🐛 Bug Report: MCP `InstrumentedStreamWriter.send()` assumes dict-shaped `request.result` and can fail for object/model results

Which component is this bug for?

Traceloop SDK

📜 Description

In opentelemetry-instrumentation-mcp, InstrumentedStreamWriter.send() assumes request.result is always a dict:

if "isError" in request.result:
    if request.result["isError"] is True:
        span.set_status(
            Status(
                StatusCode.ERROR,
                f"{request.result['content'][0]['text']}",
            )
        )

Impact

• Response-stream error handling can fail at runtime
• Error spans may not be set correctly
• Behavior is inconsistent across MCP instrumentation paths

Possible errors

• TypeError: argument of type 'X' is not iterable
• TypeError: 'X' object is not subscriptable
• KeyError: 'content'
• IndexError: list index out of range

File

• packages/opentelemetry-instrumentation-mcp/opentelemetry/instrumentation/mcp/instrumentation.py

Method

• InstrumentedStreamWriter.send()

Proposed fix
Normalize request.result access before reading fields:

• use .get(...) for dicts
• use getattr(...) for objects

👟 Reproduction steps

1. Use MCP instrumentation with a response/result object that exposes fields via attributes rather than dict keys.
2. Trigger a response where the MCP result represents an error.
3. Exercise the InstrumentedStreamWriter.send() path.
4. Observe failure or missing error handling around:

request.result["content"][0]["text"]

👍 Expected behavior

The stream writer should support both of these shapes:

dict-style

request.result["isError"]
request.result["content"][0]["text"]

object-style

request.result.isError
request.result.content[0].text

without crashing, and should always set the span status correctly for MCP error responses.

👎 Actual Behavior with Screenshots

The code currently assumes only dict-style access in InstrumentedStreamWriter.send().

This causes inconsistent handling and can break when request.result is not a dict.

🤖 Python Version

No response

📃 Provide any additional context for the Bug.

No response

👀 Have you spent some time to check if this bug has been raised before?

• I checked and didn't find similar issue

Are you willing to submit PR?

None

Start in packages/opentelemetry-instrumentation-mcp/opentelemetry/instrumentation/mcp/instrumentation.py, focusing on InstrumentedStreamWriter.send(). Reproduce the error-handling path with both dict-style and object-style request.result values, then verify that the stream writer does not crash and sets the span status correctly for MCP error responses.