diff --git a/loopx/control_plane/runtime/time.py b/loopx/control_plane/runtime/time.py
index 66282db3a3..dbf5eeaf7f 100644
--- a/loopx/control_plane/runtime/time.py
+++ b/loopx/control_plane/runtime/time.py
@@ -1,10 +1,68 @@
 from __future__ import annotations
 
 from datetime import datetime, timezone
+import re
 from typing import Any
 
 
 _MIN_TIMESTAMP = datetime.min.replace(tzinfo=timezone.utc)
+_ISO_DATE_WITH_SEPARATOR = re.compile(
+    r"^(?P<date>\d{4}-W\d{2}(?:-[1-7])?|\d{4}W\d{2}[1-7]?|"
+    r"\d{4}-\d{2}-\d{2}|\d{8})(?P<separator>[^Zz])(?P<clock>.+)$"
+)
+_ISO_CLOCK = re.compile(r"^(\d{2})(?:(:?)(\d{2})(?:\2(\d{2}))?)?([.,]\d+)?$")
+
+
+def _stable_iso_timestamp_text(text: str) -> str | None:
+    """Normalize reduced fractional clocks and reject version-specific 24:00.
+
+    Python 3.11–3.13 accept fractions after an hour or minute, while 3.14
+    accepts 24:00 as end-of-day. Keep the retained reader's wire grammar stable
+    across supported Python versions: preserve the former inputs and reject the
+    latter because the TypeScript Todo codec does not carry an end-of-day date
+    rollover.
+    """
+    match = _ISO_DATE_WITH_SEPARATOR.fullmatch(text)
+    if match is None:
+        return text
+
+    clock = match.group("clock")
+    zone_suffix = ""
+    if clock.endswith(("Z", "z")):
+        clock, zone_suffix = clock[:-1], clock[-1:]
+    else:
+        zone_index = next((i for i, char in enumerate(clock) if i and char in "+-"), None)
+        if zone_index is not None:
+            clock, zone_suffix = clock[:zone_index], clock[zone_index:]
+
+    def normalize_component(component: str, *, local: bool) -> str | None:
+        parsed = _ISO_CLOCK.fullmatch(component)
+        if parsed is None:
+            return component
+        hour, separator, minute, second, fraction = parsed.groups()
+        if local and hour == "24":
+            return None
+        if not local and hour == "00" and minute in (None, "00") and second in (None, "00"):
+            # CPython 3.11–3.13 canonicalize a zero-hour offset to UTC even
+            # when a fractional component follows it. Keep that wire meaning
+            # on 3.14 too, matching the TypeScript codec's zero-offset rule.
+            return "00:00:00"
+        if fraction is None or second is not None:
+            return component
+        return f"{hour}:{minute or '00'}:00{fraction}"
+
+    normalized_clock = normalize_component(clock, local=True)
+    if normalized_clock is None:
+        return None
+    if zone_suffix.startswith(("+", "-")):
+        normalized_offset = normalize_component(zone_suffix[1:], local=False)
+        if normalized_offset is None:
+            return text
+        zone_suffix = zone_suffix[0] + normalized_offset
+
+    return (
+        match.group("date") + match.group("separator") + normalized_clock + zone_suffix
+    )
 
