Copilot SSE streaming endpoint API specification

[nighca]

Update #2901

[gemini-code-assist]

Code Review

This pull request is a significant and well-executed migration of the main copilot's tool-use mechanism from a custom XML format to the provider's standard tool-call API. The changes span across documentation, OpenAPI specifications, and frontend logic, including a new SSE-based streaming implementation. The implementation is robust, with thorough handling of backward compatibility for persisted sessions and a comprehensive new test suite. My review found one area for improvement in the OpenAPI specification to reduce duplication.

[nighca]

这里的定义与 OpenAI-compatible API 概念一致，不过有细微的区别，如

• 结构上更简单，没有 choices 等我们不需要的概念
• 数据字段名与其他接口数据命名风格保持一致为小驼峰

[Copilot]

Pull request overview

Updates the Copilot API spec to support provider-standard tool calling and introduces a structured SSE streaming endpoint to improve tool-call reliability and client parsing.

Changes:

• Replaces inline Copilot message request bodies with a shared CopilotMessageRequest schema.
• Redefines CopilotMessage as a discriminated union (user / copilot / tool) with structured toolCalls and tool-result messages.
• Adds POST /copilot/sse/message with defined SSE event semantics and schemas for structured streaming.

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[xgopilot]

The schema refactoring is well-structured overall — the discriminated oneOf for CopilotMessage, the shared CopilotMessageRequest, and the SSE event schema hierarchy are all clean additions. Inline comments cover the notable issues: wrong HTTP status code for streaming (201 vs 200), an unenforced schema invariant on CopilotCopilotMessage, a tool_name enum vs example contradiction, int64 for a JS-facing index, and extensibility gaps in finishReason and CopilotSSEErrorEvent.reason.