Evaluating code inside the booted application is the tool that changes how an agent works on Rails: instead of reasoning about what a model returns, it runs the line and reads the answer, with the logs and the database covering the two places the truth usually hides. That reach is why the gem is development-only — it refuses to start where code reloading is off, and stays bound to localhost until you deliberately open it.
A gem added to a Rails app's development group. It runs inside the running application and serves MCP over HTTP at /tidewave/mcp on the port your app already uses, so the agent talks to the booted app rather than to a copy of its source.
- Ruby evaluated in the context of the running application, with its loaded dependencies and in-memory data — the result plus anything printed, like a Rails console the agent can use
- SQL run against the development database, so the agent can seed realistic records or check that a create action saved what it should have
- The server log read back, which is where the params, the backtrace and what actually happened after a request are
- Documentation for a class, method or constant, read from the exact gem versions pinned in Gemfile.lock rather than from training data
- The file and line where something is defined, across the app and its gems — resolved from the running app, so a method generated at runtime is found where grep would miss it
A Rails app you can add the gem to as a development-group dependency, and a client that speaks streamable HTTP — point it at http://localhost:3000/tidewave/mcp, or whichever port the app runs on. It raises rather than boot in any environment where code reloading is disabled, which normally includes production, and accepts requests from localhost only unless allow_remote_access is turned on. There are deliberately no tools for listing routes or associations: the author's position is that an agent reading those source files gets more context and can edit them in the same pass.
Build from source — clone the repository and build it