The traceparent header

traceparent: 00-{trace-id}-{span-id}-01. Client sends; server picks up as parent span.

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Auto-injected by client

A2AClient client = A2AClient.builder()
    .baseUrl(url)
    .addInterceptor(new TracePropagationInterceptor())
    .build();
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Auto-extracted at server

A2A server auto-extracts traceparent, sets it as current span parent. Whole invocation becomes a child span.