Pydantic Add a ser_json_datetime config option for

The Fix

pip install pydantic==2.11.8

Based on closed pydantic/pydantic issue #10454 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -83,7 +83,9 @@ extra_javascript:
   - 'extra/feedback.js'
   - 'extra/fluff.js'
+  - 'extra/mathjax.js'
   - 'https://samuelcolvin.github.io/mkdocs-run-code/run_code_main.js'
+  - 'https://unpkg.com/mathjax@3/es5/tex-mml-chtml.js'

repro.py

from typing import Annotated
import datetime as dt
from pydantic import BaseModel, PlainSerializer


T_SerializedDatetime = Annotated[
    dt.datetime,
    PlainSerializer(
        lambda x: x.timestamp() * 1000,
        return_type=int,
        when_used="json",
    ),
]

class Value(BaseModel):
    series: dict[T_SerializedDatetime, float]

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install pydantic==2.11.8\nWhen NOT to use: This fix is not suitable for applications that require strict ISO 8601 formatting for datetime serialization.\n\n

Why This Fix Works in Production

Trigger: - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
Mechanism: The serialization of datetime objects was inefficient, causing performance issues during large data series serialization
Why the fix works: Adds configuration options for validation and JSON serialization of temporal types, improving performance when serializing datetime as a millisecond timestamp. (first fixed release: 2.11.8).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

The serialization of datetime objects was inefficient, causing performance issues during large data series serialization
Production symptom (often without a traceback): - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)

Proof / Evidence

GitHub issue: #10454
Fix PR: https://github.com/pydantic/pydantic/pull/12068
First fixed release: 2.11.8
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.69

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“I've set the PR i have done with this implementation as ready to review, but welcome any design feedback if we want to change this…”

@ollz272 · 2024-09-24 · source

“> I've set the PR i have done with this implementation as ready to review, but welcome any design feedback if we want to change…”

@sydney-runkle · 2024-09-24 · source

“As a note, I'm happy to work on this feature, though may need a bit of guidance in this (i know bits and bobs now…”

@ollz272 · 2024-09-21 · source

“Hmm, given that we just added milliseconds_float, seconds_float options for ser_json_timedelta, perhaps we should just use a consistent pattern here?”

@sydney-runkle · 2024-09-23 · source

Failure Signature (Search String)

- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
- [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- [ ] [Data validation/parsing](https://docs.pydantic.dev/concepts/models/#basic-model-usage)

Minimal Reproduction

repro.py

from typing import Annotated
import datetime as dt
from pydantic import BaseModel, PlainSerializer


T_SerializedDatetime = Annotated[
    dt.datetime,
    PlainSerializer(
        lambda x: x.timestamp() * 1000,
        return_type=int,
        when_used="json",
    ),
]

class Value(BaseModel):
    series: dict[T_SerializedDatetime, float]

What Broke

High latency observed during serialization of datetime objects in production, impacting response times.

Why It Broke

The serialization of datetime objects was inefficient, causing performance issues during large data series serialization

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install pydantic==2.11.8

When NOT to use: This fix is not suitable for applications that require strict ISO 8601 formatting for datetime serialization.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/pydantic/pydantic/pull/12068

First fixed release: 2.11.8

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix is not suitable for applications that require strict ISO 8601 formatting for datetime serialization.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.
Track RSS + object counts after deployments; alert on monotonic growth and GC pressure.
Add a long-running test that repeats the failing call path and asserts stable memory.

Version Compatibility Table

Version	Status
2.11.8	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:configuration-error Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Data consistency OpenAPI schema Serialization

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Pydantic Add a ser_json_datetime config option for configuring how datetimes are serialized. (Fix)