Model-Shunt 🔀

A decoupled, zero-dependency, universal implementation of the Shunt model-routing pattern (originally conceived by Spotify Engineering).

Model-Shunt allows AI coding agents (Antigravity, Cursor, Windsurf, Claude Code, Aider, OpenHands, etc.) to delegate token-heavy I/O (bulk file reading/code analysis) and repetitive boilerplate generation (tests, mocks, stubs, configs) to fast, economical, or local worker models (Gemini 2.5 Flash, Groq/Llama, Ollama, DeepSeek, GPT-4o-mini). This cuts primary agent token consumption by up to 90% while keeping the main context window clean.

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⚡ Key Highlights

• Zero External Dependencies: Built with pure Python 3 standard library (urllib, json, re, argparse). No pip install, no virtual environment, and no npm required.
• Agent-Agnostic: Works transparently across any AI coding agent via standard MCP (Model Context Protocol), standalone CLI scripts, or PreToolUse lifecycle hooks.
• Dynamic Model Discovery & Auto-Routing: Queries the worker endpoint in real time to discover available models and automatically routes to the best model for the task:
  • Reader Mode (Bulk I/O): Prioritizes massive context windows and ultra-low cost (e.g., gemini-2.5-flash, llama-3.3-70b-versatile, gpt-4o-mini).
  • Writer Mode (Code Generation): Prioritizes specialized coding models (e.g., qwen2.5-coder:latest, gemini-2.5-flash, deepseek-chat).
• Bypasses Linux ARG_MAX Limits: Unlike naive implementations that pass file contents as CLI arguments (capped at ~128 KB on Linux), Model-Shunt streams corpus data over stdin, allowing analysis of hundreds of thousands of lines without buffer overflows.
• Deterministic Line Numbering (N|): Automatically prefixes every line in file blocks with its 1-based index, forcing worker models to cite verifiable, exact line numbers instead of hallucinating locations.
• Binary File Protection: Inspects byte headers to reject binary files (PDFs, images, compiled objects) before sending them to the LLM.
• Network Resilience: Automatic exponential backoff retries for rate limits (HTTP 429) and transient server errors (HTTP 503/502), with configurable timeouts and token limits.
• Map-Reduce for Oversized Corpora: When a bulk_read payload exceeds the direct limit (SHUNT_MAX_DIRECT_TOKENS, default ~200k tokens), Model-Shunt automatically splits the corpus into chunks, maps the question over each chunk (preserving absolute N| line numbers), and reduces the extracts into one cited answer. Giant single-line files (minified JSON/JS) are sliced by characters with explicit position markers. Rate-limit pacing waits out provider quota windows instead of failing.

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📁 Repository Structure

model-shunt/
├── src/model_shunt/
│   ├── worker.py              # Universal LLM worker engine with model discovery (zero-deps)
│   └── server.py              # Stdio MCP server exposing routing tools
├── bin/model-shunt.js         # npm/npx launcher shim (requires local Python 3)
├── plugin/
│   ├── .claude-plugin/        # Plugin manifest for hook-compatible agents
│   ├── hooks/                 # PreToolUse interceptor hooks (check-file-size, check-bash-read)
│   ├── scripts/               # Executable streaming CLIs (bulk-read, code-write)
│   └── skills/                # Agent skill manifests (/bulk-reader, /code-writer)
├── pyproject.toml             # PyPI packaging (uvx / pip install)
├── package.json               # npm packaging (npx)
├── config.example.json        # Configuration template
├── test_shunt.py              # Automated test suite
└── .gitignore                 # Credential and cache protection

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⚙️ Configuration

Configure your worker model via environment variables or a config.json file (placed in ~/.config/model-shunt/config.json or in the project root):

Using config.json

{
  "provider": "gemini",
  "model": "auto",
  "timeout": 90,
  "max_tokens": 8192
}

Tip: Setting "model": "auto" (or passing --auto-model in the CLI) will automatically inspect the provider's active models and pick the optimal one for reading vs writing.

Security: Do not put your API key in config.json — use environment variables instead (e.g. GEMINI_API_KEY, GROQ_API_KEY, or SHUNT_API_KEY). An api_key field exists as a last-resort fallback, but keeping secrets out of files is strongly recommended.

