Code Executor Skill

Overview

This skill teaches you (Main Claude Code) how to efficiently handle multi-tool MCP workflows by launching a subagent that writes and executes code to compose MCP tool calls.

Architecture

Main Context (YOU - no MCP servers configured)
    ↓
    Recognize multi-tool MCP workflow needed
    ↓
    Launch subagent via Task tool
    ↓
Subagent Context (has MCP servers configured)
    ↓
    Write TypeScript/Python code
    ↓
    Execute code via Bash (deno/python)
    ↓
    Code calls multiple MCP tools via local MCP client
    ↓
    Return results to YOU (main context)

Why This Architecture?

Problem: Loading all MCP tool schemas into your main context causes token bloat (141k tokens for 47 tools).

Solution:

• YOU (main) have NO MCP servers → no token bloat
• Subagent has MCP servers → isolated context
• Subagent writes code that calls multiple tools
• Results return to you

Benefits:

• ✅ 98% token reduction in main context
• ✅ Multi-tool composition in single execution
• ✅ Complex logic, loops, data transformations
• ✅ Subagent context disposed after task

When to Use This Pattern

✅ Launch Subagent With This Skill When:

1. Multi-tool MCP workflows (3+ MCP tool calls needed)

   • Example: "List files, read each one, aggregate data, store in database"
   • Example: "Fetch from multiple APIs, merge results, write report"

2. Complex data processing across MCP tools

   • Example: "Read database records, enrich with API data, filter, store results"
   • Example: "Parse all JSON files in directory, validate, deduplicate, insert to DB"

3. Conditional tool selection

   • Example: "Try primary API, fallback to secondary if it fails, then cache"
   • Example: "Route to different storage backends based on file size"

4. Parallel MCP operations

   • Example: "Fetch from 5 different APIs simultaneously"
   • Example: "Process multiple files in parallel"

5. Retry logic and error recovery

   • Example: "Retry database query with exponential backoff"
   • Example: "Try multiple data sources until one succeeds"

❌ Don't Use This Pattern When:

1. Single simple MCP tool call - Just call the tool directly
2. No MCP tools needed - Use regular task planning
3. UI/user interaction required - Use slash commands
4. Simple sequential operations - Regular tool calls are clearer

How to Launch a Subagent

When you identify a multi-tool MCP workflow, use the Task tool to launch a subagent:

Task({
  subagent_type: "general-purpose",
  prompt: `You have access to MCP servers with the following tools:
  - mcp__filesystem__readFile, writeFile, listDirectory
  - mcp__database__query, insert, update
  - [list the relevant tools for this task]

  Task: [Describe the multi-tool workflow clearly]

  Instructions:
  1. Reference the cached script pattern from scripts/[relevant-script].ts
  2. Write TypeScript code that:
     - Imports: import { callMCPTool } from '../../lib/mcp-client.ts';
     - Calls MCP tools using callMCPTool('mcp__server__tool', params)
     - Processes and transforms data as needed
     - Returns summary of results
  3. Execute the code via Bash with MCP_CONFIG_PATH set:
     MCP_CONFIG_PATH=~/.claude/subagent-mcp.json deno run --allow-read --allow-run --allow-env /tmp/script.ts
  4. Return the execution results

  Use the pattern from: [specify cached script that matches]
  - scripts/typescript/multi-tool-workflow.ts (sequential pipeline)
  - scripts/typescript/parallel-execution.ts (concurrent operations)
  - scripts/typescript/error-recovery.ts (retry logic)
  - scripts/typescript/file-processing.ts (batch file ops)
  - scripts/typescript/conditional-logic.ts (dynamic tool selection)
  - scripts/typescript/data-aggregation.ts (multi-source merging)

  Expected output: [What you need back from the subagent]`
})

Cached Script Patterns

The skill includes 12 proven script patterns (6 TypeScript + 6 Python) in the scripts/ directory. Always reference these when launching subagents:

