Synthesis Skill

VCS Provider

This skill uses VCS operations through Exarchos MCP actions (create_pr, merge_pr, list_prs, check_ci, etc.). These actions automatically detect and route to the correct VCS provider (GitHub, GitLab, Azure DevOps). No gh/glab/az commands needed — the MCP server handles provider dispatch.

Not to be confused with merge_orchestrate. This skill calls merge_pr to land a user-facing PR on main via the VCS provider — a remote operation. merge_orchestrate (@skills/merge-orchestrator/SKILL.md) is the upstream sibling: a local git merge of a subagent worktree branch onto the integration branch during the delegate → merge-pending → delegate HSM loop. Synthesize never invokes merge_orchestrate; merge-pending never invokes merge_pr.

Overview

Submit stacked PRs after review phase completes. The prepare_synthesis composite action consolidates readiness checks, stack verification, test validation, and quality signal analysis into a single call -- eliminating the multi-script coordination that historically caused synthesis failures.

Prerequisites:

• All delegated tasks complete with reviews passed (spec + quality)
• The integration branch already exists from delegation phase
• Task branches present and pushed to remote

Do NOT proceed if either review is incomplete or failed -- return to /exarchos:review first.

Entry points: Synthesis is normally reached from the review phase of feature / debug / refactor workflows. It is also reachable from oneshot workflows via the opt-in path — when a user signals "let's open a PR for this" during plan or implementing, the request_synthesize event is appended, and finalize_oneshot then resolves the choice state and transitions the workflow to synthesize. See @skills/oneshot-workflow/SKILL.md for the opt-in mechanics and synthesisPolicy semantics.

Triggers

Activate this skill when:

• User runs /exarchos:synthesize command
• All reviews have passed successfully
• Ready to submit PRs
• Oneshot workflow resolved to synthesize via finalize_oneshot

Process

Runbook: Follow the synthesis-flow runbook: exarchos_orchestrate({ action: "runbook", id: "synthesis-flow" }) If runbook unavailable, use describe to retrieve action schemas: exarchos_orchestrate({ action: "describe", actions: ["prepare_synthesis"] })

Step 1: Verify Readiness

Call the prepare_synthesis composite action to validate all preconditions in a single operation:

mcp__plugin_exarchos_exarchos__exarchos_orchestrate({
  action: "prepare_synthesis",
  featureId: "<id>"
})

This action performs:

• Phase readiness -- Confirms workflow is in the correct phase with all reviews complete
• Stack integrity -- Detects diverged branches, missing task branches, or broken parent chains and reconstructs automatically
• Test verification -- Runs npm run test:run && npm run typecheck from the stack top
• Benchmark regression -- If state.verification.hasBenchmarks is true, checks for performance regressions
• Quality signals -- Queries code_quality view for regressions and actionable hints
• Gate events -- Auto-emits gate.executed events for each check (tests, benchmarks, CodeRabbit)

For the full breakdown of individual checks the composite action performs, see references/synthesis-steps.md.

On success: All checks passed. The response includes a readiness summary with any quality hints to present to the user. Proceed to Step 2.

On failure: The response identifies which check failed and provides remediation guidance. Follow the guidance -- typically returning to /exarchos:review or /exarchos:delegate.

If any quality hint has confidenceLevel: 'actionable', present the suggestedAction to the user before proceeding.

Step 2: Write and Validate PR Descriptions

For each PR in the stack, write a structured description following references/pr-descriptions.md. Required sections: Summary, Changes, Test Plan, plus a footer. Projects can override required sections via .exarchos/pr-template.md.

Title format: <type>: <what> (max 72 chars)

Write the PR body to a temp file:

cat > /tmp/pr-body.md <<'EOF'
## Summary
[2-3 sentences: what changed, why it matters]

## Changes
- **Component** -- Description of change

## Test Plan
[Testing approach and coverage]

---
**Results:** Tests X pass · Build 0 errors
**Design:** [doc](path)
**Related:** #issue
EOF

Validate before creating the PR:

mcp__plugin_exarchos_exarchos__exarchos_orchestrate({
  action: "validate_pr_body",
  bodyFile: "/tmp/pr-body.md"
})

Do NOT call create_pr until validation passes. If validation fails, fix the body and re-validate.