Using Environment Variables

# Google Gemini (Recommended: 1M token context, high speed, ultra-low cost)
export SHUNT_PROVIDER="gemini"
export GEMINI_API_KEY="your-api-key"

# Groq (Ultra-low latency inference)
export SHUNT_PROVIDER="groq"
export GROQ_API_KEY="your-api-key"

# Ollama (100% private, local, and free)
export SHUNT_PROVIDER="ollama"
export SHUNT_BASE_URL="http://localhost:11434/v1"

# OpenAI / DeepSeek / OpenRouter / Anthropic
export SHUNT_PROVIDER="deepseek"
export DEEPSEEK_API_KEY="your-api-key"

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🛠️ Usage Modes

Mode 1: Universal MCP Server (Recommended)

Model-Shunt provides a standard stdio MCP server exposing three tools:

1. get_available_models(provider?): Discovers live models from the provider endpoint and returns recommended models for reading and code writing.
2. bulk_read(question, file_paths, model?, provider?): Reads large or multiple files and outputs concise, structured bullets with exact line citations.
3. code_write(spec, reference_path, target_path?, model?, provider?): Replicates patterns, styling, and conventions from a reference file and writes generated code directly to disk without consuming frontier agent output tokens.

Installation

MCP Registry name: mcp-name: io.github.yasmanycastillo/model-shunt

Claude Code (one-liner):

claude mcp add model-shunt -- uvx model-shunt
# Node users: claude mcp add model-shunt -- npx -y model-shunt

Any MCP client (Cursor, Windsurf, Antigravity, Claude Desktop, etc.) — add to its MCP settings. No clone, no absolute paths:

{
  "mcpServers": {
    "model-shunt": {
      "command": "uvx",
      "args": ["model-shunt"],
      "env": {
        "SHUNT_PROVIDER": "gemini",
        "SHUNT_MODEL": "auto",
        "GEMINI_API_KEY": "your-api-key"
      }
    }
  }
}

Fallback (offline / no uv / no npx): run straight from a clone with Python 3.9+ — replace "command"/"args" with "command": "python3", "args": ["/absolute/path/to/model-shunt/src/model_shunt/server.py"].

Security: by default bulk_read/code_write only operate on files inside the server's working directory (the agent workspace). Set SHUNT_ALLOWED_ROOTS (PATH-style list) to expand the sandbox.

Map-Reduce Tuning (optional)

Variable	Default	Purpose
SHUNT_MAX_DIRECT_TOKENS	200000	Payloads above this estimated size switch to map-reduce
SHUNT_CHUNK_CHARS	600000	Chunk size in characters (~150k tokens)
SHUNT_CHUNK_RETRIES	3	Retries per chunk on rate limits
SHUNT_CHUNK_RETRY_DELAY	60	Seconds to wait out a provider quota window (free-tier TPM)

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Mode 2: PreToolUse Interceptor Hooks

For agents supporting pre-execution hooks (e.g., Claude Code, custom agent loops):

1. File Read Interceptor (check-file-size):
   • If the agent attempts a whole-file read on a file exceeding the threshold (default: 350 lines, configurable via SHUNT_MIN_LINES), the hook blocks the call and instructs the agent to delegate to bulk-read.
   • Targeted reads with offset and limit are allowed, preserving surgical context for code editing.
2. Terminal Guard (check-bash-read):
   • Prevents agents from bypassing the read hook by executing commands like cat, less, or more on large files directly in the terminal context.

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Mode 3: Standalone CLI & Scripts

You can also use Model-Shunt directly from the command line or from agent bash sessions:

Discover Available Models & Recommendations

python3 src/model_shunt/worker.py --list-models --provider gemini

Run Bulk Reading Analysis

./plugin/scripts/bulk-read \
  --question "How does the token refresh cycle work?" \
  --paths src/auth.py src/tokens.py \
  --auto-model

Generate Boilerplate Directly to Disk

./plugin/scripts/code-write \
  --spec "Create unit tests for the BillingService covering charge and refund" \
  --reference tests/test_user.py \
  --target tests/test_billing.py \
  --auto-model

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🧪 Verification

Run the built-in test suite to verify your environment:

python3 test_shunt.py

The test suite validates:

• Configuration resolution, fallback cascades, and model selection.
• Binary file detection and rejection.
• Hook decisions (surgical reads allowed, large file reads blocked, bash flag parsing).
• MCP stdio protocol compliance and tool execution.
• CLI discovery flags.

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📄 License

MIT. Inspired by Spotify Engineering's Shunt architecture.