TypeScript Patterns

1. scripts/typescript/multi-tool-workflow.ts

   • Sequential 5-step pipeline: Fetch → Transform → Validate → Store → Report
   • Use for: Data processing pipelines where each step depends on previous

2. scripts/typescript/parallel-execution.ts

   • Concurrent operations with Promise.all and Promise.allSettled
   • Use for: Fetching from multiple sources simultaneously

3. scripts/typescript/error-recovery.ts

   • Retry logic with exponential backoff, fallback strategies
   • Use for: Unreliable APIs, failover patterns

4. scripts/typescript/file-processing.ts

   • Batch file operations: list → filter → process → aggregate
   • Use for: Processing multiple files, generating reports

5. scripts/typescript/conditional-logic.ts

   • Dynamic tool selection based on runtime data
   • Use for: Routing, adaptive workflows, decision trees

6. scripts/typescript/data-aggregation.ts

   • Fetch from multiple sources, transform to common format, merge
   • Use for: Combining data from multiple databases/APIs

Python Patterns

Same patterns available in scripts/python/:

• multi_tool_workflow.py
• parallel_execution.py
• error_recovery.py
• file_processing.py
• conditional_logic.py
• data_aggregation.py

Subagent Task Prompt Template

Use this template when launching subagents:

Task({
  subagent_type: "general-purpose",
  prompt: `You are a subagent with access to MCP servers. Write code to complete this task.

  ## Available MCP Tools
  [List the MCP servers/tools available - you should know these from your configuration]
  - mcp__filesystem__readFile, writeFile, listDirectory
  - mcp__database__query, insert
  - [etc...]

  ## Task
  [Clear description of what needs to be done]

  ## Instructions

  1. **Reference Cached Pattern**
     Use the pattern from: scripts/typescript/[most-similar-script].ts
     - This script shows: [what pattern it demonstrates]
     - Adapt it for: [how to modify for this task]

  2. **Write TypeScript Code**
     \`\`\`typescript
     import { callMCPTool } from '../../lib/mcp-client.ts';

     // [Step-by-step comments of what the code should do]
     \`\`\`

  3. **Tool Call Format**
     - Format: callMCPTool('mcp__<server>__<tool>', { params })
     - Example: callMCPTool('mcp__filesystem__readFile', { path: '/data/file.json' })

  4. **Execute Code**
     Run via Bash with MCP_CONFIG_PATH set:
     \`\`\`bash
     MCP_CONFIG_PATH=~/.claude/subagent-mcp.json deno run --allow-read --allow-run --allow-env /tmp/your-script.ts
     \`\`\`

  5. **Return Results**
     Provide summary of:
     - What was accomplished
     - Number of records/files processed
     - Any errors or warnings
     - Final output data

  ## Expected Output
  [Specify exactly what data/summary you need back]

  ## Error Handling
  - Wrap all MCP calls in try/catch
  - Log errors clearly
  - [Any specific error handling needed]
  `
})

Example: Complete Workflow

User Request

"Read all JSON files in /tmp/data, aggregate the data, and store in database"

Your Response (Main Claude Code)

You recognize this needs:

• Multiple filesystem operations (list, read multiple files)
• Data processing (parse JSON, aggregate)
• Database operation (insert)

You launch subagent:

Task({
  subagent_type: "general-purpose",
  prompt: `You have access to MCP servers with these tools:
  - mcp__filesystem__listDirectory
  - mcp__filesystem__readFile
  - mcp__database__insert

  Task: Read all JSON files in /tmp/data, parse them, aggregate the data, and store in database table 'aggregated_data'.

