Huginn

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What Huginn Actually Does

Keeps a registry of tools (name, schema, handler).

Lets clients list and call tools over stdio like it’s the 1980s, but JSON.

Executes handlers safely, with timeouts, aborts, and structured logs — so you can look professional while debugging and pulling your hair out.

The Sacred Tool Contract

Everything revolves around this simple contract, mess it up, and Huginn will politely do nothing and fully ignore you.

{
  name: string,
  description: string,
  inputSchema: Record<string, any>,
  handler: async (args: Record<string, any>, signal: AbortSignal) => {
    content: Array<{ type: string; text: string }>,
    isError?: boolean
  }
}

Think of it as OpenAPI’s introverted cousin — minimal, yet oddly particular.

Getting Started - Let's see if it even runs

bun install
bun install-plugins    # discover and install plugin dependencies
bun start              # starts the MCP server (stdio transport)
# dev: bun dev
# tests: bun test && bun typecheck

Press Ctrl+C to stop it like a civilized human, not by closing the terminal window.

# Install dependencies
bun install

# Start the MCP server
bun start

# Development with watch mode
bun dev

# Run tests
bun test
bun test:watch     # watch mode
bun coverage       # with coverage

# Type checking
bun typecheck

# Linting & formatting
bun lint
bun lint:fix
bun format
bun format:write

# Install MCP plugins
bun install-plugins

Adding a Tool (the Right Way)

Edit src/tools/registry.ts and drop in an object that follows the contract above.

If your tool might take longer than a tweet to execute, use the provided AbortSignal and wrap it with safeToolExecution.

// src/tools/registry.ts
{
  name: 'my_tool',
  description: 'Echo text back',
  inputSchema: { type: 'object', properties: { text: { type: 'string' } }, required: ['text'] },
  handler: async (args, signal) => ({
    content: [{ type: 'text', text: `You sent: ${String(args.text)}` }]
  })
}

Write a test in test/, because future you will forget what this tool does.

Plugin System - For the modular minded

Huginn supports a plugin architecture for organizing tools into separate modules.

Installing Plugin Dependencies

# Interactive plugin selection (default)
bun install-plugins

# Install all plugins automatically
bun install-plugins --all

# Auto-confirm dependency installation
bun install-plugins --yes

# Combine options
bun install-plugins --all --yes

Interactive Mode (Default):

• Shows all available plugins with descriptions
• Lets you select which plugins to install
• Enter numbers (e.g., "1 3"), "all", or "none"

Command Options:

• --all or -a: Install all plugins without prompting
• --yes or -y: Auto-confirm bun install step

This will:

1. Discover plugin folders at src/plugins/*/index.ts
2. Let you choose which plugin folders to install (interactive mode)
3. Add missing packages declared by that folder’s requiredPackages to your package.json
4. Optionally run bun install to actually install them
5. Generate a manifest in out/plugins-manifest.json with folder names and tool lists

Creating a Plugin

Create a directory under src/plugins/your-plugin-name/ with an index.ts file:

// src/plugins/my-plugin/index.ts
export const myTool = {
  name: 'my_tool',
  description: 'Does something useful',
  inputSchema: {
    type: 'object',
    properties: { text: { type: 'string' } },
    required: ['text']
  },
  handler: async (args, signal) => ({
    content: [{ type: 'text', text: `Processed: ${args.text}` }]
  })
};

export const anotherTool = {
  name: 'another_tool',
  description: 'Does something else useful',
  inputSchema: { type: 'object', properties: {}, additionalProperties: false },
  handler: async (_args, _signal) => ({
    content: [{ type: 'text', text: 'All good.' }]
  })
};

// The only thing the loader cares about
export const tools = [myTool, anotherTool];

// Optional: per-plugin dependencies (installed via `bun install-plugins`)
export const requiredPackages = {
  'some-package': '^1.0.0'
};

Then run bun install-plugins to register the whole folder and install its dependencies.

Generated Tools (quality-of-life, not sorcery)

• create_mcp_tool: scaffold a new plugin folder with a properly tagged tool name (generated_<base>), path-safe writes, optional dependency add, optional install (you confirm), and auto-refresh.
• remove_generated_tool: deletes only tools that were generated (header + generated_ prefix) and refreshes the registry.
• rename_generated_tool: renames a generated tool (folder + tool name), with safety checks, then refresh.
• edit_generated_tool: updates description, input_schema, handler_code, and optional requiredPackages; unsafe code is gated behind a flag with explicit warnings.

Rules are simple: you bring the logic, Huginn brings the boundaries. Stay inside src/plugins/, export tools = [...], and we’ll do the rest.

Included Plugins

code_screenshot

Render a styled PNG from a code string with syntax highlighting, line numbers, and optional line highlighting.

Args:

• code (string, required)
• language (string, default: "plaintext")
• transparent (boolean, default: false)
• highlight (string, line ranges like "3,5-7,10")
• scale (number, default: 2)
• titleOverride (string)

Notes:

• Fixed line height (18px) and preserved indentation; empty lines keep height.
• Highlight uses an inset left bar (no layout shift) and avoids first-glyph overlap.
• Very long lines are supported; the viewport auto-expands to capture full width.
• Transparent mode: rounded corners, smaller title, margin + shadow for separation.
• Iosevka is embedded when available; falls back to system monospace.

visualize_diff_image

Generate HTML or PNG from a unified diff.

Args:

• diff (string, required)
• format ("html" | "image", default: "html")
• outputType ("side-by-side" | "line-by-line", default: "side-by-side")

Built-in Management Tools

refresh_plugins

Reload plugins without restarting the server.

Schema:

• plugin (string, optional): tool name or plugin directory to refresh only that plugin.

Examples:

{ "name": "refresh_plugins", "arguments": {} }

{ "name": "refresh_plugins", "arguments": { "plugin": "code_screenshot" } }

Logging & Config - For the paranoid

Logs: logs/mcp-tools.log(JSON-lines). Override with MCP_TOOL_LOG_FILE.

Timeout: MCP_TOOL_TIMEOUT_MS (milliseconds). — Yes, you can set it to Infinity, but don’t.

Roadmap - Aka. “The Illusion of Progress”

License

MIT — go make something useful. Or something deeply cursed. Huginn doesn’t judge, but he does know.