W3C baggage header not propagated into Kafka producer record headers

[ls-adrian-chong]

Tracer Version(s)

1.62.0 (latest as of 2026-05-05), and all versions since baggage support was introduced (v1.33.0 / 2025-03-03)

Java Version(s)

17 (Amazon Corretto), also reproduced on Temurin 17

JVM Vendor

Amazon Corretto / Eclipse Temurin

Bug Report

The W3C baggage header is not injected into Kafka producer record headers, despite DD_TRACE_PROPAGATION_STYLE=datadog,tracecontext,baggage being configured. The BaggagePropagator is registered and works correctly for HTTP client instrumentations, but fails silently for Kafka.

What works:

• ot-baggage-* headers (Datadog propagation style) — propagated correctly into Kafka
• W3C baggage header for HTTP-to-HTTP propagation — works correctly
• Baggage extraction from incoming HTTP requests — works correctly (visible as span tags)

What doesn't work:

• W3C baggage header injection into Kafka producer record headers

Root Cause

In KafkaProducerInstrumentation.java (ContextPropagationAdvice), the inject call passes the span as the context:

defaultPropagator().inject(span, record.headers(), setter);

BaggagePropagator.inject() calls Baggage.fromContext(context) which does context.get(BAGGAGE_KEY). But AgentSpan.get() (in AgentSpan.java:227-229) delegates non-span keys to Context.root():

default <T> T get(@Nonnull ContextKey<T> key) {
    return SPAN_KEY == key ? (T) this : Context.root().get(key);
}

The Baggage object lives on Context.current() (the thread-local context), not on the span. So Baggage.fromContext(span) always returns null.

HTTP client instrumentations work because they pass Context.current() to the propagator, which includes both the span and the baggage.

The Fix

Replace span with Context.current() in the Kafka producer instrumentation:

+ import static datadog.context.Context.current;

// In ContextPropagationAdvice (both kafka-clients-0.11 and kafka-clients-3.8):
- defaultPropagator().inject(span, record.headers(), setter);
+ defaultPropagator().inject(current(), record.headers(), setter);

Proof

We built a patched agent with this fix and confirmed it works. Kafka consumer output with the patched agent:

baggage:sd-routing-key=test123,e2e-run-id=run456,
x-datadog-trace-id:11803532876627986230,
x-datadog-parent-id:5176844657287312305,
traceparent:00-4bf92f3577b34da6a3ce929d0e0e4736-47d7d8c9a6fd33b1-01,
tracestate:dd=s:1;p:47d7d8c9a6fd33b1;t.dm:-0;t.tid:4bf92f3577b34da6

Expected Behavior

When baggage is included in DD_TRACE_PROPAGATION_STYLE, the W3C baggage header should be injected into Kafka producer record headers, just as it is for HTTP client requests.

Reproduction Code

Minimal Spring Boot app with a Kafka producer:

@PostMapping("/send")
public ResponseEntity<String> send(@RequestBody String body) {
    kafkaTemplate.send("test-topic", "key", body);
    return ResponseEntity.ok("sent");
}

Configuration:

DD_TRACE_PROPAGATION_STYLE=datadog,tracecontext,baggage
DD_KAFKA_CLIENT_PROPAGATION_ENABLED=true

Send a request with baggage: sd-routing-key=test123 header → consume from test-topic and inspect headers → no baggage header present.

Affected Files

• dd-java-agent/instrumentation/kafka/kafka-clients-0.11/src/main/java/datadog/trace/instrumentation/kafka_clients/KafkaProducerInstrumentation.java (lines 221, 243)
• dd-java-agent/instrumentation/kafka/kafka-clients-3.8/src/main/java17/datadog/trace/instrumentation/kafka_clients38/ProducerContextPropagationAdvice.java (lines 46, 68)

Related Issues

• Baggage not being propagated in zio #6772 — Baggage not being propagated in zio (same root cause)
• Baggage injected via DD_TRACE_HEADER_BAGGAGE is not retrievable via OTEL Baggage API #10031 — Baggage injected via DD_TRACE_HEADER_BAGGAGE not retrievable via OTEL Baggage API (related context disconnect)

Impact

This blocks use cases where W3C baggage needs to flow through Kafka (e.g., routing keys for ephemeral environments, correlation IDs for end-to-end testing). Python's ddtrace handles this correctly with DD_KAFKA_PROPAGATION_ENABLED=true.