fix(plugins): return None, not the input, when a built-in plugin leaves a value unchanged

[AtulJoshi1206]

Update (Oct 17): rebased onto current main, plus one more commit that fixes the same pattern in the other two built-in plugins that had it.

• ReflectAndRetryModelPlugin: once the retry limit is exceeded with throw_exception_if_retry_exceeded=False, after_model_callback returned the error response itself. core/_finalizer.py treats any plugin result as an override, so the remaining plugins and the agent's canonical_after_model_callbacks were skipped for that response. It now returns None.
• SaveFilesAsArtifactsPlugin: with no artifact service configured, it logged that it was disabled and then returned the user message itself, which stopped every later plugin's on_user_message_callback. It now returns None. That matches its other no-op paths (no parts, no inline data), which already return None.
• Scope check: I scanned every class in src/google/adk/plugins for a method that returns one of its own parameters (result, llm_response, user_message, ...). After this change there are none left.
• Tests:
  • Updated the two tests that asserted the input came back, and renamed one to match.
  • Added test_agent_after_model_callback_runs_when_retry_limit_is_exceeded, which runs through InMemoryRunner.
  • Added test_no_artifact_service_does_not_stop_later_plugins, at the PluginManager level.
  • Both fail on main.
  • tests/unittests/plugins, flows/llm_flows, agents and runners: 3375 passed.

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Link to Issue or Description of Change

No existing issue; described below following the bug-report structure.

Describe the Bug:

MultimodalToolResultsPlugin.after_tool_callback returns the tool result itself when the result is not a Part or a list of Parts:

if not (isinstance(result, types.Part) or ...):
  return result

PluginManager stops at the first non-None plugin result. In flows/llm_flows/tools/_caller.py, the agent's canonical_after_tool_callbacks only run when the plugin result is None. Under the BasePlugin contract, None means "leave the result unchanged".

Impact:

Registering this built-in plugin silently turns off every agent-level after_tool_callback (redaction, rewriting, state updates) for every tool that returns an ordinary dict. It also turns off any plugin registered after it.

Steps to Reproduce:

An LlmAgent with a dict-returning tool, an after_tool_callback that rewrites the response, and plugins=[MultimodalToolResultsPlugin()].

Observed Behavior: the callback is never called, and the original dict is sent to the model.

Expected Behavior: the callback runs and its return value replaces the response, the same as without the plugin.

Solution:

Return None for non-part results. The other return paths of this callback already return None.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.

• All unit tests pass locally.

• Added test_agent_after_tool_callback_runs_for_non_part_results, which runs an agent end to end through InMemoryRunner with the plugin.

• Updated test_tool_returning_non_list_of_parts_is_unchanged to assert result is None. That matches its stated intent ("has this result unchanged") under the plugin contract.

• Both fail on main.

pytest tests/unittests/plugins                                  # 1134 passed
pytest tests/unittests/flows/llm_flows tests/unittests/agents   # 1841 passed, 2 xfailed

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules. (none)

🤖 Generated with Claude Code

[AtulJoshi1206]

Rebased on latest main. While re-checking this I found the same "return the input to mean unchanged" pattern in two more built-in plugins: ReflectAndRetryModelPlugin (once retries are exhausted without throwing, it skipped the agent's after_model_callback) and SaveFilesAsArtifactsPlugin (with no artifact service, it stopped later plugins). Fixed both in a second commit with tests, and updated the title and description. A scan of the plugins package shows nothing else left with this pattern.