fix(mcp): reject malformed Args on Add MCP Server instead of defaulting to []

`add_server` wrapped `json.loads(args)` in a bare `except` that fell back to
`[]`, so an Args value that is not JSON — a bare path, which is what the form's
placeholder invites people to type — registered the server and spawned the
stdio subprocess with an empty argv. Nothing surfaced the loss: the POST
returned 200 and the row persisted with `"args": []`.

The route now returns 400 for an unparseable value, and also for valid JSON of
the wrong shape: `args=5` reached `StdioServerParameters(args=5)` and raised an
unhandled TypeError in the error formatter's `" ".join(...)`.

Both form clients mirror the guard instead of leaving the user to read a 400
they cannot see. `settings.js` stops silently defaulting a bad Args value, and
`admin.js` gains the same client-side parse check plus a `res.ok` branch so a
server-side rejection is not reported as a connection failure.

Ported from public `dev` (`9d5c0319`, #6215 upstream, fixing #6211), with its
test. The `admin.js` hunks are inert on `lab` — `initMcpForm` early-returns
because that form's markup is not in this build — and are carried anyway to
keep the two lines from diverging further.

routes/mcp/mcp_routes.py

@@ -181,10 +181,17 @@ def setup_mcp_routes(mcp_manager: McpManager):
         if transport == "http" and not url:
             raise HTTPException(400, "url is required for HTTP transport")
 
-        # Parse JSON fields
-        try:
-            parsed_args = json.loads(args) if args else []
-        except json.JSONDecodeError:
+        # Parse JSON fields. args is not defaulted on a parse failure: an
+        # unparseable value is silently discarded downstream (stdio spawns
+        # with an empty argv), so the caller must be told instead.
+        if args:
+            try:
+                parsed_args = json.loads(args)
+            except json.JSONDecodeError:
+                raise HTTPException(400, "args must be valid JSON, e.g. [\"-y\", \"pkg\"]")
+            if not isinstance(parsed_args, list):
+                raise HTTPException(400, "args must be a JSON array, e.g. [\"-y\", \"pkg\"]")
+        else:
             parsed_args = []
         try:
             parsed_env = json.loads(env) if env else {}

static/js/admin.js

@@ -3142,6 +3142,7 @@ function initMcpForm() {
     if (transport === 'stdio' && !command) { msg.textContent = 'Command is required for stdio'; msg.className = 'admin-error'; return; }
     if (transport === 'sse' && !url) { msg.textContent = 'URL is required for SSE'; msg.className = 'admin-error'; return; }
     try { JSON.parse(env); } catch { msg.textContent = 'Env must be valid JSON'; msg.className = 'admin-error'; return; }
+    try { JSON.parse(args); } catch { msg.textContent = 'Args must be valid JSON, e.g. ["-y", "pkg"]'; msg.className = 'admin-error'; return; }
     const fd = new FormData();
     fd.append('name', name); fd.append('transport', transport); fd.append('command', command); fd.append('args', args); fd.append('env', env); fd.append('url', url);
     // If preset has oauthFile config, send credentials for file generation
@@ -3162,6 +3163,10 @@ function initMcpForm() {
     try {
       const res = await fetch('/api/mcp/servers', { method: 'POST', body: fd, credentials: 'same-origin' });
       const data = await res.json();
+      if (!res.ok) {
+        msg.textContent = data.detail || `Failed (${res.status})`; msg.className = 'admin-error';
+        return;
+      }
       if (data.needs_oauth) {
         msg.innerHTML = `Added ${esc(name)} — <a href="/api/mcp/oauth/authorize/${data.id}" target="_blank" style="color:var(--red);font-weight:600;">Authorize with Google</a> to connect`;
         msg.className = 'admin-success';

static/js/settings.js

@@ -5102,7 +5102,11 @@ async function initUnifiedIntegrations() {
         fd.append('transport', transport);
         if (transport === 'stdio') {
           fd.append('command', el('uf-mcp-cmd').value);
-          let args = '[]'; try { args = JSON.stringify(JSON.parse(el('uf-mcp-args').value || '[]')); } catch (_) {}
+          // Unlike env below, an unparseable args value is not silently
+          // defaulted: it would spawn the subprocess with an empty argv.
+          let args;
+          try { args = JSON.stringify(JSON.parse(el('uf-mcp-args').value || '[]')); }
+          catch (_) { el('uf-mcp-msg').textContent = 'Args must be valid JSON, e.g. ["-y", "pkg"]'; return; }
           let env  = '{}'; try { env  = JSON.stringify(JSON.parse(el('uf-mcp-env').value  || '{}')); } catch (_) {}
           fd.append('args', args);
           fd.append('env', env);

