Jotain ships a native integration with devenv, the Nix-based developer environment tool. The library lives in lisp/devenv.el (standalone and reusable — no Jotain dependencies) and is wired in by init-devenv.el. Everything is driven through the devenv CLI and degrades to a clean error when the binary is not on PATH. devenv 2.1+ is recommended; every invocation sets AI_AGENT=1 so devenv’s TUI stays out of the machine-readable output.
C-c vdevenv allow / revoke)C-c v ¶C-c v (or M-x devenv) opens a transient covering the whole surface:
| Key | Command | What it does |
|---|---|---|
t | devenv-task-run | Pick a task (annotated from devenv tasks list --json) and run it; prefix arg selects the dependency mode (before/single/after/all) |
s | devenv-script-run | Pick a script defined in devenv.nix (annotated with its description) and run it |
T | devenv-test | devenv test (enterTest, with processes started/stopped around it) |
b | devenv-build | devenv build, optionally for specific attributes |
u / d | devenv-up / devenv-down | Start/stop the project’s processes; up streams into a live buffer (or detaches, see devenv-up-detached); prefix arg picks a subset |
p | devenv-processes | Process dashboard (below) |
l | devenv-processes-logs | Tail one process’s logs |
i / / / : | devenv-info / devenv-search / devenv-eval | Introspection: environment summary, package/option search, evaluate any config attribute to pretty-printed JSON |
r | devenv-reload | Refresh the environment (below) |
e | devenv-env-global-mode | Toggle the native environment loader (below) |
m | devenv-modeline-mode | Toggle the mode-line status segment (below) |
a / x | devenv-allow / devenv-revoke | Trust / untrust the project for auto-activation (below) |
M | devenv-mcp-setup | Register the project’s MCP server with mcp.el |
U / G | devenv-update / devenv-gc | Update devenv.lock inputs / garbage-collect old generations |
$ | devenv-show-log | Every CLI invocation is recorded in *devenv log* |
The -c (--clean) and -r (--refresh-eval-cache) infixes apply to devenv-test and devenv-build.
Long-running commands (test, build, tasks, update, gc) run in compilation buffers with Nix trace matching, so M-g M-n jumps to the file.nix:LINE:COL locations in evaluation errors.
devenv-processes opens a tabulated view of the devenv process manager. Keys: s start, k stop, r restart, l/RET logs, u up, d down, g refresh. Data comes from devenv processes list (JSON when available, plain table on older devenv), fetched asynchronously so the UI never blocks.
Environment loading is done by the library’s native loader (devenv-env-global-mode, enabled by init-devenv; direnv/envrc is disabled in init-prog). Every buffer inside a trusted devenv project carries the project environment buffer-locally, so eglot servers, apheleia formatters, M-x compile, and the ghostel terminal all see devenv-provided tools with no extra configuration.
The goal is parity with a terminal devenv shell, which takes more than reading devenv print-dev-env:
devenv-env-loader = shell (default) sources the printed dev-env script in bash, shellHook included, and diffs the resulting environment against Emacs’s own. Hook-only values therefore reach Emacs too: LOCALE_ARCHIVE, MANPATH, CARGO_INSTALL_ROOT, $CARGO_HOME/bin on PATH, and anything enterShell exports. Because sourcing runs project code it is limited to projects trusted with devenv allow (see below).
devenv-env-loader = json parses devenv print-dev-env --json instead. No project code runs, but the shellHook’s effects are missing. This is also the automatic fallback when sourcing is unavailable or fails.
PATH and XDG_DATA_DIRS are layered, not replaced (devenv-env-layered-variables), matching the nix_saved_PATH restore in devenv’s own script. Without this, everything outside the project profile disappears from exec-path — the devenv CLI included, which then breaks C-c v inside the very projects it manages.
devenv-env-never-applied-variables): nix’s own ignoreVars plus the stdenv build variables devenv’s shellHook unsets. The JSON form is the raw build environment, so applying it verbatim sets TMPDIR/NIX_BUILD_TOP to the build sandbox’s /build, which does not exist on the host. That breaks every tool needing a temp directory: rust-analyzer’s proc-macro server dies with proc-macro server did not respond with data, and cargo build scripts fail.
