[feature] Mount an imported MCP router's tools beside an app's own tools at the same level #2286

The problem

An app that wraps an imported MCP server can't add its own tools at the top level. These are the only two router() overloads (apps@0.0.1-beta.34, unchanged in 0.0.1-beta.84 / Executor 2.0.0-beta.16):

router(children: Record<string, RouterChild>, options?)
router(source: RouterDeclaration, options)   // only overrides metadata

So with

tools: router(withApprovals(await mcpRouter({ url }), policy), { title: "cass" })

the MCP tools are tools.cass.cass_search and so on. To add one app-owned tool (for us, a feedback.* router), the MCP router has to move under a key: router({ sessions: mcpSource, feedback: feedbackRouter }). That renames every imported tool to tools.cass.sessions.cass_search and breaks existing callers and instructions. We deployed exactly that on self-host 2.0.0-beta.10, then reverted it for this reason.

Proposed shape

Let a router mix an imported source's tools with its own children at the same level. For example:

• router({ ...mcpSource, feedback }), or a mergeRouters(mcpSource, router({ feedback })) helper.
• Or an option such as router({ "": mcpSource, feedback }) that mounts a source at the parent path.

Name collisions could fail at evaluation, the way duplicate keys do today. Lazy and dynamic resolution of the imported source should keep working: names that don't match a static child would be forwarded to the source.

Alternatives

• Mount under a sub-key: this breaks tool paths.
• A dynamicRouter that re-lists the MCP tools and resolves app tools by hand: this duplicates mcpRouter and loses its caching and approvals wiring.
• A separate app: an extra install just to hold one tool, and cross-app calls are deferred.

Where it belongs

Executor Cloud, Self-host (Docker), Self-host (Cloudflare), CLI

Before you submit

• I searched the open issues for a duplicate.