/parallel-execution

name: parallel-execution
description: CRITICAL skill for executing multiple Task tool calls in a SINGLE message for true parallelism. Essential for efficient multi-task workflows, subagent coordination, and maximizing throughput.
license: MIT
compatibility: opencode
metadata:
  category: workflow
  audience: agents

Parallel Execution

**CRITICAL**: This skill teaches how to execute multiple tasks simultaneously for maximum efficiency.

The Fundamental Rule

> **ALL Task calls MUST be in a SINGLE assistant message for true parallelism.**

If Task calls are in separate messages, they run SEQUENTIALLY, not in parallel.

---

Why Parallel Execution Matters

Sequential (SLOW - AVOID)

Message 1: Start Task A
           ↓ wait for completion
Message 2: Start Task B
           ↓ wait for completion
Message 3: Start Task C
           ↓ wait for completion

Total time = A + B + C = 90 seconds (if each takes 30s)

Parallel (FAST - USE THIS)

Message 1: Start Task A ─┐
           Start Task B ─┼─ All run simultaneously
           Start Task C ─┘

Total time ≈ max(A, B, C) = 30 seconds

**Speedup: 3x faster with 3 parallel tasks**

---

How to Execute in Parallel

Step 1: Identify Independent Tasks

Tasks are independent when:

• They don't depend on each other's output
• They don't modify the same files
• They can run in any order

Step 2: Launch ALL Tasks in ONE Message

<!-- CORRECT: All tasks in single message = PARALLEL -->
<task>
  <description>Analyze authentication module</description>
  <prompt>Review src/auth for security patterns...</prompt>
</task>

<task>
  <description>Analyze API layer</description>
  <prompt>Review src/api for REST best practices...</prompt>
</task>

<task>
  <description>Analyze database layer</description>
  <prompt>Review src/db for query optimization...</prompt>
</task>

Step 3: Collect and Synthesize Results

After all tasks complete, combine their findings into a unified response.

---

Parallelization Patterns

Pattern 1: Task-Based Parallelization

When you have N independent tasks, spawn N subagents:

Implementation Plan:
1. Implement auth module
2. Create API endpoints
3. Add database schema
4. Write unit tests
5. Update documentation

Launch 5 parallel subagents:
├─ Subagent 1: Implement auth module
├─ Subagent 2: Create API endpoints
├─ Subagent 3: Add database schema
├─ Subagent 4: Write unit tests
└─ Subagent 5: Update documentation

**All 5 in ONE message!**

Pattern 2: Directory-Based Parallelization

Analyze different directories simultaneously:

Codebase Structure:
├── src/auth/
├── src/api/
├── src/db/
└── src/ui/

Launch 4 parallel subagents:
├─ Subagent 1: Analyze src/auth
├─ Subagent 2: Analyze src/api
├─ Subagent 3: Analyze src/db
└─ Subagent 4: Analyze src/ui

Pattern 3: Perspective-Based Parallelization

Review from multiple angles at once:

Code Review Perspectives:
- Security vulnerabilities
- Performance bottlenecks
- Test coverage gaps
- Architecture patterns

Launch 4 parallel subagents:
├─ Subagent 1: Security review
├─ Subagent 2: Performance analysis
├─ Subagent 3: Test coverage review
└─ Subagent 4: Architecture assessment

Pattern 4: Adversarial Verification

Use conflicting mandates for thorough review:

Verification Subagents (all parallel):
├─ Syntax & Type Checker
├─ Test Runner
├─ Lint & Style Checker
├─ Security Scanner
└─ Build Validator

Then (sequential, after above complete):
├─ False Positive Filter
├─ Missing Issues Finder
└─ Context Validator

---

TodoWrite Integration

When using parallel execution, mark ALL parallel tasks as `in_progress` simultaneously:

Before Launching Parallel Tasks

{
  "todos": [
    { "content": "Analyze auth module", "status": "in_progress", "activeForm": "Analyzing auth module" },
    { "content": "Analyze API layer", "status": "in_progress", "activeForm": "Analyzing API layer" },
    { "content": "Analyze database layer", "status": "in_progress", "activeForm": "Analyzing database layer" },
    { "content": "Synthesize findings", "status": "pending", "activeForm": "Synthesizing findings" }
  ]
}

After Each Task Completes

Mark as completed as results come in:

{
  "todos": [
    { "content": "Analyze auth module", "status": "completed", "activeForm": "Analyzing auth module" },
    { "content": "Analyze API layer", "status": "completed", "activeForm": "Analyzing API layer" },
    { "content": "Analyze database layer", "status": "in_progress", "activeForm": "Analyzing database layer" },
    { "content": "Synthesize findings", "status": "pending", "activeForm": "Synthesizing findings" }
  ]
}

---

When to Parallelize

Good Candidates

| Scenario | Parallel Approach | |----------|-------------------| | Multiple independent analyses | One subagent per analysis | | Multi-file processing | One subagent per file/directory | | Different review perspectives | One subagent per perspective | | Multiple independent features | One subagent per feature | | Exploratory research | Multiple search strategies |

When NOT to Parallelize

| Scenario | Why Sequential | |----------|----------------| | Tasks with dependencies | B needs A's output | | Same file modifications | Risk of conflicts | | Sequential workflows | Order matters (commit → push → PR) | | Shared state | Race conditions | | Limited resources | Overwhelming the system |

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Performance Impact

| # Parallel Tasks | Sequential Time | Parallel Time | Speedup | |------------------|-----------------|---------------|---------| | 2 | 60s | 30s | 2x | | 3 | 90s | 30s | 3x | | 5 | 150s | 30s | 5x | | 10 | 300s | 30s | 10x |

*Assuming each task takes ~30 seconds*

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Common Mistakes

Mistake 1: Separate Messages

WRONG (Sequential):
Message 1: "I'll start analyzing the auth module..."
           <task>Analyze auth</task>
Message 2: "Now let me analyze the API..."
           <t