feat(mcp): expose plain query parameter for auto-expand search

[fxstein]

Problem

The SDK search() method accepts either query (plain string → auto-expanded by the trained model) or queries (pre-expanded typed sub-queries). The CLI qmd query "text" uses the query path, triggering the full pipeline: expansion → parallel BM25 + vector → RRF → reranking.

However, the MCP query tool only exposes the queries path via the searches parameter. There is no way to send a plain text query through MCP and get the full auto-expand pipeline.

The tool description Strategy table even says: "Unknown vocabulary | Use a standalone natural-language query (no typed lines) so the server can auto-expand it" — but the tool schema doesn't accept a standalone query.

Fix

Add an optional query string parameter to the MCP query tool. When provided, route to store.search({ query }) which calls hybridQuery() with auto-expansion. query and searches are mutually exclusive — providing both returns an error.

The searches parameter is made optional (was required). Validation ensures at least one of query or searches is provided.

Tool description Strategy table updated to recommend query as the default approach.

Testing

Built and tested locally against a 7,791 document index:

Query type	Latency	Results	Notes
query: "how does authentication work" (cold)	961ms	5	Expansion model loading
query: "how does authentication work" (warm)	259ms	5	Cached — 3.7× faster
query: "DEC-0054 sub-agent spawn protocol"	273ms	5	93% top hit ✅
searches: [{type: "lex", query: "\"DEC-0054\""}]	197ms	5	Existing path, unchanged
searches: [{type: "vec", query: "..."}]	622ms	5	Existing path, unchanged

Existing searches parameter works identically to before — no breaking changes.

Environment

• QMD: v2.0.1
• Platform: macOS (Apple Silicon)
• Node: v24.2.0