMP_RE.match(entry["created_at"]), entry

backend/tests/test_utils_time.py

@@ -0,0 +1,90 @@
+"""Tests for ``deerflow.utils.time``."""
+
+from __future__ import annotations
+
+import re
+from datetime import UTC, datetime, timedelta, timezone
+
+from deerflow.utils.time import coerce_iso, now_iso
+
+_ISO_RE = re.compile(r"^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}")
+
+
+def test_now_iso_is_utc_iso8601() -> None:
+    value = now_iso()
+    assert _ISO_RE.match(value), value
+    parsed = datetime.fromisoformat(value)
+    assert parsed.tzinfo is not None
+    assert parsed.tzinfo.utcoffset(parsed) == UTC.utcoffset(parsed)
+
+
+def test_coerce_iso_passes_iso_through() -> None:
+    iso = "2026-04-27T01:13:30.411334+00:00"
+    assert coerce_iso(iso) == iso
+
+
+def test_coerce_iso_converts_unix_float_string() -> None:
+    legacy = "1777252410.411327"
+    out = coerce_iso(legacy)
+    assert _ISO_RE.match(out), out
+    # Round-trip: parsed timestamp matches the original epoch.
+    parsed = datetime.fromisoformat(out)
+    assert abs(parsed.timestamp() - 1777252410.411327) < 1e-3
+
+
+def test_coerce_iso_converts_unix_int_string() -> None:
+    out = coerce_iso("1700000000")
+    assert _ISO_RE.match(out), out
+
+
+def test_coerce_iso_converts_numeric_types() -> None:
+    out_float = coerce_iso(1777252410.411327)
+    out_int = coerce_iso(1700000000)
+    assert _ISO_RE.match(out_float)
+    assert _ISO_RE.match(out_int)
+
+
+def test_coerce_iso_handles_empty_and_none() -> None:
+    assert coerce_iso(None) == ""
+    assert coerce_iso("") == ""
+
+
+def test_coerce_iso_does_not_misinterpret_short_numeric() -> None:
+    # A 4-digit year should never be parsed as a unix timestamp; only
+    # 10-digit unix-second strings match the legacy pattern.
+    assert coerce_iso("2026") == "2026"
+
+
+def test_coerce_iso_handles_unparseable_string() -> None:
+    assert coerce_iso("not-a-timestamp") == "not-a-timestamp"
+
+
+def test_coerce_iso_rejects_bool() -> None:
+    # ``bool`` is a subclass of ``int`` — must not be treated as epoch 0/1.
+    assert coerce_iso(True) == "True"
+    assert coerce_iso(False) == "False"
+
+
+def test_coerce_iso_handles_tz_aware_datetime() -> None:
+    # str(datetime) would emit a space separator; coerce_iso must use ``T``.
+    dt = datetime(2026, 4, 27, 1, 13, 30, 411327, tzinfo=UTC)
+    out = coerce_iso(dt)
+    assert out == "2026-04-27T01:13:30.411327+00:00"
+    assert "T" in out and " " not in out
+
+
+def test_coerce_iso_handles_tz_naive_datetime_as_utc() -> None:
+    dt = datetime(2026, 4, 27, 1, 13, 30, 411327)
+    out = coerce_iso(dt)
+    assert out == "2026-04-27T01:13:30.411327+00:00"
+    parsed = datetime.fromisoformat(out)
+    assert parsed.tzinfo is not None
+    assert parsed.utcoffset() == timedelta(0)
+
+
+def test_coerce_iso_normalises_non_utc_datetime_to_utc() -> None:
+    # +08:00 wall-clock 09:13 == UTC 01:13.
+    plus_eight = timezone(timedelta(hours=8))
+    dt = datetime(2026, 4, 27, 9, 13, 30, 411327, tzinfo=plus_eight)
+    out = coerce_iso(dt)
+    assert out == "2026-04-27T01:13:30.411327+00:00"