Pydantic model_json_schema() generates an unexpected

The Fix

pip install pydantic==2.9.2

Based on closed pydantic/pydantic issue #10397 · 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

@@ -1899,7 +1899,7 @@ def get_title_from_name(self, name: str) -> str:
             The title.
         """
-        return name.title().replace('_', ' ')
+        return name.title().replace('_', ' ').strip()

repro.py

from pydantic import BaseModel, Field

class MyModel(BaseModel):
    private_field: int = Field(alias='_private_field', serialization_alias='_private_field')


MyModel.model_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==2.9.2\nWhen NOT to use: Do not apply this fix if the leading space is intentionally required for specific formatting.\n\n

Why This Fix Works in Production

Trigger: model_json_schema() generates an unexpected leading space in title when using a serialization_alias with a `_` prefix
Mechanism: Leading whitespace is introduced when generating titles from serialization aliases with a '_' prefix
Why the fix works: Strips leading whitespace from JSON Schema title generation when using serialization aliases. (first fixed release: 2.9.2).

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).
Leading whitespace is introduced when generating titles from serialization aliases with a '_' prefix
Production symptom (often without a traceback): model_json_schema() generates an unexpected leading space in title when using a serialization_alias with a `_` prefix

Proof / Evidence

GitHub issue: #10397
Fix PR: https://github.com/pydantic/pydantic/pull/10404
First fixed release: 2.9.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.63

Verified Execution

We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.

Status: PASS
Ran: 2026-02-11T16:52:29Z
Package: pydantic
Fixed: 2.9.2
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

Discussion

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

Jump to Sources Open on GitHub

“Hi @eyurtsev, Thanks for bringing this to our attention! I've opened a PR with a fix 👍”

@sydney-runkle · 2024-09-13 · source

Failure Signature (Search String)

model_json_schema() generates an unexpected leading space in title when using a serialization_alias with a `_` prefix
When generating model_schema() with a serialization alias the automatic title is getting an unexpected leading space. I would expect either the `_` to be present or else to be

Copy-friendly signature

signature.txt

Failure Signature
-----------------
model_json_schema() generates an unexpected leading space in title when using a serialization_alias with a `_` prefix
When generating model_schema()  with a serialization alias the automatic title is getting an unexpected leading space. I would expect either the `_` to be present or else to be stripped rather than replaced with a space.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
model_json_schema() generates an unexpected leading space in title when using a serialization_alias with a `_` prefix
When generating model_schema()  with a serialization alias the automatic title is getting an unexpected leading space. I would expect either the `_` to be present or else to be stripped rather than replaced with a space.

Minimal Reproduction

repro.py

from pydantic import BaseModel, Field

class MyModel(BaseModel):
    private_field: int = Field(alias='_private_field', serialization_alias='_private_field')


MyModel.model_json_schema()

Environment

Python: 3.11
Pydantic: 2

What Broke

Generated JSON Schema titles contain unexpected leading spaces, affecting API responses.

Why It Broke

Leading whitespace is introduced when generating titles from serialization aliases with a '_' prefix

Fix Options (Details)

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

pip install pydantic==2.9.2

When NOT to use: Do not apply this fix if the leading space is intentionally required for specific formatting.

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/10404

First fixed release: 2.9.2

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

When NOT to Use This Fix

Do not apply this fix if the leading space is intentionally required for specific formatting.

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
2.9.2	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 model_json_schema() generates an unexpected leading space in title when using a (Fix)