feat(#651): auto-recover BLOCKED-by-merge-conflict tasks via worktree rebase (Option A)

[lwgray]

Why

PR #653 marked tasks BLOCKED on merge failure but left them unrecoverable. verify-snake-8 (test71, 2026-05-25) hit one such conflict and the runner spawned 49 ephemeral agents before the 20-min stall watchdog gave up — roughly $25-50 of wasted spawn cost from one recoverable conflict. On a commercial customer's API key the wasted spend could be $200-500 per stall event.

This PR implements Option A from Larry's 2026-05-25 decision: auto-recovery via worktree rebase. The proper architectural prevention (#206 baton arbitration) is the next sprint of work; this is the safety net that ships fast.

Most conflicts in DAG-based parallel work are stale-base false conflicts. Rebase resolves these mechanically. Real content conflicts abort the rebase and leave the task BLOCKED.

What changed

_attempt_merge_recovery(task, state) in src/marcus_mcp/tools/task.py:

1. Reads source_context.merge_conflict (set by PR fix(#651): mark task BLOCKED when worktree merge to main fails #653's helper)
2. Resolves worktree path: <project_root>/../worktrees/<agent_id>
3. git rebase main in the worktree
4. On clean rebase: checkout main + defensive reset + git merge --ff-only <branch> + kanban → DONE + clear merge_conflict from source_context
5. On rebase conflict: git rebase --abort to clean state; leave BLOCKED; return None

_sweep_blocked_merge_conflicts(state) iterates state.project_tasks and runs recovery on each BLOCKED+merge_conflict task.

Wired into request_next_task: when no optimal_task found, sweep fires before returning no_task_ready. If recovery unblocks downstream tasks, refresh state + re-select for the current agent. Eliminates the spawn-thrash cycle.

Bright-line check

Marcus rebasing the agent's branch onto main is environment shaping, not HOW dictation. Same primitive as git pull --rebase. Agent's code unchanged; Marcus just integrates cleanly against advanced main. Passes Invariant #2.

Test plan

Automated (green)

• 5 new tests in tests/unit/marcus_mcp/test_merge_conflict_recovery.py using real tmp_path git repos
• 408 unit tests pass across tests/unit/marcus_mcp + tests/unit/mcp
• mypy src/marcus_mcp/tools/task.py — clean

End-to-end (reviewer to run)

• Re-run a spec that triggers a conflict (verify-snake-8 setup). Expected: conflict happens, task briefly BLOCKED, next poll's sweep recovers via rebase, task → DONE, downstream unblocks, run completes. Spawn count <10, not 49.

Empirical motivation

From verify-snake-8 (test71) runner log:

[02:19:16] start
[03:14:52]   spawned agent_unicorn_1_41    ← spawn #41 chasing the blocked task
[03:15:27]   spawned agent_unicorn_2_42
... 8 more spawns ...
[03:19:15] STALL: no task-count change for 20 min — tearing down
[03:20:15] run finished — spawned 49 ephemeral agents

This PR eliminates that failure mode.

What this PR does NOT do

• Prevent conflicts (that's Implement Baton Arbitration for resource-level write locking #206 baton arbitration, next sprint)
• Handle real content conflicts automatically (leaves task BLOCKED for human)
• Address the L6 cliff "games don't work" problem (PR feat(verification): Phase A + Phase B soft rollout — Marcus authors verifications, gate prefers them, falls back to agent (#636) #642 / behavioral verification)

Related

• PR fix(#651): mark task BLOCKED when worktree merge to main fails #653 — safety net this builds on (BLOCKED on merge fail, defensive reset, memory honesty)
• PR perf(cache): bump foundation contract cache TTL from 60s to 600s #655 — cache TTL bump
• Implement Baton Arbitration for resource-level write locking #206 — Baton Arbitration (prevention, next sprint)
• bug: verify-snake-5 ships with stale-state bug in setNextDirection — LEFT arrow permanently blocked (build verification doesn't catch behavioral correctness) #654 — behavioral verification gap (separate L6 cliff concern)

🤖 Generated with Claude Code

[claude]

