feat: support using fetchEventSource for subscriptions

[goleary]

Describe the feature you'd like to request

EventSource is lacking a ton of functionality that makes auth a giant PITA

• headers are not supported
• cross origin cookies are not supported.

Describe the solution you'd like to see

fetchEventSource fixes these shortcomings of EventSource.

Would be awesome to be able to utilize this out of the box in tRPC.

Describe alternate solutions

We use tRPC with header auth, but now to use subscriptions we have to either expose our tokens via a get param or implement a subscription specific token process.

Additional information

No response

👨‍👧‍👦 Contributing

• 🙋‍♂️ Yes, I'd be down to file a PR implementing this feature!

[KATT]

Yeah, I want to get rid of EventSource too. I agree with everything

[KATT]

I'd do a new httpLink, make sure it works with resolveResponse, and deprecate the old one (ensuring that there's functionality pairity)

[ravikant-diwakar]

Technical Implementation Approach

This is an excellent enhancement for tRPC's subscription architecture. Here's my proposed technical solution:

Core Implementation Strategy

1. New HTTP Link Architecture

   • Create httpFetchEventSourceLink alongside existing httpSubscriptionLink
   • Implement as a drop-in replacement with enhanced auth capabilities
   • Maintain backward compatibility with existing resolveResponse pipeline

2. TypeScript Interface Design

interface FetchEventSourceLinkOptions<TRouter extends AnyRouter = AnyRouter> 
  extends HTTPLinkOptions<TRouter> {
  fetchEventSource?: typeof fetchEventSource;
  headers?: HeadersInit | (() => HeadersInit | Promise<HeadersInit>);
  credentials?: RequestCredentials;
  onRetry?: (err: any, retryCount: number) => number | false;
}

3. Authentication Flow Enhancement

   • Support dynamic header injection for JWT/Bearer tokens
   • Enable cookie-based authentication for cross-origin scenarios
   • Implement retry logic with token refresh capability

4. Migration Path

   • Deprecate EventSource-based implementation with clear migration guide
   • Provide feature parity testing suite
   • Add comprehensive TypeScript types for new subscription client

Key Benefits

• Solves header-based auth limitations
• Enables cross-origin cookie support
• Maintains type safety throughout subscription lifecycle
• Provides better error handling and retry mechanisms

Would be happy to contribute to the implementation if this approach aligns with the team's vision. The complexity involves deep integration with tRPC's link system and subscription resolver pipeline.

[KATT]

@ravikant-diwakar if you gonna have ai write for you, tell it to be concise. I've already said that I accept prs and the constraints in one sentence. If you have actual questions once/if you start work on it, please let me know