bug: Implementation of the SSE Specification is incorrect (Typescript SDK)

[plettj]

The Typescript SDK, specifically the @ag-ui/client package, currently ignores any event text that doesn't start with data: , while the SSE spec says it should be checking only for data:, ignoring the space.

Problem

AG-UI's Implementation: The Typescript SDK's HttpAgent uses processSSEEvent which scans for data: (with a space) here, ignoring any blocks like data:{} without the space:

if (line.startsWith("data: ")) {
  // Extract data content (remove 'data: ' prefix)
  dataLines.push(line.slice(6));
}

SSE Specification: The official SSE spec from WHATWG states that the space after the colon should always be ignored. From the docs:

The following stream fires two identical events:

data:test

data: test

This is because the space after the colon is ignored if present.

Solution

Implement the official SSE specification by not requiring a space after the colon, and ignoring exactly one space if it does exist.

Concerns

My only concern with implementing the spec more precisely is backward compatibility.

There's one failure case: if an application's backend happens to send both data: and data: in their streams, and the frontend that uses @ag-ui/client relies on not receiving the data: events.

---

If you're curious why this bug is important, well, my team is using Gin on our backend which uses manucorporat/sse for SSE... which doesn't add a space after its colons. That's an 85K+ starred project which relies on an implementation that was last edited 10 years ago. Truly the pinnacle of software engineering :)