fix: emit reasoning role for AG-UI reasoning messages

[by291 (by291)]

Summary

• emit the protocol-defined reasoning role for REASONING_MESSAGE_START events
• avoid forwarding the internal assistant response role into AG-UI reasoning messages
• add a handler-level regression assertion over the emitted SSE payload

Problem

Agent response updates normally use the assistant role, including updates that contain TextReasoningContent. The AG-UI hosting conversion forwarded that role into REASONING_MESSAGE_START, producing role assistant.

AG-UI requires this event role to be the literal reasoning. Strict clients such as @ag-ui/client reject the current payload during schema validation. The protocol decision is also documented by ag-ui-protocol/ag-ui#1416.

Testing

• go test ./...
• go test -race ./...
• go vet ./...

The regression test was verified to fail before the production change with actual assistant and expected reasoning.

[Copilot]

Pull request overview

This PR fixes AG-UI hosting SSE output so REASONING_MESSAGE_START events always emit the protocol-defined role ("reasoning") rather than forwarding the agent update’s internal role (commonly "assistant"), and adds a regression assertion to prevent reintroducing the mismatch that breaks strict AG-UI clients.

Changes:

• Emit role: "reasoning" for REASONING_MESSAGE_START regardless of the agent update role.
• Stop defaulting reasoning-message roles to "assistant" in the hosting conversion path.
• Strengthen TestHandler_ReasoningContent_EmitsReasoningEvents to decode SSE data: frames and assert the emitted role.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
provider/aguiprovider/hosting_events.go	Forces REASONING_MESSAGE_START role to the protocol role (reasoning).
provider/aguiprovider/hosting_test.go	Adds a handler-level regression assertion that the SSE payload uses role: "reasoning" for reasoning start events.

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[github-actions]

Go API Consistency Review

Scope: user-visible behavior (SSE protocol output)
Changed Go contract: None (no exported API changes). The internal updatesToAGUIEvents function now emits role: "reasoning" (via aguiTypes.RoleReasoning) instead of forwarding the internal assistant role for REASONING_MESSAGE_START SSE events.
Upstream evidence reviewed: The AG-UI protocol specification (ag-ui-protocol/ag-ui#1416) is the normative reference for the REASONING_MESSAGE_START event role field. Searched microsoft/agent-framework for Python/.NET AG-UI SSE hosting equivalents — no matching implementation found; AG-UI hosting is currently Go-specific.
Result: Aligned — no parity issues found.

This PR is a protocol compliance fix. The AG-UI spec requires REASONING_MESSAGE_START to carry role: "reasoning"; the previous Go code incorrectly forwarded the internal assistant role. The fix matches the authoritative protocol contract. No exported Go API surface changed, so public-api-change label is not warranted. parity-approved label remains correct.

Generated by Go API Consistency Review Agent · sonnet46 · 15.4 AIC · ⌖ 4.99 AIC · ⊞ 6.4K · ◷