Julia REPL over MCP

The mcp__julia-repl__* tools drive a Julia process that stays alive for the whole session, so startup and compilation cost is paid once rather than on every julia -e. dotfiles/claude/settings.json starts the server and dotfiles/julia/setup-packages.jl installs it into the @AgentREPL shared environment.

Invoking it

Tool	Use
`eval`	run code (`code`)
`revise`	reload edited files (`action="revise"`)
`info`	Julia version, active project, worker pid, user variables
`activate`	switch project (`path`), then `pkg` `action="instantiate"`
`pkg`	`add`, `rm`, `status`, `update`, `test`, `develop`
`session`	isolated named sessions (`action`, `name`)
`reset`	kill the worker and spawn a clean one

The server activates the session's working directory on startup, so in a package repo the package is already the active project. log_viewer opens a terminal in the human's tmux, so ask before calling it.

Hot or cold

Iterate hot, verify cold.

Reach for a fresh process, either reset or plain julia --project=. in Bash, when:

verifying anything before a commit, so no result rests on session state
measuring startup, TTFX, or precompilation
a reload leaves behaviour that no longer makes sense

Everything else belongs in the warm session.

Revise

Call revise with action="revise" after editing a .jl file, then re-eval. Method changes reload, and on Julia 1.12 struct and const changes reload too, verified here with Revise 3.16 and the revise_structs preference that setup-packages.jl sets. reset remains the fix when a reload leaves the session inconsistent. A script outside a package needs revise with action="includet" once.

Tests

Narrow the filter while iterating, then run the full suite in a fresh process before finishing.

@run_package_tests filter = ti -> contains(ti.filename, "growth")
@run_package_tests filter = ti -> ti.name == "growth rate stays positive"
@run_package_tests filter = ti -> :integration in ti.tags

pkg with action="test" is the cold run, since Pkg.test starts its own process.

One worker per client

The server speaks STDIO and spawns a private worker per client, so Claude Code, pi and Neovim each get their own session and cannot share one. The human tmux and vim-slime REPL is separate again, and stays that way.

Each client costs about 1 GB resident before any package loads (measured: 581 MB server plus 428 MB worker), so on a shared host drop sessions you no longer need with session action="destroy".