checks/skill-build.nix

@@ -11,6 +11,30 @@ let
   discovered = discoverSkills ./fixtures/valid;
   built = map (skill: mkSkill { inherit pkgs; inherit (skill) name src; }) discovered;
 
+  commandSrc = pkgs.runCommandLocal "command-skill-src" { } ''
+    mkdir -p "$out"
+    printf '%s\n' '---' 'name: command-skill' 'description: Fixture skill with a command.' '---' \
+      > "$out/SKILL.md"
+    cat > "$out/run-command" <<'EOF'
+#!/usr/bin/env sh
+: "''${TOOL:=tool}" # mkSkill:TOOL
+exec "$TOOL" "$@"
+EOF
+    chmod +x "$out/run-command"
+  '';
+
+  commandSkill = mkSkill {
+    inherit pkgs;
+    name = "command-skill";
+    src = commandSrc;
+    commands = [
+      {
+        path = "run-command";
+        env.TOOL = "${pkgs.coreutils}/bin/printf";
+      }
+    ];
+  };
+
   collision = builtins.tryEval (discoverSkills ./fixtures/collision);
 
   discoveredNames = lib.sort (a: b: a < b) (map (skill: skill.name) discovered);
@@ -24,6 +48,7 @@ pkgs.runCommandLocal "skill-build-check"
     pairs = lib.concatStringsSep " " (map (skill: "${skill.skillName}=${skill}") built);
     collisionSucceeded = if collision.success then "yes" else "no";
     expectedNames = lib.concatStringsSep " " discoveredNames;
+    inherit commandSkill;
   }
   ''
     fail() { echo "FAIL: $1" >&2; exit 1; }
@@ -52,5 +77,11 @@ pkgs.runCommandLocal "skill-build-check"
     test -f "$beta_path/tools/SKILL.md" \
       || fail "beta's asset subfolder did not ride along into \$out"
 
+    echo "command tier: declared command env defaults are patched"
+    test -x "$commandSkill/run-command" \
+      || fail "declared command is not executable"
+    grep -qx ': "''${TOOL:=${pkgs.coreutils}/bin/printf}" # mkSkill:TOOL' "$commandSkill/run-command" \
+      || fail "declared command env default was not patched"
+
     touch "$out"
   ''

lib/mk-skill.nix

@@ -3,7 +3,45 @@
   pkgs,
   src,
   name ? builtins.baseNameOf (toString src),
+  commands ? [ ],
 }:
+let
+  lib = pkgs.lib;
+
+  patchEnvDefault = command: envName: value:
+    let
+      marker = "# mkSkill:${envName}";
+      replacement = '': "''${${envName}:=${value}}" ${marker}'';
+    in
+    ''
+      if ! awk -v marker=${lib.escapeShellArg marker} -v replacement=${lib.escapeShellArg replacement} '
+        index($0, marker) { print replacement; found = 1; next }
+        { print }
+        END { if (!found) exit 42 }
+      ' "$_mkskill_command" > "$_mkskill_command.tmp"; then
+        rm -f "$_mkskill_command.tmp"
+        echo "mkSkill: command '${command.path}' has no ${marker} marker" >&2
+        exit 1
+      fi
+      mv "$_mkskill_command.tmp" "$_mkskill_command"
+      chmod +x "$_mkskill_command"
+    '';
+
+  patchCommand = command:
+    let
+      env = command.env or { };
+    in
+    ''
+      _mkskill_command="$out/${command.path}"
+      test -f "$_mkskill_command" \
+        || { echo "mkSkill: command '${command.path}' does not exist" >&2; exit 1; }
+      test -x "$_mkskill_command" \
+        || { echo "mkSkill: command '${command.path}' is not executable" >&2; exit 1; }
+      ${lib.concatStringsSep "\n" (lib.mapAttrsToList (patchEnvDefault command) env)}
+    '';
+
+  patchCommands = lib.concatStringsSep "\n" (map patchCommand commands);
+in
 pkgs.stdenvNoCC.mkDerivation {
   inherit name src;
 
@@ -18,6 +56,7 @@ pkgs.stdenvNoCC.mkDerivation {
     cp -R "$src"/. "$out/"
     test -f "$out/SKILL.md" \
       || { echo "mkSkill: skill '${name}' has no SKILL.md at its root" >&2; exit 1; }
+${patchCommands}
     runHook postInstall
   '';