Hermes Agent lost MCP tool discovery when a tool-list event exceeded one MiB

An MCP connection can pass its handshake and still become unusable when the client receives the list of tools it is allowed to call. The Hermes Agent report describes server responses containing thousands of tool descriptions, including a several-megabyte Cloudflare list. A one-megabyte SSE event decoder limit introduced along the dependency path rejected the subsequent tools/list event and surfaced a generic stream-ended error, despite earlier healthy initialization.

The failure is a framing and compatibility issue, not proof of an attacker obtaining tool authority. It also must not be merged with an unrelated ten-megabyte HTTP body guard. The amount of data in one SSE event, the cumulative response body, the number of tools and the nesting of each tool schema consume resources at different stages of parsing. Retry loops can compound the incident by disabling a server after repeated opaque errors.

The MCP SDK later offered configurable event limits, but the supplied issue did not verify a completed Hermes application fix. A sound reproduction covers a small registry, a single event just above the threshold, and a paginated or streaming alternative. The correct response should identify the violated limit and preserve connectivity for unaffected servers rather than silently treating tool discovery as a disconnected transport.