parallel-task-execution

Parallel Task Execution

Claude Code can execute multiple independent tool calls in a single response. This dramatically reduces wall-clock time for tasks that do not depend on each other.

When to Use

• Multiple files need to be read, searched, or analyzed independently
• User wants to speed up a multi-step workflow
• Several subagents can investigate different aspects simultaneously
• Independent operations (read + grep + glob) can run concurrently
• User asks about fan-out/fan-in or parallel execution

Core Patterns

Identifying Independent Operations

Two operations are independent when neither needs the other's result. Test by asking: "Can I start operation B without knowing the result of operation A?"

# INDEPENDENT (run in parallel):
- Read file A and Read file B
- Grep for pattern X and Grep for pattern Y
- Analyze auth module and Analyze database module

# DEPENDENT (run sequentially):
- Find the config file, THEN read it
- Read the test file, THEN modify it based on contents
- Get the function signature, THEN find all call sites

Parallel Tool Calls

Make multiple tool calls in a single response block:

# Single response with 3 parallel calls:

Call 1 - Read:
  file_path: "src/auth/middleware.ts"

Call 2 - Read:
  file_path: "src/auth/tokens.ts"

Call 3 - Read:
  file_path: "src/auth/permissions.ts"

All three files are read concurrently. Results arrive together for synthesis.

Fan-Out with Subagents

Launch multiple Agent tool calls for independent investigations:

# Fan-out: 3 parallel subagents

Agent 1:
  prompt: "Analyze all React components in src/components/
           for accessibility issues (missing aria labels,
           no keyboard handlers). Report findings as a list."

Agent 2:
  prompt: "Check all API routes in src/api/ for proper
           error handling. Report which routes lack try/catch
           or return generic error messages."

Agent 3:
  prompt: "Review all database queries in src/db/
           for N+1 query problems. Report file paths and
           the specific query patterns."