Cache MCP tools with invalidation

[ktsaou]

Problem

MCP tools are listed on every prompt build, which calls listTools per MCP server sequentially. A slow MCP server can slow all LLM requests.

Solution

• Add a per-mcp-server cache of raw MCP listTools results in MCP state.
• Cache validity is tracked by a per-mcp-server sequence counter.
• On a cache hit, rebuild AI SDK Tool objects from cached MCP tool definitions so the behavior matches a fresh listTools fetch.
• Empty tool lists are treated as valid cache entries.

Invalidation events (existing flows only)

• MCP ToolsChanged notification (now consumed to invalidate that server).
• MCP add/connect/disconnect (existing endpoints).
• MCP listTools failure (existing behavior: mark failed + remove client) also invalidates cache.
• Instance disposal clears caches.

Behavior details

• If cache is valid, MCP.tools returns immediately without calling MCP listTools.
• If cache is invalid or missing, MCP.tools blocks and refreshes (same as current behavior for a fresh fetch).
• Failure behavior is unchanged: failed status + client removal.

Tests

• Added MCP tools cache tests (cache hit, ToolsChanged invalidation, empty tools).
• Ran: bun test test/mcp/tools-cache.test.ts (from packages/opencode).

Notes

• No new config watcher; config reload behavior remains unchanged.