  Instructions:
  1. Use the pattern from scripts/typescript/file-processing.ts
     - It shows: batch file operations with filtering and aggregation
     - Adapt it to: read JSON files specifically and store results in DB

  2. Write TypeScript code that:
     - Lists files in /tmp/data
     - Filters for .json extension
     - Reads each JSON file
     - Parses and aggregates the data
     - Stores aggregated results in database

  3. Import the MCP client:
     import { callMCPTool } from '../../lib/mcp-client.ts';

  4. Execute via:
     MCP_CONFIG_PATH=~/.claude/subagent-mcp.json deno run --allow-read --allow-run --allow-env /tmp/aggregate-files.ts

  5. Return: Number of files processed and records stored

  Expected output: "Processed X files, stored Y records in database"`
})

Subagent Executes

1. Reads cached pattern from scripts/typescript/file-processing.ts
2. Writes adapted code:

import { callMCPTool } from '../../lib/mcp-client.ts';

const files = await callMCPTool('mcp__filesystem__listDirectory', {
  path: '/tmp/data'
});

const jsonFiles = files.filter(f => f.name.endsWith('.json'));
const allData = [];

for (const file of jsonFiles) {
  const content = await callMCPTool('mcp__filesystem__readFile', {
    path: file.path
  });
  allData.push(...JSON.parse(content));
}

const aggregated = {
  total_records: allData.length,
  data: allData,
  timestamp: new Date().toISOString()
};

await callMCPTool('mcp__database__insert', {
  table: 'aggregated_data',
  record: aggregated
});

console.log(`Processed ${jsonFiles.length} files, ${allData.length} records`);

3. Executes code via Bash
4. Returns: "Processed 15 files, 1,247 records stored in database"

You Receive Results

You report to user: "✓ Successfully processed 15 JSON files from /tmp/data and stored 1,247 aggregated records in the database."

Tool Naming Convention

Format: mcp__<server>__<tool>

Examples:

• mcp__filesystem__readFile
• mcp__database__query
• mcp__github__createPullRequest
• mcp__postgres__insert

The <server> name comes from the subagent's MCP configuration file (.mcp.json or ~/.claude/subagent-mcp.json).

Choosing TypeScript vs Python

TypeScript (Deno):

• ✅ Strong typing with interfaces
• ✅ Better for complex data transformations
• ✅ Native async/await, Promise.all
• ✅ Faster execution
• ❌ Requires Deno installed

Python:

• ✅ More familiar to some developers
• ✅ Better for data science/ML tasks
• ✅ Rich standard library
• ✅ Type hints with TypedDict
• ❌ Slightly slower

Default recommendation: Use TypeScript unless subagent explicitly needs Python libraries.

MCP Client Library

The subagent uses a local MCP client library to call tools:

TypeScript: lib/mcp-client.ts

import { callMCPTool, callMCPToolsParallel } from '../../lib/mcp-client.ts';

// Single tool call
const result = await callMCPTool('mcp__filesystem__readFile', {
  path: '/data/file.json'
});

// Parallel calls
const [file1, file2, file3] = await callMCPToolsParallel([
  { tool: 'mcp__filesystem__readFile', parameters: { path: '/data/1.json' } },
  { tool: 'mcp__filesystem__readFile', parameters: { path: '/data/2.json' } },
  { tool: 'mcp__filesystem__readFile', parameters: { path: '/data/3.json' } }
]);

Python: lib/mcp_client.py

from lib.mcp_client import call_mcp_tool, call_mcp_tools_parallel

# Single tool call
result = await call_mcp_tool('mcp__filesystem__readFile', {
    'path': '/data/file.json'
})

# Parallel calls
results = await call_mcp_tools_parallel([
    {'tool': 'mcp__filesystem__readFile', 'parameters': {'path': '/data/1.json'}},
    {'tool': 'mcp__filesystem__readFile', 'parameters': {'path': '/data/2.json'}},
    {'tool': 'mcp__filesystem__readFile', 'parameters': {'path': '/data/3.json'}}
])

Templates

For simpler tasks, reference the templates in templates/:

• basic-typescript.template.ts - Single tool call skeleton
• basic-python.template.py - Single tool call skeleton
• multi-tool.template.ts - Multiple tool composition
• multi-tool.template.py - Multiple tool composition

Tell the subagent: "Use the pattern from templates/basic-typescript.template.ts"

