The Fix
pip install pydantic==2.6.4
Based on closed pydantic/pydantic issue #9147 · PR/commit linked
Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.
@@ -126,7 +126,7 @@ class TypeAdapter(Generic[T]):
@overload
def __init__(
- self,
+ self: TypeAdapter[T],
type: type[T],
repro.py
from pydantic import AnyHttpUrl, TypeAdapter
def ensure_url(url: AnyHttpUrl | str) -> AnyHttpUrl:
if isinstance(url, str):
return TypeAdapter(AnyHttpUrl).validate_python(url) # pyright: ignore [reportCallIssue, reportReturnType]
return url
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==2.6.4\nWhen NOT to use: This fix should not be applied if backward compatibility with existing types is a concern.\n\n
Why This Fix Works in Production
- Trigger: - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
- Mechanism: TypeAdapter's typing was incompatible with special forms, causing issues with PyRight
- Why the fix works: Makes TypeAdapter's typing compatible with special forms, addressing typing issues with PyRight. (first fixed release: 2.6.4).
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
- TypeAdapter's typing was incompatible with special forms, causing issues with PyRight
- Production symptom (often without a traceback): - [ ] [Compatibility between releases](https://docs.pydantic.dev/changelog/)
Proof / Evidence
- GitHub issue: #9147
- Fix PR: https://github.com/pydantic/pydantic/pull/8923
- First fixed release: 2.6.4
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.68
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“This should be fixed on main because of our recent work. See https://github.com/pydantic/pydantic/issues/10783 for more details :)”
“> For now, let's have the subclasses replace the exiting types in pydantic, then we can move them to pydantic-extra-types when we do a V3…”
“Duplicate of https://github.com/pydantic/pydantic/discussions/7094”
“@Viicos, Any suggestions for how we might fix 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)
Minimal Reproduction
repro.py
from pydantic import AnyHttpUrl, TypeAdapter
def ensure_url(url: AnyHttpUrl | str) -> AnyHttpUrl:
if isinstance(url, str):
return TypeAdapter(AnyHttpUrl).validate_python(url) # pyright: ignore [reportCallIssue, reportReturnType]
return url
What Broke
Typing issues led to incorrect type validation and potential runtime errors.
Why It Broke
TypeAdapter's typing was incompatible with special forms, causing issues with PyRight
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==2.6.4
When NOT to use: This fix should not be applied if backward compatibility with existing types is a concern.
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/8923
First fixed release: 2.6.4
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be applied if backward compatibility with existing types is a concern.
- 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.
Version Compatibility Table
| Version | Status |
|---|---|
| 2.6.4 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.