Pydantic Recursive type aliases should just work (Fix)

The Fix

pip install pydantic==2.11.0

Based on closed pydantic/pydantic issue #10873 · 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

@@ -15,6 +15,8 @@
 from typing_extensions import ParamSpec, TypeAliasType, TypeIs, deprecated, get_args, get_origin
 
+from pydantic.version import version_short
+
 from ._namespace_utils import GlobalsNamespace, MappingNamespace, NsResolver, get_module_ns_of

repro.py

# module1.py

JsonData: TypeAlias = 'str | int | float | None | list[JsonData] | dict[str, JsonData]'

# module2.py

from module1 import JsonData
from pydantic import BaseModel

class Model(BaseModel):
    a: JsonData

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 should not be applied if the application relies on the previous behavior of recursion handling.\n\n

Why This Fix Works in Production

Trigger: Users experienced crashes or unexpected behavior when using recursive type aliases.
Mechanism: Recursion errors during type evaluation were not handled properly in the library
Why the fix works: Improves exception message when encountering recursion errors during type evaluation, addressing issue #10873. (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

Triggered by an upgrade/regression window: 3.12 breaks; 2.11.0 is the first fixed release.
Shows up under Python 3.11 in real deployments (not just unit tests).
Recursion errors during type evaluation were not handled properly in the library
Production symptom (often without a traceback): Users experienced crashes or unexpected behavior when using recursive type aliases.

Proof / Evidence

GitHub issue: #10873
Fix PR: https://github.com/pydantic/pydantic/pull/11356
First fixed release: 2.11.0
Affected versions: 3.12
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.70

Discussion

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

Jump to Sources Open on GitHub

“@jogo-openai, if you are using 3.12+: If using Python 3.11-:”

@Viicos · 2025-01-27 · source

“@Viicos I was trying to follow along for the TypeAliasType route with this example but had trouble sorting out out. What is the workaround?”

@jogo-openai · 2025-01-23 · source

“I don't think it is possible to safely support this”

@Viicos · 2024-11-22 · source

Failure Signature (Search String)

Users experienced crashes or unexpected behavior when using recursive type aliases.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Users experienced crashes or unexpected behavior when using recursive type aliases.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Users experienced crashes or unexpected behavior when using recursive type aliases.

Minimal Reproduction

repro.py

# module1.py

JsonData: TypeAlias = 'str | int | float | None | list[JsonData] | dict[str, JsonData]'

# module2.py

from module1 import JsonData
from pydantic import BaseModel

class Model(BaseModel):
    a: JsonData

Environment

Python: 3.11

What Broke

Users experienced crashes or unexpected behavior when using recursive type aliases.

Why It Broke

Recursion errors during type evaluation were not handled properly in the library

Fix Options (Details)

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

pip install pydantic==2.11.0

When NOT to use: This fix should not be applied if the application relies on the previous behavior of recursion handling.

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/11356

First fixed release: 2.11.0

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

When NOT to Use This Fix

This fix should not be applied if the application relies on the previous behavior of recursion handling.

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
3.12	Broken
2.11.0	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.