Watson

Code intelligence for Elixir/Phoenix projects. Builds a searchable call graph for LLM coding agents.

Installation

Add watson as a dev dependency in mix.exs:

def deps do
  [
    {:watson, "~> 0.1.0", only: :dev, runtime: false}
  ]
end

Then fetch dependencies:

mix deps.get

Claude Code Setup

Add to your ~/.claude/mcp.json:

{
  "mcpServers": {
    "watson": {
      "command": "mix",
      "args": ["watson.mcp"],
      "cwd": "/path/to/your/elixir/project"
    }
  }
}

Auto-Indexing

Watson automatically indexes your project when you use any query tool. The index is:

• Created on first use - no manual setup required
• Updated incrementally - only changed files are re-indexed
• Persisted in .watson/ - survives restarts

You can force a full rebuild with the index tool if needed, but it's rarely necessary.

Tools

Code Navigation

Tool	Description
function_definition	Find where a function is defined. Returns file path, line numbers, visibility, and whether it's a macro.
function_references	Find all call sites for a function. Returns file, line, and calling function for each invocation.
function_callers	Find functions that call a given function (traverse up the call graph). Use for impact analysis.
function_callees	Find functions called by a given function (traverse down the call graph).

Type Information

Tool	Description
function_spec	Get the @spec type signature for a function. Returns parameter types and return type.
module_types	List all type definitions (@type, @typep, @opaque, @callback) in a module.
type_errors	Get compiler type errors and warnings from Elixir's type checker (1.17+).

Phoenix/Ecto

Tool	Description
routes	List all Phoenix routes. Returns HTTP verb, path, controller, and action.
schema	Get Ecto schema structure: table name, fields with types, and associations.

Analysis

Tool	Description
impact_analysis	Analyze what's affected by changing files. Returns affected modules and suggested test files.
index	Force a full rebuild of the code index. Usually not needed due to auto-indexing.
project_info	Get project and index information for debugging. Returns Watson version, Elixir/OTP versions, mix environment, and index statistics.

MFA Format

Functions are specified in Module.function/arity format:

MyApp.Accounts.get_user/1
Phoenix.Controller.render/3