config-set

Config set

Persist an engine-wide configuration value. The single key the engine reads today is diff.tool — the command /skill-engine:review prints when surfacing how to inspect a proposed-vs-live diff. The default value is git diff --no-index --color; users with a preferred diff tool override it once and the choice persists across sessions.

Argument shape

/skill-engine:config-set <key> "<value>"

Example invocations:

/skill-engine:config-set diff.tool "delta"
/skill-engine:config-set diff.tool "git diff --no-index --color"
/skill-engine:config-set diff.tool "code --diff"

The <value> is taken verbatim; quote it so shell-splitting does not eat embedded spaces. The engine does not validate that the configured command exists on the user's PATH — that is the user's responsibility. A misconfigured command surfaces as a runtime failure when /skill-engine:review prints it and the user runs it.

Resolution of the config file

The config file lives at $CLAUDE_PLUGIN_DATA/config.json. This is the same plugin-data tree the existing SessionStart hook (declared in plugin/skill-engine/.claude-plugin/plugin.json) writes state/current.json to. The directory is created if absent.

When $CLAUDE_PLUGIN_DATA is unset, surface:

$CLAUDE_PLUGIN_DATA is unset. The engine's plugin-data tree is not reachable from this shell. Run the skill from a Claude Code session where the plugin is installed, or set $CLAUDE_PLUGIN_DATA to the engine's plugin-data directory.

Exit non-zero. Do not write a config file to a fallback location — the engine's other surfaces (the SessionStart hook, /skill-engine:review) all key off $CLAUDE_PLUGIN_DATA, and writing to a different path would create a config that the readers never find.

The write

Read the existing config.json (or initialize an empty object if absent). Merge the new key/value into it, preserving any other keys present. Write back with schema_version: 1 if not already set.

Example file shape after /skill-engine:config-set diff.tool "delta":

{
  "schema_version": 1,
  "diff.tool": "delta"
}

The flat key shape (diff.tool as one string, not nested) keeps the file shallow and matches the argument the user typed. Future keys (diff.binary, cache.location, etc.) follow the same shape.

Exit message

Set diff.tool = "<value>" in $CLAUDE_PLUGIN_DATA/config.json.

What this skill does NOT do

• It does not list current values. A separate config-list skill would be the symmetric surface; it is out of scope for the v0.3.0 review workflow.
• It does not unset values. Hand-edit config.json to remove a key, or set it back to the engine default.
• It does not validate the diff tool against the user's shell environment. The engine treats <value> as opaque.
• It does not gate keys against a schema. Any <key> accepted by the argument parser is written; readers (/skill-engine:review) read the keys they know and ignore the rest.
• It does not enforce read-only-ness on user-configured diff tools. The doctrine constraint "no engine git mutations" binds the engine, not user config (the git.readonly lint codifies engine-codebase read-only-ness).