Nothing limits how fast an agent can call #417

Background

Tool calls are handled as they arrive. There is no rate limit, no concurrency
cap and no queue, so an agent in a loop — which is the normal failure mode of an
agent, not an exotic one — can issue calls as fast as it can generate them.

With paid_call in the tool list, that loop costs money (L210). With
bazaar_search it costs an unbounded number of outbound connections from the
host the server runs on.

What to build

• A concurrency cap on in-flight tool calls, and a rate limit per tool.
• Exceeding it should be a structured error that tells the agent to slow down,
  with a retry hint — not a silent queue that makes the agent think it is hanging.
• Expensive tools deserve tighter limits than cheap ones; make the limit
  per-tool rather than global.
• Count rejections in a metric so a looping agent is visible rather than merely
  expensive.

Acceptance criteria

• In-flight tool calls are capped
• A per-tool rate limit exists and is configurable
• Exceeding a limit returns a structured error with a retry hint
• Rejections are counted in a metric
• A test proves a burst is throttled rather than queued indefinitely

Drips Wave · Complexity: Intermediate · 150 points

Repo: Divinity-Labs/Lens. The MCP server is src/mcp/server.ts
(BazaarMcpServer) with error codes in src/mcp/codes.ts. It is started by
npm run mcp:server → examples/mcp-server/run.ts, and there is an agent
example at examples/mcp-server/agent.ts. Tests are tests/mcp.test.ts.

Run the tests: npm test. The tests are vitest, not jest. A single file
is npx vitest run tests/mcp.test.ts.

Load test (prices) is already failing on main. Not your PR.

This surface signs with a real mainnet key. examples/mcp-server/run.ts
requires MCP_AGENT_SECRET_KEY_MAINNET and builds a signer for
stellar:pubnet. Anything you change here can move real money, so prefer
failing closed, and never log, echo or test with a real secret key.

Never elide an address, a contract id or a key in code, comments or
fixtures. A previous batch shortened a contract id and five contributors
invented plausible-looking ids to fill the gap. Write it in full or not at all.

An agent is the least able caller to notice a wrong answer. It will reason
over whatever it is handed, confidently. Prefer a structured error over a
best-effort result.

Required: Before submitting, join the contributor Telegram so your work can be tracked and counted toward the Stellar Wave: https://t.me/+fxHXq8f1SwlkZDBk