# Review by Principal Systems Engineer (Google/Stripe caliber) (Round 5)

## Review Complete

**VERDICT: approve**

**FINDINGS:**
- **[src/colonyos/config.py]**: Both `ParallelImplementConfig.enabled` and `DEFAULTS` correctly flipped to `False`; warning on explicit `True` is appropriately non-blocking
- **[src/colonyos/orchestrator.py:593-657]**: Single-task prompt builder with dual-constraint boundaries and context trimming is well-designed for agent isolation
- **[src/colonyos/orchestrator.py:712-960]**: Sequential runner correctly implements DAG-aware execution with per-task commits, failure tracking, transitive blocking, selective staging, and subprocess timeouts
- **[src/colonyos/orchestrator.py:3976-4060]**: Fallback structure is clean with proper early returns and diagnostic logging of which mode was attempted
- **[src/colonyos/orchestrator.py:790-795]**: Transitive blocking relies on topological order invariant — correct but non-obvious; consider adding a clarifying comment (INFO, non-blocking)
- **[tests/test_sequential_implement.py]**: 32 tests / 922 lines covering all critical paths including security (selective staging, sanitization, timeouts)
- **[tests/test_orchestrator.py]**: 1 pre-existing flake (`TestBaseBranchValidation::test_invalid_base_branch_raises`) — not introduced by this branch

**SYNTHESIS:**
This implementation is approved. The architecture makes the correct trade-off: sequential execution eliminates an entire class of nondeterministic failures (merge conflicts) at the cost of wall-clock time, which is the right default for an autonomous system where reliability matters more than speed. The code is clean, follows existing patterns, and handles failure modes defensively — failed git commands don't cascade, agent exceptions are caught, transitive dependents are blocked without orphaning independent tasks. The test coverage is thorough (922 lines for ~250 lines of implementation) and includes security-focused assertions that verify subprocess arguments, not just return values. All 10 PRD requirements are satisfied, all prior review findings are addressed, and zero regressions are introduced. Ship it.
