diff --git a/src/integrations.mjs b/src/integrations.mjs
index 186b16b..b05bc0a 100644
--- a/src/integrations.mjs
+++ b/src/integrations.mjs
@@ -88,6 +88,19 @@ export function parseMcp(tokens) {
     }
   }
 
+  // A token still starting with `-` at this point 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 server NAME or its command and gets spliced
+  // straight into every engine's own `mcp add` argv. Everything after `--` is
+  // the user's command line and is deliberately not second-guessed.
+  const stray = [name, cmdParts ? null : target]
+    .find((t) => typeof t === "string" && t.startsWith("-"));
+  if (stray) {
+    return {
+      error: `unknown mcp flag "${stray}" — mcp takes --name, -t/--transport, -e/--env, and -H/--header; put a command's own flags after --`,
+    };
+  }
+
   if (verb === "install" && !name) {
     if (target && isRemoteTarget(target)) name = deriveName(target);
     else return { error: "a stdio command server needs an explicit --name" };
diff --git a/test/mcp-stray-flag.test.mjs b/test/mcp-stray-flag.test.mjs
new file mode 100644
index 0000000..52ce561
--- /dev/null
+++ b/test/mcp-stray-flag.test.mjs
@@ -0,0 +1,122 @@
+// An unsupported flag before `--` used to be swallowed as a positional, which
+// meant it became the server NAME (and its value became the command). The spec
+// is spliced verbatim into every engine's native `mcp add` argv, so
+// `mcp add -s user https://…` shipped `claude mcp add -s user -s -- user https://…`
+// to Claude: a server called `-s`, a command called `user`, and the real URL
+// demoted to an argument. Reject the stray flag instead — but only before `--`,
+// because everything after it is the user's own command line.
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { parseMcp } from "../src/integrations.mjs";
+import { planMcpAdd } from "../src/mcp.mjs";
+
+// ---------- the bug ----------
+
+test("an engine-native scope flag is rejected, not turned into the server name", () => {
+  const { error, spec } = parseMcp(["add", "-s", "user", "https://mcp.sentry.dev/mcp"]);
+  assert.equal(spec, undefined);
+  assert.match(error, /unknown mcp flag "-s"/);
+});
+
+test("a long unsupported flag is rejected too", () => {
+  const { error, spec } = parseMcp(["add", "--scope", "user", "https://mcp.example.com/mcp"]);
+  assert.equal(spec, undefined);
+  assert.match(error, /unknown mcp flag "--scope"/);
+});
+
+test("a misspelled supported flag is rejected rather than silently accepted", () => {
+  const { error } = parseMcp(["add", "--transportt", "http", "https://mcp.example.com/mcp"]);
+  assert.match(error, /unknown mcp flag "--transportt"/);
+});
+
+test("a stray flag in the command position is rejected", () => {
+  const { error, spec } = parseMcp(["add", "tools", "--verbose", "npx"]);
+  assert.equal(spec, undefined);
+  assert.match(error, /unknown mcp flag "--verbose"/);
+});
+
+test("install reports the stray flag, not a misleading missing-name error", () => {
+  const { error } = parseMcp(["install", "--scope", "user", "https://mcp.example.com/mcp"]);
+  assert.match(error, /unknown mcp flag "--scope"/);
+  assert.doesNotMatch(error, /explicit --name/);
+});
+
+test("the error names the flags mcp does take, and points at --", () => {
+  const { error } = parseMcp(["add", "-s", "user", "https://mcp.example.com/mcp"]);
+  for (const flag of ["--name", "--transport", "--env", "--header"]) {
+    assert.ok(error.includes(flag), `expected the error to mention ${flag}`);
+  }
+  assert.match(error, /after --/);
+});
+
+test("no stray flag ever reaches an engine's native argv", () => {
+  // The end-to-end consequence: nothing to plan, so nothing to splice.
+  const parsed = parseMcp(["add", "-s", "user", "https://mcp.sentry.dev/mcp"]);
+  assert.equal(parsed.spec, undefined);
+  assert.throws(() => planMcpAdd(parsed.spec));
+});
+
+test("a stray flag cannot smuggle a bogus name past the catalog lookup", () => {
+  const { error } = parseMcp(["add", "--scope", "porkbun"]);
+  assert.match(error, /unknown mcp flag "--scope"/);
+});
+
+// ---------- controls: the opposite direction ----------
+
+test("a command's own flags after -- are still passed through untouched", () => {
+  const { spec } = parseMcp(["add", "tools", "--", "npx", "-y", "srv", "--port", "3000"]);
+  assert.equal(spec.name, "tools");
+  assert.equal(spec.target, "npx");
+  assert.deepEqual(spec.args, ["-y", "srv", "--port", "3000"]);
+});
+
+test("a command's own flags after the target are still arguments, not strays", () => {
+  const { spec, error } = parseMcp(["add", "tools", "npx", "-y", "srv"]);
+  assert.equal(error, undefined);
+  assert.equal(spec.target, "npx");
+  assert.deepEqual(spec.args, ["-y", "srv"]);
+});
+
+test("every supported flag still parses exactly as before", () => {
+  const { spec } = parseMcp([
+    "install", "https://x.dev/mcp",
+    "--name", "x", "-t", "http", "-e", "K=v", "-H", "A: b",
+  ]);
+  assert.deepEqual(spec, {
+    name: "x",
+    target: "https://x.dev/mcp",
+    args: [],
+    transport: "http",
+    env: [["K", "v"]],
+    headers: ["A: b"],
+  });
+});
+
+test("the catalog shortcut is unaffected", () => {
+  const { spec, catalog, error } = parseMcp(["add", "porkbun"]);
+  assert.equal(error, undefined);
+  assert.equal(catalog.key, "porkbun");
+  assert.equal(spec.name, "porkbun");
+  assert.equal(spec.target, "npx");
+});
+
+test("a bare URL install still derives its name", () => {
+  const { spec, error } = parseMcp(["install", "https://mcp.sentry.dev/mcp"]);
+  assert.equal(error, undefined);
+  assert.equal(spec.target, "https://mcp.sentry.dev/mcp");
+  assert.ok(spec.name);
+});
+
+test("the pre-existing missing-value guards still fire first", () => {
+  assert.match(parseMcp(["install", "https://x.dev/mcp", "--name"]).error, /--name requires a value/);
+  assert.match(parseMcp(["install", "https://x.dev/mcp", "--transport", "--"]).error, /--transport requires a value/);
+});
+
+test("an unknown verb still reports an unknown verb, not an unknown flag", () => {
+  assert.match(parseMcp(["bogus"]).error, /unknown mcp verb/);
+});
+
+test("a stdio install with no name still asks for --name", () => {
+  assert.match(parseMcp(["install", "--", "npx", "srv"]).error, /explicit --name/);
+});