fix(skills): reject an unknown flag instead of installing it as the skill source

src/integrations.mjs

@@ -240,6 +240,17 @@ export async function skillCommand(tokens, { run, installedSet } = {}) {
     }
     else if (!source) source = rest[i];
   }
+  // A source still starting with `-` was never consumed as a flag, so it is a
+  // typo or an engine-native flag moshcode does not take (`-s user`). Left
+  // alone it becomes the skill SOURCE and is spliced straight into every
+  // engine's own argv — `gemini skills install -s --scope user`, and a
+  // `git clone --depth 1 -s <dest>` where `-s` (`--shared`) makes git read the
+  // destination as the repository — while the URL the user actually typed is
+  // dropped on the floor. Same guard `mcp` already applies to its own spec.
+  if (source?.startsWith("-")) {
+    console.log(err(`unknown skill flag "${source}" — skill install takes --name; a source that really starts with "-" must be written as ./${source}`));
+    return 1;
+  }
   if (!source) { console.log(err("usage: /skill install <git-url|path> [--name <name>]")); return 1; }
 
   const spec = { source, name: skillName(source, name) };

test/skill-stray-flag.test.mjs

@@ -0,0 +1,121 @@
+// `skill install` took the first unconsumed token as the source, so a flag it
+// does not know became the source itself and the real URL was dropped: for
+// `skill install -s user https://github.com/o/r.git` the source is `-s`, and
+// `user` and the URL are never looked at again. The source is spliced verbatim
+// into each engine's native argv, so gemini got `skills install -s --scope user`
+// and Claude got `git clone --depth 1 -s <skills-dir>/s` — where `-s` is git's
+// own `--shared`, which makes git read the *destination* as the repository
+// ("fatal: repository '…/skills/s' does not exist"). `mcp` already rejects a
+// stray flag for exactly this reason (test/mcp-stray-flag.test.mjs); this is
+// the same guard on the same kind of splice.
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { ENGINES } from "../src/engines.mjs";
+import { skillCommand } from "../src/integrations.mjs";
+
+const ALL = new Set(Object.keys(ENGINES));
+const URL = "https://github.com/example/real-skill.git";
+
+/** Run `fn` with console.log captured; returns { code, out }. */
+async function capture(fn) {
+  const log = console.log;
+  const out = [];
+  console.log = (...args) => out.push(args.join(" "));
+  try { return { code: await fn(), out: out.join("\n") }; }
+  finally { console.log = log; }
+}
+
+/** A `run` that records every argv it is handed and always succeeds. */
+function spy() {
+  const calls = [];
+  const run = async (cmd, args) => { calls.push([cmd, ...args].join(" ")); return { ok: true, code: 0 }; };
+  return { calls, run };
+}
+
+// --- the bug -----------------------------------------------------------------
+
+test("an engine-native scope flag is rejected instead of becoming the source", async () => {
+  const { run, calls } = spy();
+  const { code, out } = await capture(() => skillCommand(["install", "-s", "user", URL], { run, installedSet: ALL }));
+  assert.equal(code, 1, "a stray flag was accepted as the skill source");
+  assert.match(out, /unknown skill flag "-s"/);
+  assert.deepEqual(calls, [], "no engine should be run at all");
+});
+
+test("a misspelled --name is rejected, not installed as a skill called nmae", async () => {
+  const { run, calls } = spy();
+  const { code, out } = await capture(() => skillCommand(["install", "--nmae", "my-skill", URL], { run, installedSet: ALL }));
+  assert.equal(code, 1);
+  assert.match(out, /unknown skill flag "--nmae"/);
+  assert.deepEqual(calls, [], "no engine should be run at all");
+});
+
+test("the stray flag never reaches git's or gemini's argv", async () => {
+  // The end-to-end consequence: `-s` is git's --shared, so the clone would have
+  // read its own destination as the repository.
+  const { run, calls } = spy();
+  await capture(() => skillCommand(["install", "-s", "user", URL], { run, installedSet: ALL }));
+  assert.ok(!calls.some((c) => c.includes(" -s")), `a stray flag was spliced into ${calls.join(" | ")}`);
+});
+
+test("the user's real source is never silently dropped in favour of a flag", async () => {
+  const { run, calls } = spy();
+  const { code } = await capture(() => skillCommand(["install", "--scope", "user", URL], { run, installedSet: ALL }));
+  assert.equal(code, 1);
+  assert.ok(!calls.some((c) => c.includes(URL)), "nothing should have been installed");
+});
+
+test("the error names the flag skill install does take, and how to escape a real one", async () => {
+  const { run } = spy();
+  const { out } = await capture(() => skillCommand(["install", "-s", "user", URL], { run, installedSet: ALL }));
+  assert.match(out, /--name/);
+  assert.match(out, /\.\/-s/);
+});
+
+// --- controls: the opposite direction ---------------------------------------
+
+test("a normal git URL still installs across the skills engines", async () => {
+  const { run, calls } = spy();
+  const { code } = await capture(() => skillCommand(["install", URL], { run, installedSet: ALL }));
+  assert.equal(code, 0);
+  assert.ok(calls.some((c) => c.startsWith("git clone") && c.includes(URL)), `expected a clone of the source, got ${calls.join(" | ")}`);
+  assert.ok(calls.some((c) => c.startsWith(`${ENGINES.gemini.bin} skills install ${URL}`)), `expected gemini to be handed the source, got ${calls.join(" | ")}`);
+});
+
+test("--name still parses and still names the skill", async () => {
+  const { run, calls } = spy();
+  const { code } = await capture(() => skillCommand(["install", URL, "--name", "renamed"], { run, installedSet: ALL }));
+  assert.equal(code, 0);
+  assert.ok(calls.some((c) => c.includes("/renamed")), `expected the clone to land in .../renamed, got ${calls.join(" | ")}`);
+});
+
+test("a local path source is untouched by the guard", async () => {
+  const { run, calls } = spy();
+  const { code } = await capture(() => skillCommand(["install", "./my-skill"], { run, installedSet: ALL }));
+  assert.equal(code, 0);
+  assert.ok(calls.some((c) => c.includes("./my-skill")), `expected the path to survive, got ${calls.join(" | ")}`);
+});
+
+test("the pre-existing --name missing-value guard still fires first", async () => {
+  const { run } = spy();
+  const { code, out } = await capture(() => skillCommand(["install", URL, "--name"], { run, installedSet: ALL }));
+  assert.equal(code, 1);
+  assert.match(out, /--name requires a value/);
+  assert.doesNotMatch(out, /unknown skill flag/);
+});
+
+test("no source at all still reports usage, not an unknown flag", async () => {
+  const { run } = spy();
+  const { code, out } = await capture(() => skillCommand(["install"], { run, installedSet: ALL }));
+  assert.equal(code, 1);
+  assert.match(out, /usage: \/skill install/);
+  assert.doesNotMatch(out, /unknown skill flag/);
+});
+
+test("an unknown verb still reports an unknown verb", async () => {
+  const { run } = spy();
+  const { code, out } = await capture(() => skillCommand(["bogus"], { run, installedSet: ALL }));
+  assert.equal(code, 1);
+  assert.match(out, /unknown skill verb/);
+});