How MCP Tools Behave

邱敬幃 Pardn Chiu · Last updated 2026-10-01

How tools from connected MCP servers are named, capped, confirmed and kept alive.

Tool naming

MCP-exposed tools are auto-registered with the format:

mcp__<server_name>__<tool_name>

Example: mcp__github__create_issue, mcp__sqlite-notes__read_query.

Result size cap

Each MCP tool result is capped at 1 MiB. When exceeded, the result is truncated with the marker:

[mcp output truncated: <total> bytes total, <kept> kept]

This avoids OpenAI Responses API's 10 MB single-tool-output limit triggering a same-signature retry storm. SQLite SELECT * on a large table will hit this — add LIMIT / WHERE.

Confirm behavior

MCP tools route through the most conservative defaults:

Every MCP tool call goes through the same confirm gate as a built-in tool, under whatever permission mode the request carries
No per-server read_only toggle — Agenvoy does not extend trust to third-party servers because their behavior is unverifiable (a Slack MCP could silently send messages, a Filesystem MCP could silently write files)

Trust is granted per tool, not per server: /mcp → server → tools (or POST /v1/allowlist with a tool block) replaces the auto-approve entries for one prefix, leaving unrelated rules alone. Every entry must start with that prefix, and prefix* collapses the rest into a whole-server grant. The list lives in ~/.config/agenvoy/allow_tool.

Lifecycle

Startup: app.NewMCP calls mcp.New(ctx, sid), then RegisterAll(ctx), then starts Watch(ctx), before the agent registry is built; clients close on shutdown
Live tool refresh: clients subscribe to the server's notifications/tools/list_changed and re-register that server's tools when its catalog changes — no restart needed for a server that adds or drops tools
Server instructions: whatever a server declares as its instructions is surfaced into the agent system prompt, so per-server usage rules reach the model
Per-server failures: a start or tool-list failure logs a warning and skips that server; core functionality never blocks
Manual recovery: POST /v1/mcp/reconnect reconnects every client and re-registers tools; /mcp → server → reconnect in the TUI does the same for one server