The Fix
pip install pydantic==1.10.16
Based on closed pydantic/pydantic issue #9381 · 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%.
@@ -1444,7 +1444,6 @@ def _sequence_schema(self, sequence_type: Any) -> core_schema.CoreSchema:
item_type_schema = self.generate_schema(item_type)
list_schema = core_schema.list_schema(item_type_schema)
- metadata = build_metadata_dict(initial_metadata={'known_metadata_as': typing.Sequence})
python_schema = core_schema.is_instance_schema(typing.Sequence, cls_repr='Sequence')
repro.py
from pydantic import BaseModel, EmailStr, StringConstraints
from typing import Annotated
EStr = Annotated[EmailStr, StringConstraints(to_lower=True, strip_whitespace=True)]
class Foo(BaseModel):
bar: EStr
Foo(bar="[email protected]")
>> Foo(bar='[email protected]')
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==1.10.16\nWhen NOT to use: Do not use this fix if the behavior of string constraints needs to remain unchanged.\n\n
Why This Fix Works in Production
- Trigger: I see the argument for changing it, but it's a very annoying breaking change to understand, so my instinct is we don't change this, even in a major update.
- Mechanism: The string constraints were not applied correctly after EmailStr validation
- Why the fix works: Fixes the annotation application logic related to string constraints, addressing the issue where only one of the string constraints was applied. (first fixed release: 1.10.16).
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 string constraints were not applied correctly after EmailStr validation
- Production symptom (often without a traceback): I see the argument for changing it, but it's a very annoying breaking change to understand, so my instinct is we don't change this, even in a major update.
Proof / Evidence
- GitHub issue: #9381
- Fix PR: https://github.com/pydantic/pydantic/pull/9623
- First fixed release: 1.10.16
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.75
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.58
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“I'll note, I think this is related to https://github.com/pydantic/pydantic/issues/8577 (where this issue was originally discussed)”
“Another example that might be relevant Result”
“@SDAravind, Thanks for reporting this in a separate issue. I've added this to our upcoming milestone!”
“The email validation process relies on the email_validator package”
Failure Signature (Search String)
- I see the argument for changing it, but it's a very annoying breaking change to understand, so my instinct is we don't change this, even in a major update.
- As for the reason - my guess is that the best explanation is "because that's how it was first done" and there hasn't been a strong (enough) argument to change it.
Copy-friendly signature
signature.txt
Failure Signature
-----------------
I see the argument for changing it, but it's a very annoying breaking change to understand, so my instinct is we don't change this, even in a major update.
As for the reason - my guess is that the best explanation is "because that's how it was first done" and there hasn't been a strong (enough) argument to change it.
Error Message
Signature-only (no traceback captured)
error.txt
Error Message
-------------
I see the argument for changing it, but it's a very annoying breaking change to understand, so my instinct is we don't change this, even in a major update.
As for the reason - my guess is that the best explanation is "because that's how it was first done" and there hasn't been a strong (enough) argument to change it.
Minimal Reproduction
repro.py
from pydantic import BaseModel, EmailStr, StringConstraints
from typing import Annotated
EStr = Annotated[EmailStr, StringConstraints(to_lower=True, strip_whitespace=True)]
class Foo(BaseModel):
bar: EStr
Foo(bar="[email protected]")
>> Foo(bar='[email protected]')
What Broke
Email addresses were not normalized as expected, leading to inconsistent validation results.
Why It Broke
The string constraints were not applied correctly after EmailStr validation
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==1.10.16
When NOT to use: Do not use this fix if the behavior of string constraints needs to remain unchanged.
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/9623
First fixed release: 1.10.16
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- Do not use this fix if the behavior of string constraints needs to remain unchanged.
- 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 |
|---|---|
| 1.10.16 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.