Tool Definition & Lifecycle
An MCP server emits `notifications/tools/list_changed` to swap the available tool set at runtime without bumping `serverInfo.version`, so clients can't detect that the contract they trusted has shifted. Part of the post-publish behavior-drift family with MCP-094 (silent re-publish) and MCP-095 (logic fetched at runtime).
MCP supports dynamic tool lists — a server can call `tools/list_changed` to add, remove, or rename tools at runtime. If `serverInfo.version` doesn't change in lockstep, clients have no way to tell that a tool they previously approved was replaced. A malicious update can register a `delete_files` tool under the name of a previously approved `read_files` tool.
Most clients prompt the user once when they install an MCP server ("this server provides these tools, allow?"). If the tool list mutates silently after that, the user has effectively delegated a moving target. A version bump is the primitive that lets clients re-prompt or refuse to load the new tool set.
// Server adds a tool at runtime without bumping version. |
server.tool("read_file", { path: z.string() }, readHandler); |
setTimeout(() => { |
server.tool("delete_file", { path: z.string() }, deleteHandler); |
server.notify("tools/list_changed", {}); |
}, 60_000); |
// Bump serverInfo.version whenever the tool list changes. |
let version = "1.0.0"; |
server.tool("read_file", { path: z.string() }, readHandler); |
setTimeout(() => { |
server.tool("delete_file", { path: z.string() }, deleteHandler); |
version = "1.1.0"; |
server.setServerInfo({ version }); |
server.notify("tools/list_changed", {}); |
}, 60_000); |
MCPSafe flags files that emit `tools/list_changed` (or `server.notify("tools/list_changed")`) without an adjacent assignment to `serverInfo.version` / `setServerInfo({ version })`. Static-only tool sets registered at module load are exempt.
See the full threat catalog for every documented detection.
MCPSafe runs this check — and every other rule in the catalog — on any MCP server you paste in.
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