MCP tools
Tool calls in the stream and in traces
How tool calls show up: tool_call and tool_result SSE events, the Playground, and the tool.execute spans in Tracing.
Reference1 min readUpdated 23 October 2026
Every tool call an agent makes is visible three ways: in the API's event stream as it happens, beside the reply in the Playground, and in the request's trace.
In the stream
POST /v1/chat/stream sends a tool_call event when the agent calls a tool and a tool_result event with what came back, then carries on with delta events for the answer.
data: {"type": "tool_call", "id": "call_1", "name": "get_order", "input": {"order_id": "10482"}}data: {"type": "tool_result", "id": "call_1", "name": "get_order", "output": "{\"status\": \"shipped\", \"eta\": \"2026-10-27\"}"}data: {"type": "delta", "content": "Your order shipped and should arrive on 27 October."}| Event | Fields |
|---|---|
tool_call | id (pairs the call with its result), name, input (the arguments the model chose) |
tool_result | id, name, output (the full output; the model itself sees at most 2,500 characters) |
for (const e of events) { if (!e.startsWith("data: ")) continue const event = JSON.parse(e.slice(6)) if (event.type === "tool_call") console.log("calling", event.name, event.input) if (event.type === "tool_result") console.log("result of", event.name, event.output.length, "chars") if (event.type === "delta") process.stdout.write(event.content)}Full event list: Streaming events.
In the Playground
Each Playground reply lists the tools it called. The details panel's Tools tab shows the agent's MCP tools and their parameters, and Log shows each call.
In Tracing
| Span | What it records |
|---|---|
mcp.discovery | Loading the agent's MCP servers and tools: mcp.servers.count, mcp.tools.count |
agentic.round[n] | One round of the model deciding, calling tools and answering |
tool.execute | One tool call: gen_ai.tool.name, gen_ai.tool.call.id, agentic.round, tool.source (mcp, rag or dedup), tool.success, tool.output.length, plus the input and output as span events with personal details redacted |
Filter the Tracing list by has tools to find replies that used them. See Trace reference.
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