FastAPI Discriminated Unions Break When Wrapped in

The Fix

pip install fastapi==0.128.4

Based on closed fastapi/fastapi issue #14508 · 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

@@ -18,7 +18,7 @@
 from fastapi.openapi.constants import REF_TEMPLATE
 from fastapi.types import IncEx, ModelNameMap, UnionType
-from pydantic import BaseModel, ConfigDict, TypeAdapter, create_model
+from pydantic import BaseModel, ConfigDict, Field, TypeAdapter, create_model
 from pydantic import PydanticSchemaGenerationError as PydanticSchemaGenerationError

repro.py

from typing import Annotated, Union
from pydantic import BaseModel, Tag, Discriminator
from fastapi import FastAPI, Body

class Cat(BaseModel):
    pet_type: str = "cat"
    meows: int

class Dog(BaseModel):
    pet_type: str = "dog"
    barks: float

def get_pet_type(v):
    if isinstance(v, dict):
        return v.get("pet_type", "")
    return getattr(v, "pet_type", "")

# Define discriminated union
Pet = Annotated[
    Union[
        Annotated[Cat, Tag("cat")],
        Annotated[Dog, Tag("dog")]
    ],
    Discriminator(get_pet_type)
]

app = FastAPI()

# ✅ THIS WORKS in 0.124.1+
@app.post("/pet/works")
async def create_pet_works(pet: Pet = Body(...)):
    return pet

# ❌ THIS BREAKS in 0.124.1+ (worked in 0.124.0)
@app.post("/pet/broken")
async def create_pet_broken(pet: Annotated[Pet, Body(...)]):
    return pet

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: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\nOption B — Safe version pin\npip install fastapi==0.124.0\nWhen NOT to use: Do not use if you need features or security fixes in newer releases.\n\nOption C — Workaround\nUse pet: Pet = Body(...) instead of pet: Annotated[Pet,\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: pydantic.errors.PydanticUserError: `Tag` not provided for choice { 'type': 'tagged-union', 'choices': { 'cat': {'type':…
Mechanism: Fix support for tagged union with discriminator inside of `Annotated` with `Body()` in FastAPI.
Why the fix works: Fix support for tagged union with discriminator inside of `Annotated` with `Body()` in FastAPI. (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

Triggered by an upgrade/regression window: 0.124.1 breaks; 0.128.4 is the first fixed release.
Surfaces as: pydantic.errors.PydanticUserError: `Tag` not provided for choice {\n 'type': 'tagged-union',\n 'choices': {\n 'cat': {'type': 'definition-ref', 'metadata':\n…

Proof / Evidence

GitHub issue: #14508
Fix PR: https://github.com/fastapi/fastapi/pull/14512
First fixed release: 0.128.4
Affected versions: 0.124.1
Reproduced locally: No (not executed)
Last verified: 2026-02-08
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.40

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: fastapi
Fixed: 0.128.4
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

“This is fixed in https://github.com/fastapi/fastapi/pull/14512, it is now available in FastAPI 0.124.3 :tada:”

@tiangolo · 2025-12-12 · confirmation · source

Failure Signature (Search String)

pydantic.errors.PydanticUserError: `Tag` not provided for choice {\n 'type': 'tagged-union',\n 'choices': {\n 'cat': {'type': 'definition-ref', 'metadata':\n

Error Message

Stack trace

error.txt

Error Message
-------------
pydantic.errors.PydanticUserError: `Tag` not provided for choice {\n       'type': 'tagged-union',\n       'choices': {\n         'cat': {'type': 'definition-ref', 'metadata':\n   {'pydantic_internal_union_tag_key': 'cat'}},\n         'dog': {'type': 'definition-ref', 'metadata':\n   {'pydantic_internal_union_tag_key': 'dog'}}\n       },\n       'discriminator': <function get_pet_type at 0x...>\n     } used with `Discriminator`

Minimal Reproduction

repro.py

from typing import Annotated, Union
from pydantic import BaseModel, Tag, Discriminator
from fastapi import FastAPI, Body

class Cat(BaseModel):
    pet_type: str = "cat"
    meows: int

class Dog(BaseModel):
    pet_type: str = "dog"
    barks: float

def get_pet_type(v):
    if isinstance(v, dict):
        return v.get("pet_type", "")
    return getattr(v, "pet_type", "")

# Define discriminated union
Pet = Annotated[
    Union[
        Annotated[Cat, Tag("cat")],
        Annotated[Dog, Tag("dog")]
    ],
    Discriminator(get_pet_type)
]

app = FastAPI()

# ✅ THIS WORKS in 0.124.1+
@app.post("/pet/works")
async def create_pet_works(pet: Pet = Body(...)):
    return pet

# ❌ THIS BREAKS in 0.124.1+ (worked in 0.124.0)
@app.post("/pet/broken")
async def create_pet_broken(pet: Annotated[Pet, Body(...)]):
    return pet

Fix Options (Details)

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

pip install fastapi==0.128.4

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option B — Safe version pin Backward-compatible pin

pip install fastapi==0.124.0

When NOT to use: Do not use if you need features or security fixes in newer releases.

Use when you can’t upgrade immediately. Plan a follow-up to upgrade (pins can accumulate security/compat debt).

Option C — Workaround Temporary workaround

Use pet: Pet = Body(...) instead of pet: Annotated[Pet,

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.

Fix reference: https://github.com/fastapi/fastapi/pull/14512

First fixed release: 0.128.4

Last verified: 2026-02-08. Validate in your environment.

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.
Do not use if you need features or security fixes in newer releases.

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
0.124.0	Working
0.124.1	Working
0.124.1	Broken
0.128.4	Fixed

Related Issues

No related fixes found.

Cluster: fastapi:configuration-error FastAPI hub FastAPI best practices All hubs All clusters

Related clusters: Data consistency Rate limit Race condition

Sources

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

FastAPI Discriminated Unions Break When Wrapped in Annotated[Union, Body(...)] in FastAPI 0.124.1+ (Fix)