FunctionDeclaration.from_callable drops enum and properties from Optional[...] parameters

@Venkaiahbabuneelam

FunctionDeclaration.from_callable keeps only the type of the inner schema when a parameter is Optional[X]. An Optional[Literal["c", "f"]] parameter becomes a nullable string with no enum. An Optional[Address] parameter becomes a bare nullable object with no properties or required. Address | None keeps them, but Literal["c", "f"] | None loses its enum too.

import json
from typing import Literal, Optional
import pydantic
from google import genai
from google.genai import types

client = genai.Client(api_key="not-a-real-key")  # no request is sent

class Address(pydantic.BaseModel):
  city: str

def unit_plain(unit: Literal["c", "f"]) -> str: ...
def unit_optional(unit: Optional[Literal["c", "f"]] = None) -> str: ...
def addr_optional(addr: Optional[Address] = None) -> str: ...
def addrs_optional(addrs: Optional[list[Address]] = None) -> str: ...
def unit_pipe(unit: Literal["c", "f"] | None = None) -> str: ...
def addr_pipe(addr: Address | None = None) -> str: ...

for f in [unit_plain, unit_optional, addr_optional, addrs_optional, unit_pipe, addr_pipe]:
  fd = types.FunctionDeclaration.from_callable(client=client, callable=f)
  (name, s), = fd.parameters.properties.items()
  print(f"{f.__name__:15}", json.dumps(s.model_dump(exclude_none=True)))

$ python -I genai_fd3.py
unit_plain      {"enum": ["c", "f"], "type": "STRING"}
unit_optional   {"nullable": true, "type": "STRING"}
addr_optional   {"nullable": true, "type": "OBJECT"}
addrs_optional  {"items": {"properties": {"city": {"type": "STRING"}}, "required": ["city"], "type": "OBJECT"}, "nullable": true, "type": "ARRAY"}
unit_pipe       {"nullable": true, "type": "STRING"}
addr_pipe       {"nullable": true, "properties": {"city": {"type": "STRING"}}, "required": ["city"], "type": "OBJECT"}
# google-genai 2.29.0 at 6a6501c (main, 2026-10-11), CPython 3.12.3, Linux x86_64

The single-member collapse in the Union branch of _parse_schema_from_parameter (_automatic_function_calling_util.py:282) copies schema.any_of[0].type and nothing else. Optional[list[...]] survives only because the lines above it copy items by hand. Copying the whole member fixes both:

       if len(schema.any_of) == 1:  # param: Union[List, None] -> Array
-        schema.type = schema.any_of[0].type
-        schema.any_of = None
+        schema = schema.any_of[0].model_copy(
+            update={'nullable': schema.nullable}
+        )

$ python -I genai_fd3.py   # with the patch
unit_plain      {"enum": ["c", "f"], "type": "STRING"}
unit_optional   {"enum": ["c", "f"], "nullable": true, "type": "STRING"}
addr_optional   {"nullable": true, "properties": {"city": {"type": "STRING"}}, "required": ["city"], "type": "OBJECT"}
addrs_optional  {"items": {"properties": {"city": {"type": "STRING"}}, "required": ["city"], "type": "OBJECT"}, "nullable": true, "type": "ARRAY"}
unit_pipe       {"enum": ["c", "f"], "nullable": true, "type": "STRING"}
addr_pipe       {"nullable": true, "properties": {"city": {"type": "STRING"}}, "required": ["city"], "type": "OBJECT"}

$ python -m pytest -q google/genai/tests/types/test_types.py google/genai/tests/types/test_future.py
95 passed, 6 skipped, 2 warnings     # same count before and after the patch
# not run: any request to the live API

This is the default use_json_schema=False path, so it hits direct from_callable callers. A function passed straight into tools=[...] takes use_json_schema=True (_transformers.py:961), where open #3074 and #3093 work. @Venkaiahbabuneelam, CONTRIBUTING.md says outside pull requests are not taken yet, so the diff above is the whole proposal. Would a test for these signatures in tests/types/test_types.py help it land?

@chrikrah

Hi @chrikrah,

Thank you for bringing this to our attention, We have confirmed that types.FunctionDeclaration.from_callable drops enum, properties, and required fields on Optional[...] and Literal[...] | None parameters, We truly appreciate you flagging this issue, we will file a bug internally.
Thanks

could you set use_json_schema = True? parsing a callable into json schema is the recommended way for the latest API

chore: log warning if users don't use json schema for FunctionDeclara…

@yyyu-google

@yyyu-google yes, use_json_schema=True works for us and we will switch to it. The three Literal signatures keep their enum, and Address arrives as a $ref with properties and required under $defs.

$ python genai_fd3.py 1   # report's script, from_callable(..., use_json_schema=True), parameters_json_schema["properties"]
unit_plain      {"unit": {"enum": ["c", "f"], "type": "string"}}
unit_optional   {"unit": {"anyOf": [{"enum": ["c", "f"], "type": "string"}, {"type": "null"}], "default": null}}
addr_optional   {"addr": {"anyOf": [{"$ref": "#/$defs/Address"}, {"type": "null"}], "default": null}}
addrs_optional  {"addrs": {"anyOf": [{"items": {"$ref": "#/$defs/Address"}, "type": "array"}, {"type": "null"}], "default": null}}
unit_pipe       {"unit": {"anyOf": [{"enum": ["c", "f"], "type": "string"}, {"type": "null"}], "default": null}}
addr_pipe       {"addr": {"anyOf": [{"$ref": "#/$defs/Address"}, {"type": "null"}], "default": null}}

$ python genai_fd3.py 0   # same script, default path, parameters.properties
use_json_schema is not True, which is not recommended. Parsing a callable into JSON schema is preferred.
unit_plain      {"enum": ["c", "f"], "type": "STRING"}
unit_optional   {"nullable": true, "type": "STRING"}
addr_optional   {"nullable": true, "type": "OBJECT"}
addrs_optional  {"items": {"properties": {"city": {"type": "STRING"}}, "required": ["city"], "type": "OBJECT"}, "nullable": true, "type": "ARRAY"}
unit_pipe       {"nullable": true, "type": "STRING"}
addr_pipe       {"nullable": true, "properties": {"city": {"type": "STRING"}}, "required": ["city"], "type": "OBJECT"}
# google-genai at 9727c40 (head of #3175), CPython 3.12.3, no request sent

At the same head the default path disagrees with itself: a bare Literal keeps its enum and Address | None keeps properties, while Optional[Address] and both nullable Literal spellings lose them. That is the bug confirmed in triage above, and #3175 adds the warning without touching it.

Would you take the three-line collapse fix from the report alongside #3175, or should #3175 say Refs #3105 so this stays open for it?

chore: log warning if users don't use json schema for FunctionDeclara…

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FunctionDeclaration.from_callable drops enum and properties from Optional[...] parameters #3105

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FunctionDeclaration.from_callable drops enum and properties from Optional[...] parameters #3105

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Venkaiahbabuneelam commented on Oct 13, 2026

yyyu-google commented on Oct 20, 2026

chrikrah commented on Oct 22, 2026

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