fix(runners): honor before_run_callback early-exit for Workflow runs

[garyzava]

Summary

In the Workflow run path (Runner._run_node_async), the before_run plugin
callback was awaited but its return value was discarded. Per the plugin contract,
returning a Content from before_run_callback should halt the run and end it
with that content — but for Workflow root agents the value was ignored and
execution continued into the workflow.

This captures the callback result and, when it is a Content, emits it as the
early-exit event and stops — matching the existing behavior of the non-workflow
run path (run_async).

Fixes #6013

What changed

• src/google/adk/runners.py — in _run_node_async, capture
  run_before_run_callback(...)'s result; if it's a types.Content, build the
  early-exit Event, append it (subject to _should_append_event), yield it, and
  return — instead of unconditionally proceeding. Mirrors the non-workflow path.
• tests/unittests/workflow/test_workflow_failures.py — regression test: a plugin
  whose before_run_callback returns Content halts the workflow and the node
  never executes.

Why this approach

The non-workflow path already implements the documented early-exit contract
(capture the result, emit an early-exit event, skip execution). This change makes
the Workflow path consistent with it rather than introducing new behavior, so the
fix is minimal and the intended semantics are unambiguous.

How I tested

$ pytest tests/unittests/workflow/test_workflow_failures.py -q
23 passed

The new test (test_workflow_halts_when_before_run_callback_returns_content)
fails before the change (the node executes; no early-exit event) and passes
after. Existing workflow-failure tests and the non-workflow plugin tests
(tests/unittests/test_runners.py -k plugin) still pass. pyink and isort
report no changes.

Backward compatibility

No behavior change when before_run_callback returns None (the common case) —
the workflow runs exactly as before. Only the documented halt-on-Content case is
affected, and it now matches the non-workflow path.

[adk-bot]

🔍 ADK Pull Request Analysis: PR #6032

Title: fix(runners): honor before_run_callback early-exit for Workflow runs
Author: @garyzava
Status: open
Impact: 72 additions, 2 deletions across 2 files

Executive Summary

1. Core Objective: Swallowing of before_run_callback returning values was occurring during root workflow executions (_run_node_async). This PR fixes the bug by capturing the callback's return value and gracefully halting/early-exiting with the returned Content, aligning the workflow run path with the non-workflow sequential execution path.
2. Justification & Value: [Justified Fix] - Restores a core plugin capability of halting execution to workflow agents, preventing unwanted LLM requests, charge consumption, and downstream orchestration side effects.
3. Alignment with Principles: [Pass with Nits] - Exceptional structural alignment with zero breaking changes, with very minor type-hinting/styling nits on the newly introduced unit-test callback.
4. Recommendation: [Approve with Nits]

Detailed Findings & Analysis

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

• Context & Background: Issue #6013 reported that ADK 2.0 ignores before_run_callback returning a value for workflows. Because a Workflow is a BaseNode rather than a standard BaseAgent, root agent execution is routed through _run_node_async where the callback return value was awaited but discarded.
• Implementation Mechanism: In runners.py:L531-L532, changed await ic.plugin_manager.run_before_run_callback(invocation_context=ic) to capture the return value as early_exit_result. If isinstance(early_exit_result, types.Content), it halts, constructs an early-exit Event (setting author='model'), applies custom run-config metadata, appends it to the sessions (if _should_append_event allows), yields the event, and returns. This accurately mirrors the standard sequential path implementation in runners.py:L1373-L1393.
• Affected Surface: Zero breaking changes to public APIs. Changes are fully internal to the orchestration runner path.

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

• Workspace Verification:
  • Investigated baseline workspace file runners.py.
  • Confirmed that the baseline codebase strictly ignores the result of the before_run invocation:

    # Run before_run callbacks
    await ic.plugin_manager.run_before_run_callback(invocation_context=ic)

• Value Assessment: Extremely high value addition. Bypassing execution is highly critical to preventing unnecessary downstream LLM network calls, helping developers minimize execution costs and side-effects.
• Alternative Approaches: No cleaner alternative exists. Replicating the working sequential branch's early-exit semantics in the active node runtime branch creates unified execution parity.
• Scope & Depth: [Systematic Fix] & [Root Cause] - This is a robust framework-level fix addressing the underlying architectural gap across execution path divisions, rather than adding a superficial or localized band-aid.

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

• Public API & Visibility Boundaries:
  • Status: [Pass]
  • Analysis: No signature alterations or breaking changes to existing components.
• Code Quality, Typing & Conventions:
  • Status: [Nits]
  • Analysis:
    • In test_workflow_failures.py, the mock callback before_run_callback lacks typing annotations for arguments and return types. For full styling compliance, it should be annotated:

      async def before_run_callback(
          self, *, invocation_context: InvocationContext
      ) -> types.Content:

    • While runners.py includes from __future__ import annotations, the test file test_workflow_failures.py does not contain it. Since it is an existing file and was only incrementally modified, this is not a violation (but worth clean-up if/where convenient).
• Robustness & Edge Cases:
  • Status: [Pass]
  • Analysis: Uses isinstance(early_exit_result, types.Content) for clean type validation, preventing arbitrary issues or object mismatch bugs.
• Test Integrity & Quality:
  • Status: [Pass]
  • Analysis: Adds a dedicated regression test test_workflow_halts_when_before_run_callback_returns_content. It verifies behavior from public interfaces, avoids mocking core engine structures, declares test helper instances (_RecordingNode, _HaltPlugin) inline/local to the test case, and cleanly follows the Arrange-Act-Assert pattern.

---

Summary of Triage & Findings

• CLA Status: Verified and signed successfully (cla/google status check is SUCCESS).
• PR Structure: Well-defined, localized, and cleanly designed.
• Testing Quality: Excellent behavior-focused test.
• Final Action Verdict: Approve with Nits. Request the contributor to add type hints for the newly introduced before_run_callback argument and return type in the test mock class.

[rohityan]

Hi @garyzava , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. Can you meanwhile address the adk bot nits suggestions.

[rohityan]

Hi @GWeale , can you please review this.