batchy - SKILL.md Agent Skill

name: batchy description: Send non-urgent tasks to Claude's Batch API at 50% off. Use for code reviews, documentation, analysis, refactoring plans, or any work that can wait ~1hr. Trigger with "/batchy", "batch this", or "send to batch". argument-hint: [task description or "check"/"status"/"list"]

You have access to the claude-batch MCP server with these tools:

send_to_batch — submit a prompt (use packet_path for large prompts, packet_text for short ones)
batch_status — check a job by ID
batch_fetch — download results of a completed job
batch_list — list all tracked jobs
batch_poll_once — poll all pending jobs and auto-fetch completed ones

Backends

The toolkit supports two batch backends:

Anthropic — direct Anthropic Message Batches API (requires ANTHROPIC_API_KEY)
Vertex — Google Cloud Vertex AI BatchPredictionJobs (requires VERTEX_PROJECT, VERTEX_LOCATION, VERTEX_GCS_BUCKET)

send_to_batch accepts a backend parameter: "auto" (default), "anthropic", or "vertex". With "auto", Anthropic is preferred when ANTHROPIC_API_KEY is set; otherwise Vertex is used. Both backends use the same model string (e.g., claude-opus-4-6).

Vertex job IDs are long resource paths like projects/my-project/locations/us-central1/batchPredictionJobs/123456789, while Anthropic job IDs look like msgbatch_abc123.

Pre-flight check

Always call batch_list first before building any prompt. This:

Verifies the MCP server is connected and working
Shows you the current state of all jobs (avoids duplicate submissions)
Reveals any completed jobs the user might want to see first

If batch_list fails, stop and tell the user: "The batch MCP server isn't responding. Run uv run ~/.claude/mcp/claude_batch_mcp.py list to debug."

Submitting work — the packet_path pattern

For any non-trivial prompt (which is almost all batch work), assemble the prompt as a file on disk and pass it via packet_path. Never inline large content as packet_text — it wastes context window tokens and can hit argument size limits.

Step-by-step process

Gather context — The batch model has NO access to the codebase or conversation history. The prompt must be completely self-contained.

IMPORTANT: Don't waste tokens reading files!
- If the user says "upload this file" or "include these files", use cat directly in bash to append them to the batch prompt
- Only use Read tool if you need to understand the code to craft the prompt (e.g., to write targeted questions)
- When including entire files/modules for batch review, use bash cat without reading them first
Write the prompt to a temp file using bash:

# Single file task
cat > /tmp/batch_prompt.md << 'PROMPT_EOF'
You are an expert software engineer.

## Task
Review the following code for security vulnerabilities, focusing on injection attacks, auth bypass, and data leaks.

## Code
PROMPT_EOF

# Append source files directly with cat (DON'T use Read tool first!)
echo '### src/auth/login.ts' >> /tmp/batch_prompt.md
echo '```typescript' >> /tmp/batch_prompt.md
cat src/auth/login.ts >> /tmp/batch_prompt.md
echo '```' >> /tmp/batch_prompt.md

echo '### src/auth/session.ts' >> /tmp/batch_prompt.md
echo '```typescript' >> /tmp/batch_prompt.md
cat src/auth/session.ts >> /tmp/batch_prompt.md
echo '```' >> /tmp/batch_prompt.md

# Append instructions
cat >> /tmp/batch_prompt.md << 'PROMPT_EOF'

## Instructions
- List each vulnerability with severity (Critical/High/Medium/Low)
- Include the file path and line number
- Provide a fix for each issue
- Respond in markdown
PROMPT_EOF

For multi-file tasks, use a loop:

cat > /tmp/batch_prompt.md << 'PROMPT_EOF'
You are an expert software engineer. Generate comprehensive unit tests for the following source files.

## Source Files
PROMPT_EOF

# Include all source files from a directory
for f in src/services/*.ts; do
    echo "### $f" >> /tmp/batch_prompt.md
    echo '```typescript' >> /tmp/batch_prompt.md
    cat "$f" >> /tmp/batch_prompt.md
    echo '```' >> /tmp/batch_prompt.md
    echo "" >> /tmp/batch_prompt.md
done

cat >> /tmp/batch_prompt.md << 'PROMPT_EOF'

## Instructions
- Use Jest as the testing framework
- Aim for >90% line coverage
- Include edge cases and error handling tests
- Mock external dependencies
- Output each test file with its target path as a header
PROMPT_EOF

Submit with packet_path:

Call send_to_batch with:

packet_path: /tmp/batch_prompt.md
backend: "auto" (uses Vertex when configured, otherwise Anthropic — or force "anthropic" / "vertex")
label: A descriptive label like "security-review-auth" or "test-gen-services"

Report to user — Tell the user:
- The job ID
- Results typically arrive within 1 hour
- Their status bar will show when it completes
- They can check with /batchy check

Prompt template structure

You are an expert software engineer. [Specific role if needed.]

## Task
[Clear description of what to do]

## Context
### path/to/file1.rs
```rust
[full file contents]

path/to/file2.rs

[full file contents]

Instructions

[Specific output format, constraints, what to focus on]


Keep prompts focused. If a task covers many files or multiple distinct concerns, split into separate batch jobs with clear labels.

## Checking results

When the user says "check", "status", "list", or asks about batch results:

1. Call `batch_poll_once` to check for any newly completed jobs — this handles both Anthropic and Vertex backends. For Vertex jobs, this is the **only** way to poll (the statusline background poller only handles Anthropic jobs).
2. Call `batch_list` to show all jobs with their states
3. **Read results from disk** instead of calling `batch_fetch` — this saves tokens since `batch_fetch` returns the full text through MCP:
   - Job results live at `~/.claude/batches/results/<job_id>.md`
   - For Vertex jobs, the job ID contains slashes so the filename uses underscores: e.g., `projects_my-project_locations_us-central1_batchPredictionJobs_123.md`
   - Read the file directly: `cat ~/.claude/batches/results/<job_id>.md`
   - This is especially important for large results
4. Present the results to the user in a readable format

**Status bar awareness**: The user's status bar shows batch job counts (pending/done/failed), but only for Anthropic jobs. Vertex jobs won't appear in the status bar — always use `batch_poll_once` to check them.

## Cost awareness

Batch API = **50% off** standard pricing:
- Claude Opus 4: $7.50 / $37.50 per million tokens (input/output)
- Claude Sonnet 4: $1.50 / $7.50 per million tokens (input/output)

Typical turnaround: under 1 hour. Max: 24 hours.

**Good batch candidates**: code reviews, documentation generation, architecture analysis, test generation, refactoring plans, security audits, bulk file analysis, API documentation, changelog generation.

**Bad batch candidates**: anything the user needs answered right now, interactive debugging, quick questions, tasks requiring back-and-forth.

## Error handling

- If `send_to_batch` fails with a connection error, the MCP server may not be running. Suggest: "Try restarting Claude Code, or run `uv run ~/.claude/mcp/claude_batch_mcp.py list` to check."
- If a job is stuck in "submitted" for >2 hours, suggest running `batch_poll_once` and checking `batch_status` with the job ID.
- If results are empty or show errors, check the raw JSONL at `~/.claude/batches/results/<job_id>.raw.jsonl`.