The Fix
pip install pydantic==2.9.1
Based on closed pydantic/pydantic issue #9439 · PR/commit linked
@@ -212,6 +212,16 @@ print(my_model.model_dump_json()) # (3)!
3. Using [`model_dump_json`][pydantic.main.BaseModel.model_dump_json], `x` is serialized as a `string`, and `y` is serialized as a `float` because of the custom serializer applied.
+### [`complex`][]
+
+* Validation: Pydantic supports `complex` types or `str` values that can be converted to a `complex` type.
fix.md
Option A — Upgrade to fixed release\npip install pydantic==2.9.1\nWhen NOT to use: Do not use this fix if you require strict type validation for non-fraction types.\n\n
Why This Fix Works in Production
- Trigger: - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- Mechanism: Adds support for Python's `fractions.Fraction` as a natively supported type in Pydantic, addressing issue #9439.
- Why the fix works: Adds support for Python's `fractions.Fraction` as a natively supported type in Pydantic, addressing issue #9439. (first fixed release: 2.9.1).
Why This Breaks in Prod
- Production symptom (often without a traceback): - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
Proof / Evidence
- GitHub issue: #9439
- Fix PR: https://github.com/pydantic/pydantic/pull/10318
- First fixed release: 2.9.1
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.64
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Hi @etienne-monier, Seems like a great feature request, and sort of goes along with the complex number support request. PRs welcome - we'd want to…”
“@etienne-monier are you willing to make PR? Same issue happened to me, and if you don't want to add Fraction support, I may do it…”
“> @etienne-monier are you willing to make PR? Same issue happened to me, and if you don't want to add Fraction support, I may do…”
“I walk back my previous comment here - we can start with an implementation in Python only. I'll take a stab at this.”
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)
What Broke
Users cannot validate or serialize fractions, leading to data handling issues.
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==2.9.1
When NOT to use: Do not use this fix if you require strict type validation for non-fraction types.
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)
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/10318
First fixed release: 2.9.1
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- Do not use this fix if you require strict type validation for non-fraction types.
- 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.
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.
Version Compatibility Table
| Version | Status |
|---|---|
| 2.9.1 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.