Assay

Incremental Dialyzer for modern Elixir tooling. Assay reads Dialyzer settings directly from your host app's mix.exs, runs the incremental engine, filters warnings via dialyzer_ignore.exs, and emits multiple output formats suited for humans, CI, editors, and LLM-driven tools.

Features

• Incremental Dialyzer runs via mix assay
• Watch mode (mix assay.watch) with debounced re-analysis
• JSON/CLI formatters (text, github, sarif, lsp, ndjson, llm)
• dialyzer_ignore.exs filtering with per-warning decorations
• Igniter-powered installer (mix assay.install) that configures apps/ warning_apps in the host project
• JSON-RPC daemon (mix assay.daemon) plus an MCP server (mix assay.mcp) for editor/LSP/agent integrations

Installation

Using Igniter (Recommended)

The easiest way to install Assay is using the Igniter-powered installer. First, add both Assay and Igniter to your dependencies:

def deps do
  [
    {:assay, "~> 0.5", runtime: false, only: [:dev, :test]},
    {:igniter, "~> 0.6", optional: false}
  ]
end

Important: Igniter must be in your mix.exs dependencies (not optional) for the installer to work.

Then run the installer:

mix deps.get
mix assay.install --yes

The installer will:

• Detect project and dependency apps
• Configure apps and warning_apps in your mix.exs
• Create a .gitignore entry for _build/assay
• Create a dialyzer_ignore.exs file
• Optionally generate CI workflow files (GitHub Actions or GitLab CI)

Manual Installation

If you prefer not to use Igniter, you can configure Assay manually:

1. Add Assay to your dependencies:

def deps do
  [
    {:assay, "~> 0.5", runtime: false, only: [:dev, :test]}
  ]
end

2. Add configuration to your mix.exs:

def project do
  [
    # ... other config ...
    assay: [
      dialyzer: [
        apps: :project_plus_deps,  # or explicit list
        warning_apps: :project      # or explicit list
      ]
    ]
  ]
end

3. Create a dialyzer_ignore.exs file (optional):

# dialyzer_ignore.exs
[]

4. Add _build/assay to your .gitignore (optional but recommended).

Configuration

Symbolic Selectors

Assay supports symbolic selectors for apps and warning_apps:

• :project - All project applications (umbrella: all apps from Mix.Project.apps_paths(), single-app: Mix.Project.config()[:app])
• :project_plus_deps - Project apps + dependencies + base OTP libraries
• :current - Current Mix project app only (useful for umbrella projects)
• :current_plus_deps - Current app + dependencies + base OTP libraries

Example Configuration

# In mix.exs
def project do
  [
    app: :my_app,
    assay: [
      dialyzer: [
        # Analyze project apps + dependencies
        apps: :project_plus_deps,
        # Only show warnings for project apps
        warning_apps: :project
      ]
    ]
  ]
end

You can also mix symbolic selectors with explicit app names:

assay: [
  dialyzer: [
    apps: [:project_plus_deps, :custom_app],
    warning_apps: [:project, :another_app]
  ]
]

Usage

One-off analysis

mix assay
mix assay --print-config
mix assay --format github --format sarif
mix assay --no-compile          # skip compilation (project must already be compiled)

Exit codes: 0 (clean), 1 (warnings), 2 (error).

Watch mode

mix assay.watch

Re-runs incremental Dialyzer on file changes and streams formatted output.

JSON-RPC daemon

mix assay.daemon

Speaks JSON-RPC over stdio. Supported methods:

• assay/analyze – run incremental Dialyzer
• assay/getStatus, assay/getConfig, assay/setConfig
• assay/shutdown

MCP server

mix assay.mcp

Implements the Model Context Protocol (initialize, tools/list, tools/call) and exposes a single tool, assay.analyze, which returns the same structured diagnostics as the daemon. Requests/responses use the standard MCP/LSP framing: each JSON payload must be prefixed with Content-Length: <bytes>\r\n\r\n.

Pretty-printing Dialyzer terms

Add erlex to your host project's deps (e.g. {:erlex, "~> 0.2", optional: true}) and pass --format elixir to mix assay to render Dialyzer's Erlang detail blocks as Elixir-looking maps/structs (e.g. %Ecto.Changeset{}) while keeping plain output available when the default text format is used.

Troubleshooting

macOS: "Too many open files" error

On macOS, Dialyzer's incremental mode opens many files in parallel, which can exceed the default open file limit. If you encounter errors related to file limits on larger projects, increase the limit before running Assay:

ulimit -n 4096  # or higher for very large projects
mix assay

To make this permanent, add it to your shell configuration file (e.g., ~/.zshrc):

ulimit -n 4096

Development

• mix test – unit tests (including daemon + MCP simulations)
• mix credo – linting/style checks
• mix format – formatter
• mix assay.watch – dogfooding watch mode on the library itself

Status

Assay currently focuses on incremental Dialyzer runs, watch mode, the Igniter installer, and daemon/MCP integrations. Future milestones include richer pass-through Dialyzer flag support, additional output formats, and expanded editor tooling.