Execution Skill

Activate this skill during Phase 3 (Execution) of Maestro orchestration. This skill defines how Maestro executes implementation phases through native subagent delegation.

Execution Mode Gate

Step 0 — Express bypass (early return)

If workflow_mode is express in the current session, STOP HERE. Do not proceed to the execution mode gate. Do not prompt the user. Do not resolve execution mode. Express always dispatches sequentially. Return to the Express Workflow and continue from the delegation step.

Step 1 — Read the configured mode

Read MAESTRO_EXECUTION_MODE (default: ask).

• If parallel: call update_session with { execution_mode: 'parallel', execution_backend: 'native' } to record in session state. Skip to delegation.
• If sequential: call update_session with { execution_mode: 'sequential', execution_backend: 'native' } to record in session state. Skip to delegation.
• If ask: proceed to Step 2.

Step 2 — Analyze the implementation plan

Before prompting the user, analyze the approved plan to generate a recommendation:

1. Count total phases in the plan

2. Count phases marked parallel: true (parallelizable phases)

3. Count distinct parallel batches (groups of parallelizable phases at the same dependency depth)

4. Count sequential-only phases (phases with blocked_by dependencies that prevent parallelization)

5. Cross-check file ownership across all phases. If any two phases share a file in their files arrays, those phases CANNOT be parallel-eligible — subtract them from the parallelizable count. Report each overlap as an Overlapping-file Warning in the prompt.

6. If validate_plan was called during planning and returned a parallelization_profile, use its parallel_eligible and effective_batches counts as the authoritative source for items 1-5 above. These are computed from actual dependency depths and override any manual flag-based counts. If parallelization_profile is not available, use the counts from items 1-5 as-is.

Record these counts — they feed into the prompt.

Step 3 — Determine the recommendation

• If parallelizable phases ≤ 1 → auto-select sequential. Call update_session with { execution_mode: 'sequential', execution_backend: 'native' }. Inform the user: "All phases are sequential — no parallel batches available." Skip to delegation. Do NOT prompt with a choice. Do NOT call ask_user. Do NOT present options. (Parallelism requires at least 2 phases at the same dependency depth; a single parallel-eligible phase has nothing to batch with.)

When parallelizable phases ≤ 1, there is NO choice to make. Auto-select sequential and skip directly to delegation. Do not show a picker. </ANTI-PATTERN>

• If parallelizable phases > 50% of total phases → recommend parallel
• If parallelizable phases ≤ 50% but > 1 → recommend sequential (limited benefit)
• The recommended option appears first in the ask_user options list with "(Recommended)" appended to its label. The non-recommended option MUST NOT include "(Recommended)" in its label.

Step 4 — Prompt the user

Call ask_user with type: 'choice' using exactly one of these option sets:

When recommending parallel: options: - label: "Parallel (Recommended)" description: "Spawn child agents for each ready batch where file ownership does not overlap." - label: "Sequential (High Precision)" description: "Spawn one child agent at a time in dependency order."

When recommending sequential: options: - label: "Sequential (Recommended)" description: "Spawn one child agent at a time in dependency order." - label: "Parallel" description: "Spawn child agents for each ready batch where file ownership does not overlap."

Only ONE option receives the "(Recommended)" suffix. Never both. </ANTI-PATTERN>

Prompt the user for a choice using the user-prompt tool from runtime context. Replace [N], [M], and [B] with actual counts from Step 2. The prompt should convey the execution mode analysis and offer two options as described above.

Step 5 — Record and proceed

1. Call update_session with the selected execution_mode and execution_backend: native
2. The tool atomically persists both fields
3. Use the selected mode for the remainder of the session unless the user changes it

Mode-specific behavior

• If parallel is selected and a ready batch has only one phase, execute it sequentially
• If sequential is selected, preserve plan order even when phases are parallel-safe

Safety fallback

If execution_mode is not present in session state at the point where delegation is about to begin, STOP. Do not default to sequential. Return to this gate and resolve it. This catches any edge case where the gate was skipped.

State File Access

When MCP state tools (get_session_status, update_session, transition_phase) are available, prefer them for state operations. They provide structured I/O and atomic transitions.

When MCP tools are not available, state lives inside <MAESTRO_STATE_DIR> and is accessible through read_file and write_file.

Helper scripts remain available for shell-injected command prompts:

node <runtime-script-root>/read-state.js <relative-path>
node <runtime-script-root>/read-active-session.js

Hook Lifecycle During Execution

Hooks fire automatically at agent boundaries. The orchestrator does not invoke them directly.

The hooks system tracks which agent is currently executing. Before each agent dispatch, a hook resolves the active agent identity from the required Agent: header first, then falls back to legacy env/regex detection, and injects compact session context. After completion, a hook validates that the response contains both Task Report and Downstream Context; it requests one retry on the first malformed response.

The hook state directory under ${MAESTRO_HOOKS_DIR:-<os.tmpdir()>/maestro-hooks-<uid>}/<session-id>/ is transient and separate from orchestration state.

Sequential Execution Protocol

For a sequential phase:

1. Verify all blocked_by dependencies are completed
2. Mark the phase in_progress
3. Update current_phase
4. Set current_batch: null
5. Update the progress-tracking tool (use the tool names from get_runtime_context) before delegation
6. Delegate to the assigned agent with the required header and full context
7. Parse the returned handoff
8. Update session state
9. Mark the phase completed or failed
10. Update the progress-tracking tool after the state update

Native Parallel Execution Protocol

Use native parallel execution only for sibling phases at the same dependency depth with non-overlapping file ownership.

Batch Rules

1. Verify all blocking phases for every phase in the batch are completed
2. Slice the ready batch into the current dispatch chunk using MAESTRO_MAX_CONCURRENT
3. Mark only the current chunk phases in_progress
4. Set current_batch in session state for that chunk
5. Write one in-progress todo item for the chunk
6. In the next turn, emit only agent tool calls for that chunk
7. Do not mix shell commands, validation commands, file writes, or narration between those agent calls
8. MAESTRO_MAX_CONCURRENT=0 means emit the entire ready batch in one turn

Native Constraints

• The runtime only parallelizes contiguou