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devenv integration

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.

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Entry point: C-c v

C-c v (or M-x devenv) opens a transient covering the whole surface: 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.

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Process dashboard

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.

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Environment loading and reload

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.
  • Derivation-only variables are dropped (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.

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Auto-activation trust (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.

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Mode-line status

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.

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LSP for devenv.nix

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.

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MCP server for the AI stack

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 mcp.el 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.

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Customization

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).
Last modified on August 23, 2026