tests/test_mcp_add_server_args_validation.py

@@ -0,0 +1,147 @@
+"""Regression test for issue #6211: a malformed Args value on the "Add MCP
+Server" form must not be silently discarded into an empty argv.
+
+routes/mcp/mcp_routes.py's add_server() wrapped json.loads(args) in a bare
+except that fell back to `[]`, so a non-JSON Args value registered the
+server as "Connected" while forwarding no arguments to the spawned stdio
+subprocess at all, with no error surfaced anywhere.
+"""
+import asyncio
+import json
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+from fastapi import HTTPException
+
+from routes.mcp import mcp_routes
+
+
+class _FakeSession:
+    """Stands in for core.database.SessionLocal(); add_server only adds+commits."""
+
+    def __init__(self):
+        self.added = []
+
+    def add(self, obj):
+        self.added.append(obj)
+
+    def commit(self):
+        pass
+
+    def close(self):
+        pass
+
+
+def _add_server(monkeypatch):
+    """Register add_server on the shared module-level router and return the
+    freshly-added route's raw endpoint function, bypassing HTTP/Form parsing
+    (require_admin is the only other thing the function touches via `request`).
+
+    Callers must pass every Form(...) parameter add_server reads past the args
+    check (url, oauth_file, oauth_config): calling the endpoint directly skips
+    FastAPI's dependency resolution, so an omitted one arrives as the Form
+    marker object itself rather than its declared default, and later code
+    (e.g. `if oauth_file:`) reads that marker as truthy.
+    """
+    monkeypatch.setattr(mcp_routes, "require_admin", lambda request: None)
+    manager = MagicMock()
+    manager.connect_server = AsyncMock(return_value=True)
+    manager.get_server_status = MagicMock(return_value={"status": "connected", "tool_count": 1})
+    router = mcp_routes.setup_mcp_routes(manager)
+    # setup_mcp_routes appends new APIRoute objects to the shared router on
+    # every call, so take the LAST "add_server" route: the one just registered
+    # with our fake manager, not an earlier registration from importing app.py.
+    route = [r for r in router.routes if getattr(r, "name", None) == "add_server"][-1]
+    return route.endpoint, manager
+
+
+def test_add_server_rejects_malformed_args_instead_of_defaulting(monkeypatch):
+    add_server, manager = _add_server(monkeypatch)
+    monkeypatch.setattr(mcp_routes, "SessionLocal", lambda: (_ for _ in ()).throw(
+        AssertionError("must not reach the DB when args is rejected")))
+
+    with pytest.raises(HTTPException) as exc:
+        asyncio.run(add_server(
+            request=None,
+            name="filesystem",
+            transport="stdio",
+            command="mcp-server-filesystem",
+            args="/app/data/jarvis-files",  # the exact value from issue #6211
+            env="{}",
+            url=None,
+            oauth_file=None,
+            oauth_config=None,
+        ))
+
+    assert exc.value.status_code == 400
+    manager.connect_server.assert_not_called()
+
+
+def test_add_server_still_accepts_valid_json_args(monkeypatch):
+    add_server, manager = _add_server(monkeypatch)
+    fake_session = _FakeSession()
+    monkeypatch.setattr(mcp_routes, "SessionLocal", lambda: fake_session)
+
+    result = asyncio.run(add_server(
+        request=None,
+        name="filesystem",
+        transport="stdio",
+        command="mcp-server-filesystem",
+        args=json.dumps(["/app/data/jarvis-files"]),
+        env="{}",
+        url=None,
+        oauth_file=None,
+        oauth_config=None,
+    ))
+
+    assert result["connected"] is True
+    manager.connect_server.assert_awaited_once()
+    assert manager.connect_server.call_args.kwargs["args"] == ["/app/data/jarvis-files"]
+    assert fake_session.added[0].args == json.dumps(["/app/data/jarvis-files"])
+
+
+def test_add_server_rejects_valid_json_args_that_is_not_a_list(monkeypatch):
+    """Valid JSON that is not a list (e.g. args=5) must not reach
+    StdioServerParameters(args=5), which raises an unhandled TypeError when
+    the error formatter later does " ".join([command, *args])."""
+    add_server, manager = _add_server(monkeypatch)
+    monkeypatch.setattr(mcp_routes, "SessionLocal", lambda: (_ for _ in ()).throw(
+        AssertionError("must not reach the DB when args has the wrong shape")))
+
+    with pytest.raises(HTTPException) as exc:
+        asyncio.run(add_server(
+            request=None,
+            name="filesystem",
+            transport="stdio",
+            command="mcp-server-filesystem",
+            args="5",
+            env="{}",
+            url=None,
+            oauth_file=None,
+            oauth_config=None,
+        ))
+
+    assert exc.value.status_code == 400
+    manager.connect_server.assert_not_called()
+
+
+def test_add_server_still_defaults_empty_args_to_empty_list(monkeypatch):
+    """No behavior change for the common case of an empty Args field."""
+    add_server, manager = _add_server(monkeypatch)
+    fake_session = _FakeSession()
+    monkeypatch.setattr(mcp_routes, "SessionLocal", lambda: fake_session)
+
+    result = asyncio.run(add_server(
+        request=None,
+        name="no-args-server",
+        transport="stdio",
+        command="some-command",
+        args="",
+        env="{}",
+        url=None,
+        oauth_file=None,
+        oauth_config=None,
+    ))
+
+    assert result["connected"] is True
+    assert manager.connect_server.call_args.kwargs["args"] == []