What
Tracing is an optional feature that records what Claude Code does as timed steps called spans, for use with OpenTelemetry monitoring tools. When tracing is on and a tool call succeeds, Claude Code writes the result as a tool.output attribute on the span. A tool call is one action Claude takes, such as a web search. This covers:
- Results from MCP tools, which are tools provided by connected services
- Results from a small set of built-in tools
Output from the account memory server is redacted. Long output is cut short and marked as truncated, with its original length recorded.
Why
If you collect tracing data, tool results may now appear in it. Keep that in mind when deciding where the data goes.
Names in the bundletool.output
tool.output
Environment variables modified, high confidence
| `OTEL_LOG_TOOL_CONTENT` | Set to `1` to include tool content in the `tool.output` OpenTelemetry span event. Span attributes carry tool content under [their own gates](/docs/en/monitoring-usage#new-context-gates). Requires [tracing](/docs…see the edit
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
It is not clear which built-in tools are included or exactly what differs from the previous build.
Anthropic's documentation agrees
tool.output on Environment variables
Anthropic's release notes agree
Added MCP tool, WebFetch and WebSearch outputs to the tool.output OpenTelemetry span event when OTEL_LOG_TOOL_CONTENT=1