Pydantic The JSON schema for the BaseModel class (Fix)

The Fix

pip install pydantic==2.6.4

Based on closed pydantic/pydantic issue #8980 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Jump to Verify Open PR/Commit

@@ -2176,9 +2176,14 @@ def model_json_schema(
         The generated JSON Schema.
     """
+    from .main import BaseModel
+
     schema_generator_instance = schema_generator(by_alias=by_alias, ref_template=ref_template)

repro.py

pydantic version: 2.6.3
        pydantic-core version: 2.16.3
          pydantic-core build: profile=release pgo=true
                 install path: C:\...\venv\Lib\site-packages\pydantic
               python version: 3.12.1 (tags/v3.12.1:2305ca5, Dec  7 2023, 22:03:25) [MSC v.1937 64 bit (AMD64)]
                     platform: Windows-10-10.0.19045-SP0
             related packages: typing_extensions-4.9.0
                       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==2.6.4\nWhen NOT to use: This fix is not applicable if using BaseModel directly without subclassing.\n\n

Why This Fix Works in Production

Trigger: The JSON schema for the BaseModel class
Mechanism: The BaseModel class was incorrectly used, leading to an AssertionError
Why the fix works: Provides a more helpful error message for incorrect usage of the `model_json_schema` method in the BaseModel class. (first fixed release: 2.6.4).

Production impact:

If left unfixed, retries/timeouts can trigger duplicate external side-effects (double charges, duplicate emails, repeated writes).

Why This Breaks in Prod

The BaseModel class was incorrectly used, leading to an AssertionError
Production symptom (often without a traceback): The JSON schema for the BaseModel class

Proof / Evidence

GitHub issue: #8980
Fix PR: https://github.com/pydantic/pydantic/pull/8928
First fixed release: 2.6.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.56

Discussion

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

Jump to Sources Open on GitHub

“This is a duplicate of https://github.com/pydantic/pydantic/issues/8896, which was fixed in https://github.com/pydantic/pydantic/pull/8928.”

@Viicos · 2024-03-12 · confirmation · source

Failure Signature (Search String)

The JSON schema for the BaseModel class
On running the code in the example below I receive "AssertionError: this is a bug! please report it"

Copy-friendly signature

signature.txt

Failure Signature
-----------------
The JSON schema for the BaseModel class
On running the code in the example below I receive "AssertionError: this is a bug! please report it"

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
The JSON schema for the BaseModel class
On running the code in the example below I receive "AssertionError: this is a bug! please report it"

Minimal Reproduction

repro.py

pydantic version: 2.6.3
        pydantic-core version: 2.16.3
          pydantic-core build: profile=release pgo=true
                 install path: C:\...\venv\Lib\site-packages\pydantic
               python version: 3.12.1 (tags/v3.12.1:2305ca5, Dec  7 2023, 22:03:25) [MSC v.1937 64 bit (AMD64)]
                     platform: Windows-10-10.0.19045-SP0
             related packages: typing_extensions-4.9.0
                       commit: unknown

Environment

Pydantic: 2

What Broke

Users encountered an AssertionError when calling model_json_schema on BaseModel.

Why It Broke

The BaseModel class was incorrectly used, leading to an AssertionError

Fix Options (Details)

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

pip install pydantic==2.6.4

When NOT to use: This fix is not applicable if using BaseModel directly without subclassing.

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/8928

First fixed release: 2.6.4

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

When NOT to Use This Fix

This fix is not applicable if using BaseModel directly without subclassing.

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
2.6.4	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:duplicate-processing Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Configuration error Data consistency OpenAPI schema

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.