Pydantic ImportString fails to generate JsonSchema

The Fix

pip install pydantic==2.7.2

Based on closed pydantic/pydantic issue #9327 · 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

@@ -990,6 +990,10 @@ def __get_pydantic_core_schema__(
                 )
 
+        @classmethod
+        def __get_pydantic_json_schema__(cls, cs: CoreSchema, handler: GetJsonSchemaHandler) -> JsonSchemaValue:
+            return handler(core_schema.str_schema())

repro.py

# Issue was in FastApi docs generation, but the below produces an error that 
# looks the same
import pydantic

pydantic.TypeAdapter(pydantic.ImportString).json_schema()

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==2.7.2\nWhen NOT to use: This fix should not be used if the ImportString type is not required for JSON schema generation.\n\nOption C — Workaround\nwas to create a type-annotated version:\nWhen NOT to use: This fix should not be used if the ImportString type is not required for JSON schema generation.\n\n

Why This Fix Works in Production

Trigger: ImportString fails to generate JsonSchema - in fastapi openapi docs generation
Mechanism: Pydantic could not generate a JsonSchema for the ImportString type due to missing support
Why the fix works: Adds JSON-Schema support for the ImportString type, which previously failed to generate a schema. (first fixed release: 2.7.2).

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 3.11 in real deployments (not just unit tests).
Pydantic could not generate a JsonSchema for the ImportString type due to missing support
Production symptom (often without a traceback): ImportString fails to generate JsonSchema - in fastapi openapi docs generation

Proof / Evidence

GitHub issue: #9327
Fix PR: https://github.com/pydantic/pydantic/pull/9344
First fixed release: 2.7.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.67

Discussion

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

Jump to Sources Open on GitHub

“@amitschang, Ah, nice catch. We'll want to add json schema support to the ImportString type. PR welcome!”

@sydney-runkle · 2024-04-28 · source

“@sydney-runkle, thanks for looking! I'll see if I have some time to work on such a PR”

@amitschang · 2024-04-29 · source

“@sydney-runkle , I made a PR - I can't seem to add the necessary labels to it (ready for review and relnotes-fix), but it is…”

@amitschang · 2024-04-29 · source

Failure Signature (Search String)

ImportString fails to generate JsonSchema - in fastapi openapi docs generation
raise PydanticInvalidForJsonSchema(f'Cannot generate a JsonSchema for {error_info}')

Copy-friendly signature

signature.txt

Failure Signature
-----------------
ImportString fails to generate JsonSchema - in fastapi openapi docs generation
raise PydanticInvalidForJsonSchema(f'Cannot generate a JsonSchema for {error_info}')

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
ImportString fails to generate JsonSchema - in fastapi openapi docs generation
raise PydanticInvalidForJsonSchema(f'Cannot generate a JsonSchema for {error_info}')

Minimal Reproduction

repro.py

# Issue was in FastApi docs generation, but the below produces an error that 
# looks the same
import pydantic

pydantic.TypeAdapter(pydantic.ImportString).json_schema()

Environment

Python: 3.11
Pydantic: 2

What Broke

OpenAPI docs generation failed, causing issues with API documentation.

Why It Broke

Pydantic could not generate a JsonSchema for the ImportString type due to missing support

Fix Options (Details)

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

pip install pydantic==2.7.2

When NOT to use: This fix should not be used if the ImportString type is not required for JSON schema generation.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option C — Workaround Temporary workaround

was to create a type-annotated version:

When NOT to use: This fix should not be used if the ImportString type is not required for JSON schema generation.

Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.

Fix reference: https://github.com/pydantic/pydantic/pull/9344

First fixed release: 2.7.2

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

When NOT to Use This Fix

This fix should not be used if the ImportString type is not required for JSON schema generation.

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
2.7.2	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 fails to generate JsonSchema - in fastapi openapi docs generation (Fix)