[BOT ISSUE] OpenAI Responses API streaming spans lose output and metrics

[braintrust-bot]

Summary

The OpenAI instrumentation's SSE stream reassembly path hardcodes ChatCompletionAccumulator / ChatCompletionChunk, which is specific to the Chat Completions API. When a user calls client.responses().createStreaming(...), the Responses API emits a different set of SSE event types (response.created, response.output_text.delta, response.completed, etc.) that cannot be deserialized as ChatCompletionChunk. As a result, streaming Responses API spans end up with no output data and no usage metrics.

Non-streaming Responses API calls (client.responses().create(...)) work correctly — the InstrumentationSemConv class already handles input/output/input_tokens/output_tokens fields.

What is missing

TracingHttpClient.tagSpanFromSseBytes() (lines 205–231) needs a parallel code path that:

1. Detects Responses API SSE events (which may start with event: lines like event: response.created before the data: line, unlike Chat Completions which only has data: lines).
2. Uses the OpenAI Java SDK's ResponseAccumulator (analogous to ChatCompletionAccumulator) to reassemble ResponseStreamEvent chunks into a complete Response object.
3. Passes the assembled response JSON through the existing InstrumentationSemConv.tagOpenAIResponse() which already knows how to extract output, input_tokens, output_tokens, and reasoning_tokens from Responses API payloads.

Failure mode

Today when Responses API streaming is used:

• If the first non-empty SSE line is event: response.created (not data:), the code at line 176 falls through to the plain-JSON branch, which tries to parse the entire SSE byte stream as JSON → parse error → span has no output/metrics.
• Even if a data: line happened to come first, line 218–219 would attempt BraintrustJsonMapper.get().readValue(data, ChatCompletionChunk.class) on a Responses API event object → deserialization error → span has no output/metrics.

In both cases the error is caught and logged, but the span is silently incomplete.

Braintrust docs status

• The Java SDK README and Braintrust docs do not explicitly document Responses API streaming support: not_found
• Non-streaming Responses API is handled in code but not documented either.

Upstream sources

• OpenAI Responses API streaming docs: https://platform.openai.com/docs/guides/streaming-responses
• OpenAI Java SDK ResponseAccumulator: available in com.openai:openai-java (the SDK already depended on at [2.8.0,)), analogous to ChatCompletionAccumulator
• OpenAI Java SDK streaming example: https://github.com/openai/openai-java — ResponsesStructuredOutputsStreamingExample.java

Local files inspected

• braintrust-sdk/instrumentation/openai_2_8_0/src/main/java/dev/braintrust/instrumentation/openai/v2_8_0/TracingHttpClient.java — lines 205–231 (tagSpanFromSseBytes hardcodes ChatCompletionAccumulator)
• braintrust-sdk/src/main/java/dev/braintrust/instrumentation/InstrumentationSemConv.java — lines 99–104, 116–150 (correctly handles Responses API fields for non-streaming)
• braintrust-sdk/instrumentation/openai_2_8_0/src/test/java/.../BraintrustOpenAITest.java — has testWrapOpenAiResponses (non-streaming only, no streaming Responses API test)