[SSE] Add Spector coverage for id, retry, and reconnection

[JoshLove-msft]

Description

The SSE id and retry fields are standard parts of the SSE protocol with defined parsing and behavior. Clients should handle them unconditionally; services should not need to declare them using TypeSpec decorators.

Current SSE Spector coverage exercises event and data, but does not establish expected behavior for event IDs, reconnection delays, or resumption through Last-Event-ID.

Expected scenarios

Add language-neutral Spector scenarios covering:

• Receiving an event containing id, event, and data fields.
• Receiving a valid retry field containing ASCII digits.
• Ignoring an invalid retry field.
• Ignoring an id field containing U+0000 NULL.
• Reconnecting after the stream closes and sending the most recently received event ID in the Last-Event-ID request header.

The reconnection scenario can be scoped separately if Spector or generated clients do not currently own automatic reconnection behavior.

Emitter expectations

Emitter-specific tests should verify how event IDs are exposed and how retry/reconnection behavior is implemented. These fields should remain SSE envelope metadata and must not be included in the typed data payload.

Relevant specification

Server-Sent Events are defined by the WHATWG HTML Living Standard:

• Interpreting an event stream
• The Last-Event-ID header

Additional context

This coverage gap was identified while implementing C# SSE streaming support for #10738.

[github-actions]

🎯 Issue Triage

Feature request to extend @typespec/sse with first-class support for the SSE id: and retry: envelope fields, which are currently unrepresentable without incorrectly embedding them in the `(redacted) payload.

Category: Feature Request
Area: lib:http

🔍 Analysis

• The SSE wire format (WHATWG HTML Living Standard) defines four fields: event, data, id, and retry. The current @typespec/sse library only models event and data.
• id: enables resumption via the Last-Event-ID request header; retry: controls the client reconnect delay. Both are envelope-level concerns, not payload concerns.
• The proposal extends the @Events.data pattern with new property decorators @SSE.id and @SSE.retry, which maps cleanly to the existing decorator model.
• This was surfaced during C# SSE streaming support work ([http-client-csharp] Add support for Streaming/JSONL scenarios #10738).
• The proposed shape is well-specified and aligned with the existing library design.

📝 Suggested next steps

• Confirm the decorator naming convention (@SSE.id / @SSE.retry) aligns with library design guidelines
• Add @SSE.id and @SSE.retry property decorators to @typespec/sse (or relevant HTTP lib package)
• Expose the metadata on the SSE event model so emitters (e.g., C#, Python) can consume it
• Add http-specs test coverage for id and retry fields

Generated by Agentic Triage

[timotheeguerin]

Not really sure this is needed to be described, from what I understand this is the core part of the SSE spec with a defined schema and behavior.

If the field name is "id"
If the field value does not contain U+0000 NULL, then set the last event ID buffer to the field value. Otherwise, ignore the field.

If the field name is "retry"
If the field value consists of only [ASCII digits](https://infra.spec.whatwg.org/#ascii-digit), then interpret the field value as an integer in base ten, and set the event stream's [reconnection time](https://html.spec.whatwg.org/multipage/server-sent-events.html#concept-event-stream-reconnection-time) to that integer. Otherwise, ignore the field.

I feel like at least from the client POV this shouldn't matter and you should always be able to handle that.

[JoshLove-msft]

That makes sense. I agree these are fixed protocol fields with defined behavior, so clients should handle them unconditionally rather than requiring services to describe them with decorators.

I updated the issue to request Spector scenarios for id, valid and invalid retry, and Last-Event-ID reconnection behavior, with emitter-specific tests determining how the metadata and reconnection behavior are exposed.

--generated by Copilot