Claude finished @lwgray's task —— View job

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Code Review Complete

I've thoroughly reviewed this PR implementing auto-recovery for merge-conflict blocked tasks. Here's my comprehensive feedback:

Review Checklist:

• Read and analyze the main implementation in task.py
• Review the test coverage and quality
• Check code quality and best practices
• Identify potential bugs or issues
• Evaluate performance considerations
• Assess security concerns
• Provide final recommendations

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✅ Strengths

1. Excellent Problem Definition & Solution

• Clear motivation: ~$25-50 wasted spawn costs from 49 ephemeral agents chasing unrecoverable conflicts
• Smart architectural choice: auto-recovery via rebase handles the 90% case (stale-base conflicts) while leaving real content conflicts for human intervention
• Well-integrated into existing request_next_task flow

2. Robust Implementation

• Defensive error handling: Every subprocess call is wrapped with proper timeout and exception handling
• Clean state guarantee: git rebase --abort on conflict ensures worktree isn't left in broken state
• Proper Git operations: Defensive git reset --hard HEAD before merge, fast-forward-only merge strategy

3. Comprehensive Test Coverage

• 5 well-structured test classes covering happy path, failure cases, and edge cases
• Real Git repositories in tests (not mocked) - excellent for integration confidence
• Specific assertions on git state, kanban updates, and return values

⚠️ Issues to Address

1. Missing Error Framework Integration (Minor)

# Current code at task.py:2988-2990
except (_sp.SubprocessError, OSError) as exc:
    logger.warning("[recovery] %s: rebase subprocess error: %s", task.id, exc)
    return None

Recommendation: Use Marcus error framework for consistency:

from src.core.error_framework import KanbanIntegrationError, ErrorContext

try:
    rebase = _sp.run([...])
except (_sp.SubprocessError, OSError) as exc:
    raise KanbanIntegrationError(
        operation="merge_conflict_recovery",
        context=ErrorContext(
            operation="git_rebase",
            task_id=task.id,
            agent_id=agent_id
        )
    ) from exc

2. Subprocess Security (Minor)
The subprocess calls at lines 2981-2987 are secure (no shell=True, fixed command args), but consider adding explicit security documentation.

3. Performance Considerations (Minor)

• Git operations have 30-60s timeouts - appropriate for the use case
• Sequential processing in _sweep_blocked_merge_conflicts is fine for current scale
• Consider batching if recovery count grows significantly

🔍 Code Quality Assessment

Excellent:

• Clear function signatures with proper type hints
• Comprehensive docstrings following numpy style
• Logical error handling flow
• Good separation of concerns

Good naming and structure:

• _attempt_merge_recovery vs _sweep_blocked_merge_conflicts - clear distinction
• Helper functions are well-factored and testable

🧪 Test Quality Review

Excellent test design:

def _setup_recoverable_repo(tmp_path: Path) -> Path:
    """Build a tmp git project where rebase resolves cleanly."""

• Real Git operations instead of mocking - builds confidence
• Clear test scenarios: recoverable vs unrecoverable conflicts
• Proper cleanup verification: checks that git rebase --abort worked
• Edge case coverage: missing worktree, malformed source_context

🛡️ Security Assessment

✅ Secure:

• No shell injection risks (subprocess args are lists, not strings)
• Path traversal protected by working within known worktree structure
• No credential exposure in logs or error messages

📊 Architectural Alignment

✅ Passes Multi-Agency Invariants:

• Marcus is shaping the environment (rebasing branches), not dictating implementation HOW
• This is equivalent to git pull --rebase that agents could run themselves
• Agent code remains unchanged; only integration mechanics are automated

🎯 Final Recommendations

1. Ship as-is - this is production-ready code that solves a real cost problem
2. Consider follow-up: Integrate Marcus error framework for consistency with codebase patterns (see CLAUDE.md ERROR_HANDLING_FRAMEWORK section)
3. Monitor in production: The logging at lines 2974-2979 and 3058-3062 provides good observability

🏆 Summary

This is high-quality defensive code that solves a concrete business problem ($25-50 wasted spawn costs). The implementation is robust, well-tested, and properly integrated. The bright-line architectural check passes - this is environment coordination, not agent control.

Recommendation: APPROVE ✅

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