Devbox does not pull *-dev outputs into build environment (headers missing)

[silverhadch]

What happened?

When using nix-shell / nix develop, adding a library package implicitly makes its headers available via the dev output (through normal buildInputs semantics).

Devbox does not do this. As a result, headers are missing unless the user manually adds <package>.dev.

This breaks common C/C++ expectations and makes Devbox environments subtly incomplete compared to standard Nix shells.

---

Expected result

yaml.h should be available automatically, as it is in nix-shell / nix develop when adding libyaml.

---

Actual result

Compilation fails because headers are missing:

main.c:1:10: fatal error: yaml.h: No such file or directory

---

Workaround

Manually add the dev output:

devbox add libyaml.dev

After this, compilation succeeds.

---

Why this is a problem

• In nixpkgs, headers live in the dev output by design.
• nix-shell users reasonably expect headers to be present when a library is added.
• Devbox requires users to know about and manually add output-specific derivations, which defeats the abstraction Devbox aims to provide.
• This is especially painful for newcomers unfamiliar with outputs = [ "out" "dev" ].

---

Suggested fix

Devbox should automatically include the dev output for library packages (or at least for packages commonly used as build dependencies), matching nix-shell / mkShell behavior.

At minimum, this behavior should be documented very clearly.

---

Steps to reproduce

1. Initialize devbox:

   devbox init
   

2. Add packages:

   devbox add libyaml gcc
   

3. Create a test file:

   echo '#include <yaml.h>
   void main(){}' > main.c
   

4. Enter shell:

   devbox shell
   

5. Compile:

   gcc main.c
   

---

Command

devbox add libyaml

---

devbox.json

Without workaround

{
  "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.16.0/.schema/devbox.schema.json",
  "packages": [
    "libyaml@latest"
  ],
  "shell": {
    "init_hook": [
      "echo 'Welcome to devbox!' > /dev/null"
    ],
    "scripts": {
      "test": [
        "echo \"Error: no test specified\" && exit 1"
      ]
    }
  }
}

With workaround (devbox add libyaml.dev)

{
  "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.16.0/.schema/devbox.schema.json",
  "packages": [
    "libyaml@latest",
    "libyaml.dev"
  ],
  "shell": {
    "init_hook": [
      "echo 'Welcome to devbox!' > /dev/null"
    ],
    "scripts": {
      "test": [
        "echo \"Error: no test specified\" && exit 1"
      ]
    }
  }
}

---

Devbox version

0.16.0

---

Nix version

nix (Nix) 2.31.2+2

---

What system does this bug occur on?

Linux (x86_64)

---

Debug logs

No response

[silverhadch]

You can check if an drv has an dev output by nix eval --json nixpkgs#.outputs | jq 'index("dev") != null' or just use nix functions for that. If it returns true include them in the devbox.json.

[tpendragon]

Yup, this just happened to me too. I had to add libyaml.dev - even adding devbox global add libyaml --outputs=out,dev didn't add yaml.h.

[mikeland73]

Triage: looks hard, p2

I couldn't do a full repro: the sandbox can't fetch nixpkgs from the GitHub API, so devbox shell fails before building. The behaviour is clear from the code, though.

• Why headers are missing: the generated flake (internal/shellgen/tmpl/flake.nix.tmpl, ~L44-59) adds each cached package to mkShell.buildInputs as builtins.fetchClosure { fromPath = <store path of one output>; }.
  • Only the outputs from GetOutputsWithCache() / SystemInfo.DefaultOutputs() (internal/devpkg/package.go:716, internal/lock/package.go:88) are included. That is Nix's outputsToInstall, usually just out.
  • These are bare store paths, not derivations, so stdenv's setup hooks never see libyaml.dev. As a result the cc-wrapper doesn't add -isystem .../include to NIX_CFLAGS_COMPILE.
  • nix-shell -p libyaml passes the derivation itself, and stdenv picks up its dev output automatically.
• The --outputs=out,dev report (from @tpendragon) is worth a separate look, since that path should bring in the dev store path. It may be specific to devbox global.

Why "hard": automatically including dev changes the default outputs and the lockfile/cache paths for every library package. That affects download size and needs care, for example opt-in per package or only in the build env.

Workaround: devbox add libyaml.dev. Documenting this better is a cheap first step.

---

Generated by Claude Code