fix: register A2A routes from agent.json

[he-yufeng]

Summary

Fixes #5994.

get_fast_api_app() already imports json at module scope, but a later optional Gemini Enterprise block imported json again inside the function. That makes Python treat json as a local name for the whole function, so the earlier A2A agent.json loader can hit UnboundLocalError before A2A routes are registered.

This removes the inner import and adds a focused regression test that verifies an A2A agent.json is loaded and its routes are registered.

Testing

python -m pytest tests\unittests\cli\test_fast_api.py -q -k "a2a_agent_json_registers_routes"
python -m py_compile src\google\adk\cli\fast_api.py tests\unittests\cli\test_fast_api.py
python -m pyink --check src\google\adk\cli\fast_api.py tests\unittests\cli\test_fast_api.py
git diff --check

I also tried a broader local Windows A2A selection with a2a_agent_discovery and a2a_request_handler_uses_push_config_store; the assertions passed, but teardown hit an existing Windows TemporaryDirectory cleanup issue because those tests leave cwd inside the temp agent directory.

[adk-bot]

🔍 ADK Pull Request Analysis: PR #6000

Title: fix: register A2A routes from agent.json
Author: @he-yufeng
Status: OPEN
Impact: 79 additions, 2 deletions across 2 files

Executive Summary

1. Core Objective: Removes a redundant inner import json statement inside get_fast_api_app which was causing an UnboundLocalError when setting up A2A routes under CLI, and adds a regression unit test.
2. Justification & Value: [Justified Fix] - Resolves a critical regression in google-adk==2.2.0 that completely blocks A2A agent routing and discovery during CLI startup.
3. Alignment with Principles: [Pass] - Follows architectural best practices, does not introduce breaking API changes, and includes high-quality behavior-focused tests with minimal mocking.
4. Recommendation: Approve

Detailed Findings & Analysis

1. Objectives & Impact ("What does it do?")

• Context & Background: In google-adk==2.2.0, starting the API server with A2A enabled throws a fatal UnboundLocalError: cannot access local variable 'json' where it is not associated with a value. This is caused by Python's static scope-lookup rules: a function-local import json later in the function declares json as a local for the entire function body, shadowing the global import and causing a crash when json is used earlier in the function during A2A setup. Reference linked Issue #5994.
• Implementation Mechanism:
  • Removes the redundant local import json on line 748 of fast_api.py, allowing Python to resolve json to the module-level import at line 19 of fast_api.py.
  • Appends a new unit test test_a2a_agent_json_registers_routes to test_fast_api.py validating that the A2A app starts, loads agent.json, and registers the correct endpoint routes.
• Affected Surface: No public interfaces or CLI options are mutated. It alters only internal, private execution paths inside get_fast_api_app.

2. Justification & Value ("Is it a valid and useful change?")

• Workspace Verification:
  • Investigated fast_api.py:L719-721 where json.load(f) is invoked to parse agent settings.
  • Concurred that a secondary optional conditional block at fast_api.py:L748 has a local import json, which triggers the reported runtime crash when a2a=True.
• Value Assessment: Highly essential regression fix. It eliminates a critical blocker preventing standard developer discovery and messaging flow via A2A.
• Alternative Approaches: The PR's solution is the absolute cleanest one. Eliminating the duplicate local import and reusing the top-level import is elegant and standard.
• Scope & Depth: [Systematic Fix] & [Root Cause] (The code directly targets the underlying name-shading bug rather than just introducing an ad-hoc point catch).

3. Principle & Style Alignment Checklist ("Does it follow rules?")

• Public API & Visibility Boundaries:
  • Status: Pass
  • Analysis: No public classes, methods, or parameters are altered or exposed.
• Code Quality, Typing & Conventions:
  • Status: Pass
  • Analysis: Source file retains from __future__ import annotations at fast_api.py:L15. Strong typing is enforced, and no redundant Any is introduced.
• Robustness & Edge Cases:
  • Status: Pass
  • Analysis: Because json is imported globally at fast_api.py:L19, removing the function local import preserves safety and robustness under all conditions.
• Test Integrity & Quality:
  • Status: Pass
  • Analysis: The regression test follows the Arrange-Act-Assert pattern. It mocks out only the external A2A library boundaries/Starlette applications and database service configurations, maintaining clear logical separation blocks.