OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT=true does not populate gen_ai.prompt/gen_ai.completion span attributes

[victoraditya]

🔴 Required Information

Describe the Bug: Setting OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT=true does not populate gen_ai.prompt or gen_ai.completion attributes on generate_content spans.
These remain null in the OTLP-exported spans, making it impossible for observability backends (e.g. Dynatrace AI Observability) to display conversation content. The
gcp.vertex.agent.llm_request / gcp.vertex.agent.llm_response attributes are also null. Additionally, we observe repeated warnings: "Tried calling _add_event on an ended span."

Steps to Reproduce:

1. Deploy an ADK agent with OTLP export configured
2. Set env var OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT=true
3. Send a request to the agent
4. Query exported spans for gen_ai.prompt, gen_ai.completion
5. All attributes are null on generate_content spans

Expected Behavior: gen_ai.prompt and gen_ai.completion should contain prompt/response text per OpenTelemetry GenAI semantic conventions.

Observed Behavior:

• All prompt/response attributes are null
• Logs show: "Tried calling _add_event on an ended span." (repeated 4-5 times per invocation)
• Other span metadata works fine: gen_ai.agent.name, gen_ai.request.model, token counts, duration

Environment Details:

• ADK Library Version:
  1.28.1
• OS: Linux (GKE container)
• Python Version:
  3.12

Model Information:

• Are you using LiteLLM: No
• Which model: gemini-3-flash-preview

---

🟡 Optional Information

Regression: Unknown — first attempt at enabling prompt content capture.

Logs:

{"message": "Tried calling _add_event on an ended span.", "severity": "WARNING", "logger_name": "opentelemetry.sdk.trace"}

Additional Context: The env var is read in google/adk/telemetry/tracing.py line ~80. The _add_event on ended span warning suggests content serialization completes after the
span has already been closed by BatchSpanProcessor. Related: #4829

Minimal Reproduction Code:

from google.adk.telemetry.setup import maybe_set_otel_providers
maybe_set_otel_providers([])

With OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT=true

gen_ai.prompt and gen_ai.completion are null on all generate_content spans

How often has this issue occurred?:

• Always (100%)