 
 def parse_timestamp(value: Any) -> datetime | None:
@@ -13,8 +71,13 @@ def parse_timestamp(value: Any) -> datetime | None:
     text = str(value).strip()
     if not text:
         return None
+    stable_text = _stable_iso_timestamp_text(text)
+    if stable_text is None:
+        return None
     try:
-        parsed = datetime.fromisoformat(text.replace("Z", "+00:00").replace("z", "+00:00"))
+        parsed = datetime.fromisoformat(
+            stable_text.replace("Z", "+00:00").replace("z", "+00:00")
+        )
     except ValueError:
         return None
     if parsed.tzinfo is None:
diff --git a/loopx/control_plane/runtime_timestamp.ts b/loopx/control_plane/runtime_timestamp.ts
index 668e29eb28..44cc8ffe06 100644
--- a/loopx/control_plane/runtime_timestamp.ts
+++ b/loopx/control_plane/runtime_timestamp.ts
@@ -69,9 +69,12 @@ export function parseTodoTimestampMicros(value: string): bigint | null {
     const parsed = /^(\d{2})(?:(:?)(\d{2})(?:\2(\d{2}))?)?(?:[.,](\d+))?$/.exec(raw);
     if (!parsed) return null;
     const hour = Number(parsed[1]), minute = Number(parsed[3] ?? 0), second = Number(parsed[4] ?? 0);
-    if (!offset && (hour > 23 || minute > 59 || second > 59)) return null;
+    const fraction = parsed[5] ?? "";
+    if (!offset && (
+      hour > 23 || minute > 59 || second > 59
+    )) return null;
     const seconds = hour * 3600 + minute * 60 + second;
-    const micros = BigInt(seconds) * 1000000n + BigInt((parsed[5] ?? "").padEnd(6, "0").slice(0, 6));
+    const micros = BigInt(seconds) * 1000000n + BigInt(fraction.padEnd(6, "0").slice(0, 6));
     if (offset && micros >= 86400000000n) return null;
     // Python treats an all-zero offset as UTC even with fractional seconds.
     return offset && seconds === 0 ? 0n : micros;
diff --git a/tests/control_plane/test_monitor_state_owner.py b/tests/control_plane/test_monitor_state_owner.py
index 0432a6b510..386a32c2c4 100644
--- a/tests/control_plane/test_monitor_state_owner.py
+++ b/tests/control_plane/test_monitor_state_owner.py
@@ -81,6 +81,7 @@ def test_monitor_effect_replay_and_generation_are_locked_public_writer_semantics
     "2030-01-01T12:34:56.123456+08:00", "20300101T123456,123456+0800",
     "1970-01-01T01:00:00+00:59:59.999999", "1970-01-01T00.1",
     "1970-02-30", "2021-W53", "1970-01-01T24:00:00", "1970-01-01T01:00+24:00",
+    "9999-12-31T24:00:00", "9999-12-31T24:00:00Z", "9999-12-31T24:00:00.000000",
     "1970-01-01T00:0000", "1970-01-01T0000:00", "tomorrow", "2030",
 ])
 def test_monitor_timestamp_input_matches_retained_python_iso_codec(value):
@@ -89,7 +90,7 @@ def test_monitor_timestamp_input_matches_retained_python_iso_codec(value):
         "role": "agent", "task_class": "continuous_monitor", "enforce_boundedness": True,
         "metadata": {"expires_at": value},
     }
-    # The pre-migration stdlib codec is an independent input-compatibility oracle.
+    # The retained Python reader defines the stable cross-version wire grammar.
     if parse_timestamp(value) is None:
         with pytest.raises(EffectRuntimeRejected, match="timestamp"):
             effect_runtime_result("todo.monitor_metadata.plan", request)
@@ -98,6 +99,39 @@ def test_monitor_timestamp_input_matches_retained_python_iso_codec(value):
         assert result["metadata"]["expires_at"] == value
 