Step 3: Submit and Merge

Create PRs using the validated body and enable auto-merge. For each branch in the stack (bottom-up):

// Create PR via VCS MCP action
exarchos_orchestrate({
  action: "create_pr",
  base: "<parent-branch>",
  head: "<branch>",
  title: "<type>: <what>",
  body: "<pr-body>"
})

// Enable auto-merge
exarchos_orchestrate({
  action: "merge_pr",
  prId: "<number>",
  strategy: "squash"
})

After submission:

1. Apply benchmark label -- If verification.hasBenchmarks is true, apply label: gh pr edit <number> --add-label has-benchmarks
2. Record PR URLs -- Capture URLs via exarchos_orchestrate({ action: "list_prs", state: "open" })
3. Update state:

mcp__plugin_exarchos_exarchos__exarchos_workflow({
  action: "set", featureId: "<id>", updates: {
    "artifacts": { "pr": ["<url1>", "<url2>"] },
    "synthesis": { "mergeOrder": ["<branch1>", ...], "prUrl": ["<url1>", ...], "prFeedback": [] }
  }
})

For merge ordering strategy, see references/merge-ordering.md.

Human checkpoint: Output "Stacked PRs enqueued: [URLs]. Waiting for CI/merge queue." then PAUSE for user input: "Merge stack? (yes/no/feedback)"

• 'yes' -- PRs merge; transition to completed via /exarchos:cleanup
• 'feedback' -- Route to /exarchos:shepherd [PR_URL] to address comments, then return here
• 'no' -- Pause workflow; resume later with /exarchos:rehydrate

Event Emissions (REQUIRED)

After PRs are created and auto-merge is enabled, emit the stack.submitted event:

mcp__plugin_exarchos_exarchos__exarchos_event({ action: "append", stream: "<featureId>", event: {
  type: "stack.submitted",
  data: {
    branches: ["task-001-branch", "task-002-branch"],
    prNumbers: [101, 102]
  }
}})

During shepherd iterations (CI monitoring loop), emit after each assessment:

mcp__plugin_exarchos_exarchos__exarchos_event({ action: "append", stream: "<featureId>", event: {
  type: "shepherd.iteration",
  data: {
    iteration: 1,
    prsAssessed: 2,
    fixesApplied: 0,
    status: "all-green"
  }
}})

These events are checked by check-event-emissions during workflow validation. Missing emissions will trigger warnings.

Post-Merge Cleanup

After PRs merge, invoke cleanup:

mcp__plugin_exarchos_exarchos__exarchos_workflow({
  action: "cleanup", featureId: "<id>", mergeVerified: true,
  prUrl: ["<url>", ...], mergedBranches: ["<branch>", ...]
})

Then sync: git fetch --prune and remove worktrees.

Anti-Patterns

Don't	Do Instead
Skip review phase	Always run /exarchos:review first
Force push stack branches	Use normal push
Delete worktrees before merge	Wait for merge confirmation
Create PR with failing tests	Ensure review phase passes first
Run readiness scripts manually	Use prepare_synthesis composite action

Handling Failures

See references/troubleshooting.md for test failures, PR check failures, merge queue rejections, and MCP tool errors.

Phase Transitions and Guards

For the full transition table, consult @skills/workflow-state/references/phase-transitions.md.

Quick reference: The synthesize → completed transition requires guard pr-url-exists — set synthesis.prUrl or artifacts.pr in the same set call as phase.

Schema Discovery

Use exarchos_workflow({ action: "describe", actions: ["set", "init"] }) for parameter schemas and exarchos_workflow({ action: "describe", playbook: "feature" }) for phase transitions, guards, and playbook guidance. Use exarchos_orchestrate({ action: "describe", actions: ["prepare_synthesis"] }) for orchestrate action schemas.

Completion Criteria

• prepare_synthesis readiness check passed
• PR descriptions written per references/pr-descriptions.md
• PRs created and auto-merge enabled
• PR links provided to user
• State updated with PR URLs and merge order