output_config.format cannot take the pydantic model that output_format deprecates, and the dict workaround silently skips transform_schema

[percymcn]

Summary

On anthropic==0.121.0, passing output_format= to the beta messages resource emits

DeprecationWarning: The 'output_format' parameter is deprecated. Please use 'output_config.format' instead.
— resources/beta/messages/messages.py:4229

but output_config.format cannot accept a pydantic model, which is the input form
output_format= exists for. Following the warning literally raises TypeError, and the
natural workaround (hand it Model.model_json_schema() as a dict) silently skips
transform_schema — so migrating changes the schema that goes on the wire.

Repro

Executed as written against anthropic==0.121.0 / pydantic==2.13.4, with
httpx.Client.send patched to capture the request body and return a 401 (no API key,
nothing leaves the machine).

class Item(BaseModel):
    code: str = Field(pattern=r'^[A-Z]{3}$', min_length=3)

A — the deprecated form. Warns, and transform_schema runs:

{"type":"object","title":"Item",
 "properties":{"code":{"type":"string","title":"Code",
                       "description":"{minLength: 3, pattern: ^[A-Z]{3}$}"}},
 "additionalProperties":false,"required":["code"]}

B — the migration the warning names. output_config={"format": Item}:

TypeError: Object of type ModelMetaclass is not JSON serializable

C — the workaround, output_config={"format": {"type":"json_schema","schema": Item.model_json_schema()}}:

{"properties":{"code":{"minLength":3,"pattern":"^[A-Z]{3}$","title":"Code","type":"string"}},
 "required":["code"],"title":"Item","type":"object"}

C is not A. additionalProperties: false is gone, and minLength/pattern are no
longer folded into description. So a user who follows the deprecation notice stops
getting the normalization the SDK applies today, with no error and no warning.

Cause

transform_schema runs only on the elif branch of

if is_dict(output_format):                       # beta messages.py:1769, :3778
    transformed_output_format = cast(...)        # cast through, untransformed
elif is_given(output_format) and output_format is not None:
    schema = adapted_type.json_schema()
    transformed_output_format = JSONOutputFormatParam(schema=transform_schema(schema), ...)

output_config has no equivalent branch at all — its format is forwarded as given, so
every form of it is untransformed. Measured across create / parse / stream and both
the beta and non-beta resources: output_config.format never transforms.

Two smaller notes from the same measurement:

• The DeprecationWarning fires on the beta resource only. messages.parse,
  messages.stream and messages.count_tokens on the non-beta resource accept
  output_format= silently, so the two resources disagree about whether this parameter
  is deprecated.
• count_tokens(output_format=<dict>) is also untransformed while
  count_tokens(output_format=<model>) is transformed, so a token count taken via the
  dict form is counting a different document than the one parse() would send.

Suggested shape of a fix

Either route output_config.format through the same branch (accept a type there and
transform it), or state in the deprecation message that output_config.format takes a
pre-built json_schema dict and applies no transform — the current wording implies a
drop-in rename, and it is not one.

Test-design warning

A regression test that asserts only "the request was built" cannot see this — the
request builds fine in both A and C. It has to assert on the emitted schema. And a
fixture whose model carries no constrained fields is weaker than it looks: it will still
differ on additionalProperties, but nothing else, so the demotion half of the change
goes untested. Use a field with pattern=/min_length= as above.

Scope

No API key was used, so this is entirely about the request the SDK builds; I am not
claiming anything about how the service treats either payload.