Pydantic AssertionError when calling

The Fix

pip install pydantic==2.6.4

Based on closed pydantic/pydantic issue #8896 · 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

@@ -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: 2.6.1
        pydantic-core version: 2.16.2
          pydantic-core build: profile=release pgo=true
                 install path: /home/user/.local/lib/python3.12/site-packages/pydantic
               python version: 3.12.1 (main, Dec 18 2023, 00:00:00) [GCC 13.2.1 20231205 (Red Hat 13.2.1-6)]
                     platform: Linux-6.7.5-200.fc39.x86_64-x86_64-with-glibc2.38
             related packages: fastapi-0.109.2 pydantic-settings-2.2.1 typing_extensions-4.8.0
                       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 the method is called correctly on subclasses of BaseModel.\n\n

Why This Fix Works in Production

Trigger: AssertionError: this is a bug! please report it
Mechanism: Calling model_json_schema() on BaseModel instead of its subclasses leads to an assertion error
Why the fix works: Adds a more helpful error message when `model_json_schema()` is called on `BaseModel`, addressing issue #8896. (first fixed release: 2.6.4).

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 3.12 in real deployments (not just unit tests).
Calling model_json_schema() on BaseModel instead of its subclasses leads to an assertion error
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #8896
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: 0.43

Discussion

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

Jump to Sources Open on GitHub

“@sydney-runkle Yes, it is completely clear that calling model_json_schema() does not make a lot of sense”

@hsteinhaus · 2024-02-27 · confirmation · source

“@hsteinhaus, Thanks for reporting this. schema_json(), or the new (not deprecated) version of said function model_json_schema() are meant to be called on subclasses of BaseModel,…”

@sydney-runkle · 2024-02-27 · confirmation · source

“Reopening, we can add a more descriptive error for the case when it's being called on BaseModel.”

@sydney-runkle · 2024-02-27 · source

Failure Signature (Search String)

AssertionError: this is a bug! please report it

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/holger-local/.local/lib/python3.12/site-packages/pydantic/main.py", line 1287, in schema_json
    cls.model_json_schema(by_alias=by_alias, ref_template=ref_template),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/holger-local/.local/lib/python3.12/site-packages/pydantic/main.py", line 391, in model_json_schema
    return model_json_schema(
           ^^^^^^^^^^^^^^^^^^
  File "/home/holger-local/.local/lib/python3.12/site-packages/pydantic/json_schema.py", line 2176, in model_json_schema
    assert '__pydantic_core_schema__' in cls.__dict__, 'this is a bug! please report it'
AssertionError: this is a bug! please report it

Minimal Reproduction

repro.py

pydantic version: 2.6.1
        pydantic-core version: 2.16.2
          pydantic-core build: profile=release pgo=true
                 install path: /home/user/.local/lib/python3.12/site-packages/pydantic
               python version: 3.12.1 (main, Dec 18 2023, 00:00:00) [GCC 13.2.1 20231205 (Red Hat 13.2.1-6)]
                     platform: Linux-6.7.5-200.fc39.x86_64-x86_64-with-glibc2.38
             related packages: fastapi-0.109.2 pydantic-settings-2.2.1 typing_extensions-4.8.0
                       commit: unknown

Environment

Python: 3.12
Pydantic: 2

What Broke

Users encounter an assertion error when calling schema_json() on BaseModel, causing confusion.

Why It Broke

Calling model_json_schema() on BaseModel instead of its subclasses leads to an assertion error

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 the method is called correctly on subclasses of BaseModel.

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 the method is called correctly on subclasses of BaseModel.

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: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 AssertionError when calling BaseModel.schema_json() (Fix)