While a fetch is in flight, eglot-ensure is deferred and replayed once the environment lands, so language servers never start with the wrong PATH. If the fetch fails, the deferred buffers are released without connecting (starting a server with the login environment would be the wrong toolchain) and the message says how many are without an LSP; M-x eglot or C-c v r retries.
After changing devenv.nix, run devenv-reload (C-c v r). It invalidates the library’s caches, re-fetches the environment, and — because eglot snapshots the environment when a server starts — offers to eglot-reconnect every server serving the project so language servers pick up new PATH entries. Where direnv is in charge instead, envrc + use devenv is an equally good substrate: set devenv-env-defer-to-direnv back to t and the native loader stays out of any project with a .envrc, delegating reload to envrc-reload-all.
devenv allow / revoke) ¶devenv 2.1+ gates shell auto-activation behind a per-project trust database (~/.local/share/devenv/allowed), managed with devenv allow and devenv revoke. The integration honours the same contract: the native loader (devenv-env-global-mode) consults the internal devenv hook-should-activate check and never evaluates an untrusted devenv.nix automatically — exactly like the zsh/fish/nu shell hooks. The same gate decides whether the environment may be obtained by sourcing (which runs the project’s shellHook); an untrusted or unverifiable project falls back to the JSON loader. devenv-allow (C-c v a) trusts the project and immediately activates the loader in its buffers; devenv-revoke (C-c v x) untrusts it and drops the native environment. On devenv versions without the trust model (< 2.1) the loader behaves as before. Note that the direnv/envrc path has its own consent gate (envrc-allow → direnv allow), independent of devenv’s database.
devenv-modeline-mode (global, enabled by init-devenv at startup) adds a status segment to the mode line for buffers inside a devenv project:
devenv[on] — the buffer’s environment carries the devenv shell (loaded by envrc/direnv or the native loader; detected via DEVENV_PROFILE).
devenv[off] — devenv project, environment not loaded.
devenv[!] — auto-activation trust denied; run devenv-allow.
Outside devenv projects the segment is empty. mouse-1 on the segment opens the devenv transient. The redisplay path is subprocess-free: trust is probed once per project in the background from find-file-hook and cached.
devenv 2.0+ ships devenv lsp, a bundled nixd preconfigured with the project’s devenv module options (completion, hover docs for devenv.nix options). init-devenv routes devenv.nix and devenv.local.nix buffers to it while nil keeps serving every other Nix file — the routing entry delegates non-devenv buffers to whatever contact was registered before it.
devenv 2.0+ ships devenv mcp, a stdio MCP server exposing nixpkgs package search, devenv option search, and process control (list/status/logs/start/stop/restart). devenv-mcp-setup (C-c v M) registers it with mcpel under a per-project name (devenv-<project>); start it from M-x mcp-hub and run M-x gptel-mcp-connect to hand its tools to gptel. devenv-mcp-remove unregisters it.
The devenv customization group covers the executable name (devenv-executable), the quiet-mode environment (devenv-extra-env), global CLI flags (devenv-global-arguments), cache TTL and command timeout, detached up (devenv-up-detached), and the dashboard’s log line count (devenv-processes-log-lines).
The native loader adds: how the environment is obtained (devenv-env-loader), whether direnv keeps ownership (devenv-env-defer-to-direnv), extra variables to drop (devenv-env-ignored-variables, filtered on top of the built-in devenv-env-never-applied-variables), which variables extend rather than replace the global value (devenv-env-layered-variables), and the shell bookkeeping ignored when diffing a sourced environment (devenv-env-shell-ignored-variables).