The Fix
pip install pydantic==2.11.0
Based on closed pydantic/pydantic issue #11346 · 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%.
@@ -297,7 +297,8 @@ jobs:
- name: Run ODMantic tests
- run: pytest tests
+ # Disabled tests, as per https://github.com/art049/odmantic/issues/512:
+ run: pytest tests -k 'not test_custom_bson_serializable and not test_sync_custom_bson_serializable and not test_with_bson_serializer_override_builtin_bson'
repro.py
from typing import TypedDict
import pydantic
from typing_extensions import ReadOnly
class Foo(TypedDict):
foo: ReadOnly[str]
adapter = pydantic.TypeAdapter(Foo)
foo = adapter.validate_python({"foo": "bar"})
print(foo["foo"])
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==2.11.0\nWhen NOT to use: This fix is not suitable if strict immutability is required for typed dictionaries.\n\n
Why This Fix Works in Production
- Trigger: E pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for typing_extensions.ReadOnly[str]. Set…
- Mechanism: Added support for `typing.ReadOnly` in Pydantic to prevent schema generation errors when using typed dictionaries.
- Why the fix works: Added support for `typing.ReadOnly` in Pydantic to prevent schema generation errors when using typed dictionaries. (first fixed release: 2.11.0).
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
- Shows up under Python 3.12 in real deployments (not just unit tests).
- Surfaces as: E pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for typing_extensions.ReadOnly[str]. Set `arbitrary_types_allowed=True` in the…
Proof / Evidence
- GitHub issue: #11346
- Fix PR: https://github.com/pydantic/pydantic/pull/11479
- First fixed release: 2.11.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.47
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“For my use case, this works perfectly. I don't really need a runtime check as I know typeddicts are a special type from.”
“Support for ReadOnly will be added, however it might not work as you would expect, as Pydantic can't prevent mutations on dictionaries: For this reason,…”
Failure Signature (Search String)
- E pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for typing_extensions.ReadOnly[str]. Set `arbitrary_types_allowed=True` in the
Error Message
Stack trace
error.txt
Error Message
-------------
E pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for typing_extensions.ReadOnly[str]. Set `arbitrary_types_allowed=True` in the model_config to ignore this error or implement `__get_pydantic_core_schema__` on your type to fully support it.
E
E If you got this error by calling handler(<some type>) within `__get_pydantic_core_schema__` then you likely need to call `handler.generate_schema(<some type>)` since we do not call `__get_pydantic_core_schema__` on `<some type>` otherwise to avoid infinite recursion.
E
E For further information visit https://errors.pydantic.dev/2.10/u/schema-for-unknown-type
Minimal Reproduction
repro.py
from typing import TypedDict
import pydantic
from typing_extensions import ReadOnly
class Foo(TypedDict):
foo: ReadOnly[str]
adapter = pydantic.TypeAdapter(Foo)
foo = adapter.validate_python({"foo": "bar"})
print(foo["foo"])
Environment
- Python: 3.12
- Pydantic: 2
What Broke
Users encounter schema generation errors when using ReadOnly in FastAPI typed dictionaries.
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install pydantic==2.11.0
When NOT to use: This fix is not suitable if strict immutability is required for typed dictionaries.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/pydantic/pydantic/pull/11479
First fixed release: 2.11.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not suitable if strict immutability is required for typed dictionaries.
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 |
|---|---|
| 2.11.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.