Stream Anthropic /messages responses in E2E fake handlers

[stephentoub]

Why

The C# SDK leg in copilot-agent-runtime CI started failing with:

Failed to finalize Anthropic message stream: stream ended without producing a Message with role=assistant

The runtime recently ported Anthropic model orchestration to a native Rust transport. That transport sends /messages requests with stream: true and unconditionally drains any 2xx response as an SSE stream, then finalizes it to a Message. Our E2E fake handlers returned buffered application/json for /messages, which has no SSE events, so the newer CLI raised MissingFinalMessage.

This is a test-fidelity gap, not a production bug: real Anthropic returns text/event-stream for streaming requests. The SDK's own CI still passes because it runs against pinned CLI 1.0.67 (before the transport change), while the runtime leg builds the CLI from main.

What

Update all six SDK E2E fake request handlers so the /messages endpoint returns a proper Anthropic Messages SSE sequence (message_start -> content_block_start -> content_block_delta -> content_block_stop -> message_delta -> message_stop) when the request body requests streaming, while keeping the buffered JSON response for non-streaming requests.

Handlers updated:

• dotnet/test/E2E/CopilotRequestE2EProvider.cs
• nodejs/test/e2e/session_config.e2e.test.ts
• python/e2e/_copilot_request_helpers.py
• go/internal/e2e/copilot_request_helpers_test.go
• rust/tests/e2e/session_config.rs
• java/src/test/java/com/github/copilot/CopilotRequestTestSupport.java

The change is backward compatible: with the pinned CLI (which does not stream Anthropic), the new branch is inert; with the newer CLI it satisfies the SSE expectation. Only C# was failing today, but the other five share the same stub and would break the moment the pinned CLI is bumped past the transport change, so all six are fixed together.

Verification

• C#: dotnet build passes
• Node: tsc --noEmit clean
• Python: py_compile clean
• Java: test-compile BUILD SUCCESS, plus spotless:apply
• Go and Rust could not be compiled in this environment (no go toolchain; no clang for Rust's ring build). Both edits were reviewed by hand against the existing streaming patterns in their files.

Generated by Copilot

[Copilot]

Pull request overview

Updates the SDK E2E “fake upstream” request handlers across all language legs to correctly emulate Anthropic /messages streaming by returning a text/event-stream (SSE) sequence when the request body sets stream: true. This aligns the test stubs with the newer CLI/runtime behavior that always drains successful /messages responses as an SSE stream.

Changes:

• Add an Anthropic Messages SSE “message_start … message_stop” response path for streaming /messages requests.
• Preserve the existing buffered application/json response for non-streaming /messages requests.
• Thread request bodies into inference response builders where needed (e.g., Rust/Node/Java) to detect streaming requests.

Show a summary per file

File	Description
rust/tests/e2e/session_config.rs	Adds SSE response helpers and returns Anthropic /messages SSE when the request asks for streaming.
python/e2e/_copilot_request_helpers.py	Returns Anthropic /messages SSE body for streaming requests; keeps JSON for non-streaming.
nodejs/test/e2e/session_config.e2e.test.ts	Adds SSE builder + Anthropic stream body, switching based on "stream": true in the request body.
java/src/test/java/com/github/copilot/CopilotRequestTestSupport.java	Introduces anthropicMessageSseBody and returns SSE for streaming /messages requests.
go/internal/e2e/copilot_request_helpers_test.go	Adds buildAnthropicMessageSSEBody and uses it for streaming /messages responses.
dotnet/test/E2E/CopilotRequestE2EProvider.cs	Switches /messages handler to return Anthropic SSE sequence when stream: true is detected.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0
• Review effort level: Low

[github-actions]

Cross-SDK Consistency Review ✅

This PR does an excellent job of maintaining consistency across all six SDK implementations. A quick audit:

Verified consistent across all 6 SDKs (Node.js, Python, Go, .NET, Java, Rust)

Check	Result
All 6 SDKs updated in the same PR	✅
Identical SSE event sequence (message_start → content_block_start → content_block_delta → content_block_stop → message_delta → message_stop)	✅
Identical JSON payloads for each event	✅
Same content-type header (text/event-stream)	✅
Same SSE wire format (event: {name}\ndata: {json}\n\n)	✅
Same synthetic response text ("OK from the synthetic stream.")	✅
Stream detection keyed on "stream": true in request body	✅

Minor style note (no action required)

Five SDKs extract the synthetic text into a named constant (SYNTHETIC_TEXT, syntheticResponseText, SyntheticText, etc.). The Node.js test uses the same string inline as a literal. Functionally identical — just a style nit if you want to harmonize.

No cross-SDK consistency gaps introduced by this PR.

Generated by SDK Consistency Review Agent for issue #1868 · sonnet46 1.1M · ◷