fix: populate required fields in FunctionDeclaration json_schema fallback

[giulio-leone]

Summary

Fixes #4798 — required fields lost in FunctionDeclaration when the parameters_json_schema fallback path is used.

Root Cause

from_function_with_options() has two code paths for building function declarations:

1. Primary path (parameters_properties): calls _get_required_fields() ✅
2. Fallback path (parameters_json_schema): triggered when _parse_schema_from_parameter raises ValueError for complex union types (e.g. list[str] | None) — never called _get_required_fields() ❌

Additionally, the fallback path didn't propagate default values from inspect.Parameter to the generated Schema, so _get_required_fields() (which checks schema.default is None) would treat defaulted parameters as required.

Impact

The LLM sees all parameters as optional and may omit required ones. Observed in production: Gemini Flash omitted the mandatory query parameter when calling a tool, because the schema had no required field.

Fix

Three changes to the elif parameters_json_schema branch in from_function_with_options():

Change	Purpose
Added _get_required_fields() call	Populates required field (mirrors primary path)
Propagate non-None defaults to schema.default	Ensures _get_required_fields() correctly excludes defaulted params
Set schema.nullable=True for None-default params	Ensures _get_required_fields() correctly excludes nullable params

Test

Added regression test test_required_fields_set_in_json_schema_fallback that verifies:

• query (no default) → required ✅
• mode (default='default') → not required ✅
• tags: list[str] | None = None → not required ✅

Full test suite: 4726 passed, 0 failures

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[rohityan]

Hi @giulio-leone , Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process. Once the CLA is signed, we'll be able to proceed with the review of your PR. Thank you!

[giulio-leone]

@rohityan Thanks for the heads-up! I'll get the Google CLA signed. Will follow up once it's done.

[giulio-leone]

I have signed the Google CLA. Could you please re-run the CLA check? Thank you!

[giulio-leone]

Hi @rohityan — the CLA is now signed and passing ✅ (it was a Co-authored-by trailer issue that has been resolved). Ready for review!

[rohityan]

Hi @giulio-leone , our PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @Jacksunwei , can you please review this. LGTM

[giulio-leone]

Rebased this branch onto the latest main and force-pushed it.

Local validation I ran:

• actual repo test file under Python 3.13 using a lightweight bootstrap that bypasses the heavyweight package init but still executes the real source modules: tests/unittests/tools/test_from_function_with_options.py -k 'required_fields_set_in_json_schema_fallback or collections_type_parameter or no_params' -q -> 3 passed\n\nReal branch-vs-main proof for the exact regression path (query: str, mode: str = "default", tags: list[str] | None = None):\n- main imported adk-python-main/src/google/adk/tools/_automatic_function_calling_util.py\n - required=[]\n - mode_default=null\n - tags_nullable=null\n- this branch imported adk-python/src/google/adk/tools/_automatic_function_calling_util.py\n - required=["query"]\n - mode_default="default"\n - tags_nullable=true\n\nSo on main the json-schema fallback path still loses the required/default/nullable metadata that the LLM needs, while this rebased branch restores the correct contract for the same runtime scenario.

[giulio-leone]

Refreshed this branch onto current origin/main and re-ran the gate from rebased head 71720a02.

Validation (two consecutive clean passes, no code changes between passes):

• uv run --project /Users/giulioleone/Sviluppo/pr/work/adk-python-4812 --extra dev --extra test pyink --check src/google/adk/tools/_automatic_function_calling_util.py tests/unittests/tools/test_from_function_with_options.py
• uv run --project /Users/giulioleone/Sviluppo/pr/work/adk-python-4812 --extra dev --extra test isort --check-only src/google/adk/tools/_automatic_function_calling_util.py tests/unittests/tools/test_from_function_with_options.py
• uv run --project /Users/giulioleone/Sviluppo/pr/work/adk-python-4812 --extra dev --extra test pytest tests/unittests/tools/test_from_function_with_options.py -q
• result: 16 passed on both passes
• mypy diff check against current origin/main: branch errors 1342, main errors 1342, new errors introduced 0

Direct runtime proof for the actual fallback case from #4798 using:

complex_tool(query: str, mode: str = "default", tags: list[str] | None = None)

• rebased branch: required == ["query"], mode_default == "default", tags_nullable == true
• current origin/main: required == null, mode_default == null, tags_nullable == null

Worktree stayed clean after pass 2.

[giulio-leone]

✅ Autonomous Validation — Pass 2/2 Clean

Branch: fix/required-fields-json-schema-fallback
HEAD: 71720a02
Rebased onto: current origin/main (0 commits behind)

Test Results (double-pass, no code changes between passes)

Pass	Tests	Result
1	16/16	✅
2	16/16	✅

Runner: python -m pytest <test_file> -v (Python venv)

Validated autonomously — double-pass strict protocol