The Fix
pip install fastapi==0.128.4
Based on closed fastapi/fastapi issue #14431 · 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%.
@@ -180,8 +180,13 @@ def get_schema_from_model_field(
separate_input_output_schemas: bool = True,
) -> Dict[str, Any]:
+ computed_fields = field._type_adapter.core_schema.get("schema", {}).get(
+ "computed_fields", []
+ )
repro.py
import pytest
from fastapi import FastAPI
from starlette.testclient import TestClient
@pytest.fixture(name="client")
def get_client():
app = FastAPI(separate_input_output_schemas=False)
from pydantic import BaseModel, computed_field
class Rectangle(BaseModel):
width: int
length: int
@computed_field
@property
def area(self) -> int:
return self.width * self.length
@app.get("/")
def read_root() -> Rectangle:
return Rectangle(width=3, length=4)
client = TestClient(app)
return client
def test_get(client: TestClient):
response = client.get("/openapi.json")
data = response.json()
rectangle_model = data["components"]["schemas"]["Rectangle"]["properties"]
assert "area" in rectangle_model.keys()
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 fastapi==0.128.4\nWhen NOT to use: This fix should not be used if computed fields are not intended to be included in the OpenAPI schema.\n\n
Why This Fix Works in Production
- Trigger: FastAPI app with `separate_input_output_schemas` disabled excludes computed fields on Pydantic models from OpenAPI output
- Mechanism: Computed fields were omitted from the OpenAPI schema when `separate_input_output_schemas` was set to False
- Why the fix works: Fix OpenAPI schema support for computed fields when using `separate_input_output_schemas=False`. (first fixed release: 0.128.4).
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
- Computed fields were omitted from the OpenAPI schema when `separate_input_output_schemas` was set to False
- Production symptom (often without a traceback): FastAPI app with `separate_input_output_schemas` disabled excludes computed fields on Pydantic models from OpenAPI output
Proof / Evidence
- GitHub issue: #14431
- Fix PR: https://github.com/fastapi/fastapi/pull/13207
- First fixed release: 0.128.4
- Reproduced locally: No (not executed)
- Last verified: 2026-02-08
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.49
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Fixed by https://github.com/fastapi/fastapi/pull/13207 Available in FastAPI 0.123.4, released in the next few hours.”
Failure Signature (Search String)
- FastAPI app with `separate_input_output_schemas` disabled excludes computed fields on Pydantic models from OpenAPI output
- app = FastAPI(separate_input_output_schemas=False)
Copy-friendly signature
signature.txt
Failure Signature
-----------------
FastAPI app with `separate_input_output_schemas` disabled excludes computed fields on Pydantic models from OpenAPI output
app = FastAPI(separate_input_output_schemas=False)
Error Message
Signature-only (no traceback captured)
error.txt
Error Message
-------------
FastAPI app with `separate_input_output_schemas` disabled excludes computed fields on Pydantic models from OpenAPI output
app = FastAPI(separate_input_output_schemas=False)
Minimal Reproduction
repro.py
import pytest
from fastapi import FastAPI
from starlette.testclient import TestClient
@pytest.fixture(name="client")
def get_client():
app = FastAPI(separate_input_output_schemas=False)
from pydantic import BaseModel, computed_field
class Rectangle(BaseModel):
width: int
length: int
@computed_field
@property
def area(self) -> int:
return self.width * self.length
@app.get("/")
def read_root() -> Rectangle:
return Rectangle(width=3, length=4)
client = TestClient(app)
return client
def test_get(client: TestClient):
response = client.get("/openapi.json")
data = response.json()
rectangle_model = data["components"]["schemas"]["Rectangle"]["properties"]
assert "area" in rectangle_model.keys()
What Broke
OpenAPI output did not include computed fields, leading to incomplete API documentation.
Why It Broke
Computed fields were omitted from the OpenAPI schema when `separate_input_output_schemas` was set to False
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install fastapi==0.128.4
When NOT to use: This fix should not be used if computed fields are not intended to be included in the OpenAPI schema.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Fix reference: https://github.com/fastapi/fastapi/pull/13207
First fixed release: 0.128.4
Last verified: 2026-02-08. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if computed fields are not intended to be included in the OpenAPI schema.
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
- Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
- Pin production dependencies and upgrade only with a reproducible test that hits the failing path.
Version Compatibility Table
| Version | Status |
|---|---|
| 0.128.4 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.