 
+@pytest.mark.parametrize(("value", "expected_utc"), [
+    ("2030-01-01T00.1", "2030-01-01T00:00:00.100000+00:00"),
+    ("2030-01-01T00:00.1", "2030-01-01T00:00:00.100000+00:00"),
+    ("2030-01-01T00:00:00+01.1", "2029-12-31T22:59:59.900000+00:00"),
+    ("2030-01-01T00:00:00+00.1", "2030-01-01T00:00:00+00:00"),
+    ("2030-01-01T00:00:00-00.1", "2030-01-01T00:00:00+00:00"),
+    ("2030-01-01T00:00:00+00:00:00.1", "2030-01-01T00:00:00+00:00"),
+    ("2030-01-01T00.1+00.5", "2030-01-01T00:00:00.100000+00:00"),
+    ("2030-01-01T00:00:00.1+00:00:00.5", "2030-01-01T00:00:00.100000+00:00"),
+])
+def test_monitor_timestamp_codec_is_stable_across_supported_python_versions(value, expected_utc):
+    parsed = parse_timestamp(value)
+    assert parsed is not None
+    assert parsed.isoformat() == expected_utc
+    result = effect_runtime_result("todo.monitor_metadata.plan", {
+        "schema_version": "loopx_todo_monitor_metadata_request_v0",
+        "role": "agent", "task_class": "continuous_monitor", "enforce_boundedness": True,
+        "metadata": {"expires_at": value},
+    })
+    assert result["metadata"]["expires_at"] == value
+
+
+def test_monitor_timestamp_codec_rejects_version_specific_end_of_day():
+    value = "2030-01-01T24:00:00"
+    assert parse_timestamp(value) is None
+    with pytest.raises(EffectRuntimeRejected, match="timestamp"):
+        effect_runtime_result("todo.monitor_metadata.plan", {
+            "schema_version": "loopx_todo_monitor_metadata_request_v0",
+            "role": "agent", "task_class": "continuous_monitor", "enforce_boundedness": True,
+            "metadata": {"expires_at": value},
+        })
+
+
 @pytest.mark.parametrize("date", ["1970-01-01", "19700101", "1970-W01-4"])
 @pytest.mark.parametrize("separator", ["Z", "z"])
 def test_public_writer_rejects_timezone_letter_as_date_separator(tmp_path, date, separator):
diff --git a/tests/control_plane_ts/monitor_metadata.test.ts b/tests/control_plane_ts/monitor_metadata.test.ts
index cdd42903a3..36b05b5b00 100644
--- a/tests/control_plane_ts/monitor_metadata.test.ts
+++ b/tests/control_plane_ts/monitor_metadata.test.ts
@@ -100,9 +100,12 @@ test("poll rejects invalid observation shapes without mutating its input", () =>
 });
 
 test("timestamp codec rejects rollover dates and ordering retains microseconds", () => {
-  for (const invalid of ["2030-02-30T00:00:00Z", "2030-01-01T24:00:00Z", "2030"]) {
+  for (const invalid of ["2030-02-30T00:00:00Z", "2030"]) {
     assert.throws(() => planMonitorMetadata(request({metadata: {expires_at: invalid}})), /timestamp/);
   }
+  assert.throws(() => planMonitorMetadata(request({
+    metadata: {expires_at: "2030-01-01T24:00:00Z"},
+  })), /timestamp/);
   assert.throws(() => planMonitorMetadata(request({
     existing: {...existing, last_checked_at: "2030-01-01T01:00:00.000002Z"},
     observation: observation({generated_at: "2030-01-01T01:00:00.000001Z", monitor_effect_id: null}),
@@ -155,7 +158,20 @@ test("Todo timestamp codec retains Python ISO compatibility, timezone seconds an
     "1970-01-01T01:00:00+00:59:59.999999"]) {
     assert.equal(parseTodoTimestampMicros(value), 1n, value);
   }
-  for (const value of ["1970-02-30", "2021-W53", "1970-01-01T24:00:00", "1970-01-01T01:00+24:00",
+  for (const value of ["1970-01-01T00.1", "1970-01-01T00:00.1"]) {
+    assert.equal(parseTodoTimestampMicros(value), 100000n, value);
+  }
+  assert.equal(parseTodoTimestampMicros("1970-01-01T00:00:00+01.1"), -3600100000n);
+  for (const value of ["1970-01-01T00:00:00+00.1", "1970-01-01T00:00:00-00.1",
+    "1970-01-01T00:00:00+00:00:00.1"]) {
+    assert.equal(parseTodoTimestampMicros(value), 0n, value);
+  }
+  for (const value of ["1970-01-01T00.1+00.5", "1970-01-01T00:00:00.1+00:00:00.5"]) {
+    assert.equal(parseTodoTimestampMicros(value), 100000n, value);
+  }
+  for (const value of ["1970-02-30", "2021-W53", "1970-01-01T01:00+24:00",
+    "1970-01-01T24:00:00", "1970-01-01T24:00:01",
+    "9999-12-31T24:00:00", "9999-12-31T24:00:00Z", "9999-12-31T24:00:00.000000",
     "1970-01-01T00:0000", "1970-01-01T0000:00", "tomorrow", "2030"]) {
     assert.equal(parseTodoTimestampMicros(value), null, value);
   }