MCP (Model Context Protocol)

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What is traced

• “Was the tool listed” needs a tools/list call earlier in the same client. A call made before any list is recorded without this answer (never as “not listed”), and so is a call made when only the first page of a paged list was read.
• “Do the arguments fit” is checked in your process against the schema the server sent: required keys and JSON types only. The answer is true, false or unknown (when the schema uses something the SDK doesn’t check). false is recorded only for a definite miss.
• A result the server flagged as an error is recorded as a tool error, not as a connection failure. A closed connection or a timeout is recorded as such. Servers differ in how they report an unknown tool (an error result on some, a JSON-RPC error on others); the recorded “was the tool listed” answer is the same either way.
• Hashes are of the schema with the serializer’s noise removed ($schema and a top-level additionalProperties: false), so upgrading the library a server is built with doesn’t look like the tool list changing.

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Supported clients

import { initZespan } from "@zespan/sdk";

initZespan({ apiKey: "zsp_..." }); // that's all: clients created after this are traced

import zespan

zespan.init(api_key="zsp_...")  # that's all: ClientSession and Client are traced

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Python: one record per call

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Turn it off

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Your own MCP server

import { mcpServerTool } from "@zespan/sdk";

server.registerTool(
  "search_orders",
  { description: "Look up an order", inputSchema: { id: z.string() } },
  mcpServerTool("search_orders", async ({ id }, extra) => lookup(id), { server: "orders" }),
);

from zespan import mcp_server_tool

@mcp.tool()
@mcp_server_tool("search_orders", server="orders")
async def search_orders(id: str) -> str:
    return await lookup(id)

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Tools from MCP adapters

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Where it shows up

• The spans appear in the trace view with the span kind mcp_client (or mcp_server_tool).
• Each tools/call is also stored as an MCP tool call, with the server name and version, the JSON-RPC error code and the schema hashes.
• Zespan uses the recorded answers (tool listed, arguments valid, error code, tool error, connection failure, tool list changed together with a server version change) to say whether a failed MCP call started with the model or with the MCP server.

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Limits

• A tool the model provider runs on its own servers (a hosted MCP tool inside a model call) never passes through your client and can’t be seen.
• Whether the protocol version is recorded depends on the client library: version 1 of the TypeScript client has no accessor for it, so it is recorded only when the transport reports it.
• The OpenTelemetry conventions for MCP are still changing; Zespan maps the current attribute names and keeps the older error-code name readable.

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Next steps