Pydantic Allow partial needs `'trailing-strings'` mode,

The Fix

pip install pydantic==2.10.0

Based on closed pydantic/pydantic issue #10806 · 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%.

Open PR/Commit

@@ -163,6 +163,38 @@ Partial validation is particularly helpful when processing the output of an LLM,
 Partial validation can be enabled when using the three validation methods on `TypeAdapter`: [`TypeAdapter.validate_json()`][pydantic.TypeAdapter.validate_json], [`TypeAdapter.validate_python()`][pydantic.TypeAdapter.validate_python], and [`TypeAdapter.validate_strings()`][pydantic.TypeAdapter.validate_strings]. This allows you to parse and validation incomplete JSON, but also to validate Python objects created by parsing incomplete data of any format.
 
+The `experimental_allow_partial` flag can be passed to these methods to enable partial validation.
+It can take the following values (and is `False`, by default):
+

fix.md

Option A — Upgrade to fixed release\npip install pydantic==2.10.0\nWhen NOT to use: This fix should not be used if strict validation of complete data is required.\n\n

Why This Fix Works in Production

Trigger: Even though it's a change of behaviour,I think we should change the signature to match `pydantic_core.from_json`.
Mechanism: The introduction of allow partial validation skipped support for 'trailing-strings' mode, causing incomplete data handling issues
Why the fix works: Adds support for the 'trailing-strings' mode to the experimental_allow_partial feature in Pydantic, allowing for partial validation with trailing incomplete strings. (first fixed release: 2.10.0).

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

The introduction of allow partial validation skipped support for 'trailing-strings' mode, causing incomplete data handling issues
Production symptom (often without a traceback): Even though it's a change of behaviour,I think we should change the signature to match `pydantic_core.from_json`.

Proof / Evidence

GitHub issue: #10806
Fix PR: https://github.com/pydantic/pydantic/pull/10825
First fixed release: 2.10.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.60

Discussion

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

Jump to Sources Open on GitHub

“@samuelcolvin, We can do this in the beta”

@sydney-runkle · 2024-11-10 · source

Failure Signature (Search String)

Even though it's a change of behaviour,I think we should change the signature to match `pydantic_core.from_json`.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Even though it's a change of behaviour,I think we should change the signature to match `pydantic_core.from_json`.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Even though it's a change of behaviour,I think we should change the signature to match `pydantic_core.from_json`.

What Broke

Users experienced validation failures with incomplete JSON data leading to application errors.

Why It Broke

The introduction of allow partial validation skipped support for 'trailing-strings' mode, causing incomplete data handling issues

Fix Options (Details)

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

pip install pydantic==2.10.0

When NOT to use: This fix should not be used if strict validation of complete data is required.

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/10825

First fixed release: 2.10.0

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

When NOT to Use This Fix

This fix should not be used if strict validation of complete data is required.

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
2.10.0	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.

Pydantic Allow partial needs `'trailing-strings'` mode, same as `pydantic_core.from_json` (Fix)