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[bot] Anthropic MCP connector tool calls (mcp_tool_use/mcp_tool_result) produce broken, input-less child spans

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Issue-Typ
Bug
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Klar beschrieben
Aktivitätsstatus
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Tech-Stack
java
Bereich
observability

Rechercherichtung

Beginne in InstrumentationSemConv.addAnthropicServerSideChildSpans in braintrust-sdk/src/main/java/dev/braintrust/instrumentation/InstrumentationSemConv.java, wo Blöcke durch type dispatcht werden; lies auch ANTHROPIC_SERVER_TOOL_USE_TYPE, isAnthropicToolResultType und die call == null-Fallbacks. Registriere mcp_tool_use als Aufruftyp, damit die bestehende suffixbasierte Ergebniszuordnung es mit mcp_tool_result paart. Spiegle die bestehende server_tool_use-Abdeckung in BraintrustAnthropicServerSideSpansTest.java und führe diese Testklasse aus; fertig bedeutet, dass der MCP-Child-Span den echten Toolnamen und input_json trägt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.

Beschreibung

bug java

Summary

InstrumentationSemConv.addAnthropicServerSideChildSpans dispatches Anthropic Messages response content blocks by checking each block's "type" against the literal string "server_tool_use" (see ANTHROPIC_SERVER_TOOL_USE_TYPE and the dispatch check). Anthropic's MCP connector feature emits a different, distinct pair of block types for server-executed MCP tool calls: "mcp_tool_use" (the call) and "mcp_tool_result" (the result). "mcp_tool_use" does not match the "server_tool_use" check, so it is never registered in the call-tracking map used to pair calls with results.

Meanwhile "mcp_tool_result" does match the result-type check, which is a suffix check (type.endsWith("_tool_result") && !type.equals("tool_result")). So the result proceeds through the pairing logic, never finds its matching call, and is emitted alone with call == null.

Consequence for every MCP connector tool invocation:

  • The child span is named generically "mcp_tool" (derived by stripping "_tool_result" off the result's type) instead of the real tool name, because the fallback naming path only runs when call == null.
  • input_json is never set on the span, because the input-extraction path also returns null when call == null. The tool's name, server_name, and input arguments — which live only in the dropped mcp_tool_use block — are permanently lost from the structured child span.

The raw response body is still present in the parent LLM span's full JSON passthrough, so the data isn't gone from Braintrust entirely, but the per-tool-call child span that Braintrust's UI specifically surfaces for tool calls is broken for every MCP connector invocation — call name and input are unrecoverable from that span.

Braintrust docs status: supported

Braintrust's docs at https://www.braintrust.dev/docs/integrations/ai-providers/anthropic describe this capability for the Python SDK: tool calls (including MCP connector tool calls) are captured "with its input and result ... named after the tool" as dedicated spans. This confirms the capability is documented/expected, just not delivered correctly by the Java SDK.

Why this isn't speculative

The identical bug (same mechanism: literal match on "server_tool_use", suffix-based result matching that doesn't require a paired call) was already found and fixed in Braintrust's Python SDK — see braintrustdata/braintrust-sdk-python#797 (fixed in py-sdk-v0.44.1) — and is tracked as still-open for the Go SDK in braintrustdata/braintrust-sdk-go#107. The fix was never ported to the Java SDK.

Upstream sources

Local repo files inspected

  • braintrust-sdk/src/main/java/dev/braintrust/instrumentation/InstrumentationSemConv.java — ANTHROPIC_SERVER_TOOL_USE_TYPE constant and dispatch check; addAnthropicServerSideChildSpans (call registration loop); isAnthropicToolResultType (suffix-based result matching); emitAnthropicServerToolSpan, anthropicToolSpanName, anthropicToolSpanInput (fallback behavior when call == null).
  • braintrust-sdk/instrumentation/anthropic_2_2_0/src/test/java/dev/braintrust/instrumentation/anthropic/v2_2_0/BraintrustAnthropicServerSideSpansTest.java — only exercises web_search/server_tool_use; no mcp_tool_use/mcp_tool_result test coverage exists.
  • Repo-wide search for mcp_tool_use / mcp_tool_result / "MCP connector" — zero matches outside this finding.

Suggested fix

Recognize "mcp_tool_use" as an additional call-type alongside "server_tool_use" when populating the call-tracking map (the existing suffix-based result matching already handles "mcp_tool_result" correctly once a matching call is registered), and add a test mirroring the existing server_tool_use test for the MCP connector block pair.

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