get_propagated_context() ignores standard W3C traceparent and baggage headers on FastAPI requests unless Google-Agent-Engine-Traceparent is set #7844

🔴 Required Information

Describe the Bug:
AdkWebServer.get_fast_api_app() (src/google/adk/cli/fast_api.py) installs a @app.middleware("http") hook (context_propagation) that calls get_propagated_context(request) (src/google/adk/telemetry/_agent_engine.py:59-84) and attaches the returned OpenTelemetry context for the duration of the request.

#4767 and #4972 were closed on the assumption that this middleware extracts inbound W3C traceparent headers so adk api_server / adk web continues the caller's distributed trace. However, get_propagated_context() only runs TraceContextTextMapPropagator().extract(...) on the proprietary Google-Agent-Engine-Traceparent header:

# src/google/adk/telemetry/_agent_engine.py:59-84
def get_propagated_context(request: fastapi.Request) -> context.Context:
  """Propagates context from the request headers."""
  ctx = context.get_current()

  if _GOOGLE_TRACEPARENT_HEADER in request.headers:
    original_traceparent = request.headers[_GOOGLE_TRACEPARENT_HEADER]
    ctx = baggage.set_baggage(
        _GOOGLE_TRACEPARENT_BAGGAGE_KEY,
        original_traceparent,
        context=ctx,
    )

  if _GOOGLE_AE_TRACEPARENT_HEADER in request.headers:
    ae_traceparent = request.headers[_GOOGLE_AE_TRACEPARENT_HEADER]
    extracted_ctx = tracecontext.TraceContextTextMapPropagator().extract(
        carrier={"traceparent": ae_traceparent}, context=ctx
    )
    if extracted_ctx is not ctx:
      ctx = baggage.set_baggage(
          _TRACEPARENT_BAGGAGE_KEY, ae_traceparent, context=extracted_ctx
      )

  return ctx

As a result, when any non-Agent-Engine caller (an API gateway, load balancer, or upstream microservice) sends a standard W3C traceparent (and/or baggage) header to get_fast_api_app():

1. traceparent is only stashed in baggage under "google_traceparent" (which TopSpanProcessor reads to stamp supportID), never extracted as the parent SpanContext. Every inbound /run or /run_sse request starts a brand-new root trace ID instead of parenting under the caller's span.
2. Inbound W3C baggage headers (e.g., tenant, experiment, or evaluation metadata) are not extracted at all.

Steps to Reproduce:

1. Create a FastAPI app via get_fast_api_app(...) with an active TracerProvider and InMemorySpanExporter.
2. Send a POST request to /run with a valid W3C header:
   traceparent: 00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01
   and baggage: tenant_id=acme.
3. Inspect the exported invocation / invoke_agent spans.

Expected Behavior:

• When Google-Agent-Engine-Traceparent is present, it continues to take precedence as the extracted parent span context (preserving Vertex Agent Engine supportID behavior).
• When Google-Agent-Engine-Traceparent is absent and a standard W3C traceparent header is present, get_propagated_context() extracts traceparent (and tracestate) as the parent SpanContext so ADK spans join the caller's trace (trace_id == 0x4bf92f3577b34da6a3ce929d0e0e4736).
• Inbound W3C baggage headers are extracted via W3CBaggagePropagator without clobbering the internal google_traceparent / traceparent baggage keys.

Observed Behavior:
The exported ADK spans have a newly generated trace_id unrelated to 4bf92f3577b34da6a3ce929d0e0e4736, and inbound W3C baggage entries are dropped.

Environment Details:

• ADK Library Version: main (2.13.0+)
• Desktop OS: macOS / Linux
• Python Version: 3.11+

Model Information:

• Are you using LiteLLM: No
• Which model is being used: N/A (telemetry / FastAPI middleware layer)

🟡 Optional Information

Regression:
#4767 reported this gap and #4972 was closed assuming commit ffa057c1 resolved it via get_propagated_context, but get_propagated_context only extracts Google-Agent-Engine-Traceparent.

Minimal Reproduction Code:

from fastapi import FastAPI, Request
from fastapi.testclient import TestClient
from opentelemetry import baggage, trace
from google.adk.telemetry._agent_engine import get_propagated_context

app = FastAPI()

@app.get("/test")
def endpoint(request: Request):
  ctx = get_propagated_context(request)
  span_ctx = trace.get_current_span(ctx).get_span_context()
  return {
      "trace_id": f"{span_ctx.trace_id:032x}",
      "tenant": baggage.get_baggage("tenant_id", context=ctx),
  }

client = TestClient(app)
resp = client.get(
    "/test",
    headers={
        "traceparent": "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01",
        "baggage": "tenant_id=acme",
    },
)
# Returns trace_id="00000000000000000000000000000000" and tenant=None
print(resp.json())

How often has this issue occurred?:

• Always (100%)