Error Handling

Always instruct subagents to include error handling:

try {
  const result = await callMCPTool('mcp__database__query', {
    query: 'SELECT * FROM users'
  });
  return { success: true, data: result };
} catch (error) {
  console.error('Database query failed:', error.message);
  return { success: false, error: error.message };
}

Best Practices

When Launching Subagents

1. Be specific about tools available

   • List the exact MCP tools the subagent has access to
   • Don't assume subagent knows your MCP configuration

2. Reference cached patterns

   • Always point to the most similar cached script
   • Explain how to adapt it for the current task

3. Specify execution command

   • CRITICAL: Set MCP_CONFIG_PATH=~/.claude/subagent-mcp.json before command
   • Include the full deno run or python command
   • Include required permissions: --allow-read --allow-run --allow-env
   • Example: MCP_CONFIG_PATH=~/.claude/subagent-mcp.json deno run --allow-read --allow-run --allow-env script.ts

4. Define expected output clearly

   • What data you need back
   • What format (summary, full data, counts, etc.)

5. Include error handling instructions

   • How to handle failures
   • Whether to fail-fast or continue on errors

When Subagent Writes Code

(These are instructions for the subagent, but you should know them to guide effectively)

1. Always import MCP client first

   import { callMCPTool } from '../../lib/mcp-client.ts';
   

2. Use proper tool name format

   • Format: mcp__<server>__<tool>
   • Match exactly to MCP configuration

3. Handle errors gracefully

   • Try/catch around all MCP calls
   • Log errors clearly
   • Return error status

4. Process data efficiently

   • Use parallel calls when possible (Promise.all, asyncio.gather)
   • Stream large datasets
   • Aggregate early to reduce memory

5. Return useful summaries

   • Include counts, timestamps
   • Report errors/warnings
   • Provide actionable results

Debugging

If subagent task fails:

1. Check MCP configuration

   • Is the tool available in subagent's .mcp.json?
   • Is the server running?

2. Verify tool name format

   • Correct format: mcp__server__tool
   • Match to actual server name in config

3. Check MCP_CONFIG_PATH is set

   • CRITICAL: Bash execution must set MCP_CONFIG_PATH=~/.claude/subagent-mcp.json
   • Without this, MCP client won't find server configuration
   • Verify config file exists: ls -la ~/.claude/subagent-mcp.json

4. Check permissions

   • Deno needs: --allow-read --allow-run --allow-env
   • MCP servers may have restricted access

5. Review generated code

   • Did subagent import MCP client correctly?
   • Are tool names formatted properly?
   • Is error handling present?

6. Check execution output

   • Look at stdout/stderr from Bash execution
   • MCP errors appear in output

Security Considerations

1. Subagent has code execution - Review generated code before execution in sensitive contexts
2. MCP servers access resources - Configure MCP with least privilege
3. Filesystem access - Limit allowed paths in MCP server configuration
4. Database access - Use read-only connections where possible
5. Network access - MCP servers can make network requests

Documentation References

• Setup: See SUBAGENT_SETUP.md for configuration
• TypeScript: See TYPESCRIPT_GUIDE.md for patterns
• Python: See PYTHON_GUIDE.md for patterns
• Examples: See EXAMPLES.md for complete use cases
• API Reference: See REFERENCE.md for MCP client API

Summary

You are Main Claude Code. When you encounter a multi-tool MCP workflow:

1. ✅ Recognize the pattern - Multiple MCP tools needed, complex logic
2. ✅ Launch subagent - Use Task tool with clear instructions
3. ✅ Reference cached scripts - Point to most similar pattern
4. ✅ Specify tools available - List MCP servers/tools explicitly
5. ✅ Define expected output - What you need back from subagent
6. ✅ Receive results - Subagent executes code and returns summary
7. ✅ Report to user - Present results clearly

Key principle: You orchestrate, subagent executes. Keep your main context clean, let subagents handle the heavy MCP lifting.