Pydantic `ImportString` serialization is incorrect when

The Fix

pip install pydantic==1.10.24

Based on closed pydantic/pydantic issue #12218 · 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

@@ -1028,8 +1028,7 @@ def _serialize(v: Any) -> str:
                 elif v.name == '<stderr>':
                     return 'sys.stderr'
-            else:
-                return v
+            return v

repro.py

pydantic version: 2.11.7
        pydantic-core version: 2.33.2
          pydantic-core build: profile=release pgo=false
               python version: 3.12.11 (stable, redacted, redacted) [Clang 9999.0.0 (fc44a4fcd3c54be927c15ddd9211aca1501633e7)]
                     platform: Linux-5.10.0-smp-1107.69.0.0-x86_64-with-glibc2.27
             related packages: 
                       commit: unknown

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==1.10.24\nWhen NOT to use: Do not use this fix if the type's name attribute is expected to be handled differently.\n\n

Why This Fix Works in Production

Trigger: `ImportString` serialization is incorrect when type has `name` attribute
Mechanism: The serialization method for ImportString returned None for types with a name attribute not in handled cases
Why the fix works: Fixes the serialization issue in `ImportString` when the type has a `name` attribute, ensuring it no longer returns None for unhandled cases. (first fixed release: 1.10.24).

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 serialization method for ImportString returned None for types with a name attribute not in handled cases
Production symptom (often without a traceback): `ImportString` serialization is incorrect when type has `name` attribute

Proof / Evidence

GitHub issue: #12218
Fix PR: https://github.com/pydantic/pydantic/pull/12219
First fixed release: 1.10.24
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.65

Discussion

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

Jump to Sources Open on GitHub

“The fix is trivial. Please see PR #12219.”

@chr1sj0nes · 2025-09-04 · source

Failure Signature (Search String)

`ImportString` serialization is incorrect when type has `name` attribute

Copy-friendly signature

signature.txt

Failure Signature
-----------------
`ImportString` serialization is incorrect when type has `name` attribute

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
`ImportString` serialization is incorrect when type has `name` attribute

Minimal Reproduction

repro.py

pydantic version: 2.11.7
        pydantic-core version: 2.33.2
          pydantic-core build: profile=release pgo=false
               python version: 3.12.11 (stable, redacted, redacted) [Clang 9999.0.0 (fc44a4fcd3c54be927c15ddd9211aca1501633e7)]
                     platform: Linux-5.10.0-smp-1107.69.0.0-x86_64-with-glibc2.27
             related packages: 
                       commit: unknown

Environment

Pydantic: 2

What Broke

Serialization failures led to incorrect handling of types with a name attribute, causing potential data loss.

Why It Broke

The serialization method for ImportString returned None for types with a name attribute not in handled cases

Fix Options (Details)

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

pip install pydantic==1.10.24

When NOT to use: Do not use this fix if the type's name attribute is expected to be handled differently.

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/12219

First fixed release: 1.10.24

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

When NOT to Use This Fix

Do not use this fix if the type's name attribute is expected to be handled differently.

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
1.10.24	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.

Pydantic `ImportString` serialization is incorrect when type has `name` attribute (Fix)