Claude API SSE Streaming vs WebSockets: Which to Use

Claude's API streams responses using Server-Sent Events (SSE), not WebSockets. If you're building a chat interface or any feature where users watch tokens appear in real time, the answer to "which should I use" is largely already decided for you at the model layer — but the question still matters when you're deciding how to relay that stream to your own frontend or between backend services.

This article breaks down what SSE actually is, how it differs from WebSockets in practice, why Anthropic (and most LLM providers) chose SSE for token streaming, and when you might still reach for WebSockets elsewhere in your stack.

What SSE Actually Is

SSE is a one-way streaming protocol built directly on top of HTTP. The client opens a normal HTTP request, the server keeps the connection open, and sends a sequence of data: events over time instead of one response body. The browser's native EventSource API (or any HTTP client that can read a streaming response) handles the parsing.

A Claude API streaming response looks like this under the hood:

event: message_start
data: {"type":"message_start","message":{...}}

event: content_block_delta
data: {"type":"content_block_delta","delta":{"type":"text_delta","text":"Hello"}}

event: content_block_delta
data: {"type":"content_block_delta","delta":{"type":"text_delta","text":" there"}}

event: message_stop
data: {"type":"message_stop"}

Each line is a discrete event. The connection closes when the stream ends. No handshake, no persistent bidirectional channel — just HTTP, streamed.

What WebSockets Are, by Contrast

WebSockets establish a full-duplex connection: client and server can both send messages at any time over the same open socket. That's essential for things like multiplayer games, collaborative editors, or chat apps where either side needs to push data unprompted.

For calling an LLM, you don't need that. The pattern is always: client sends one request, server streams back tokens, done. There's no case where the model needs to interrupt you mid-generation to push unrelated data. The communication is inherently one-directional per turn, which is exactly what SSE was designed for.

Why SSE Wins for LLM Token Streaming

Simpler infrastructure. SSE is plain HTTP. It works through standard load balancers, reverse proxies, and CDNs without special configuration. WebSockets often need sticky sessions, dedicated proxy rules, and different scaling strategies because the connection is stateful and long-lived in a different way.

Built-in reconnection semantics. The EventSource API automatically retries on disconnect using the Last-Event-ID header, so clients can resume where they left off. You get this behavior for free; with WebSockets you have to build reconnection and state-resync logic yourself.

Lower overhead for one-way data. Since you only need server-to-client delivery, SSE avoids the overhead of maintaining a bidirectional socket — fewer open file descriptors per connection on the server, simpler connection pooling, easier to reason about timeouts.

Easier to debug. SSE streams are just HTTP responses. You can inspect them with curl -N, read them in browser devtools' Network tab as plain text, and log them like any other HTTP traffic. WebSocket frames require specialized tooling to inspect properly.

Firewall and proxy friendliness. Some corporate networks and older proxies block or mishandle WebSocket upgrades. SSE, being regular HTTP, almost never runs into this.

When WebSockets Still Make Sense

None of this means WebSockets are obsolete — they're the right tool when you genuinely need bidirectional, low-latency communication: voice assistants with interruption handling, real-time collaborative tools, or multiplayer state sync. If your product layers a live chat UI with typing indicators, presence, and simultaneous multi-user editing on top of Claude responses, you might run a WebSocket layer for the UI and use SSE (or a buffered call) underneath for the actual model call. The two aren't mutually exclusive — they solve different problems at different layers of your stack.

Consuming Claude's SSE Stream

If you're calling the Claude API directly, you consume the stream like this:

curl https://api.anthropic.com/v1/messages \
  -H "x-api-key: $ANTHROPIC_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -H "content-type: application/json" \
  -d '{
    "model": "claude-sonnet-4-20250514",
    "max_tokens": 1024,
    "stream": true,
    "messages": [{"role": "user", "content": "Write a haiku about rivers"}]
  }'

Your client reads content_block_delta events and appends text as it arrives. No socket upgrade, no separate protocol handshake — just a streamed HTTP response.

Where SubToAPI Fits

If you're already managing Claude access through a team or Max-style subscription rather than a raw provider key, SubToAPI exposes that access as a standard HTTPS API with the same SSE streaming model. You get sub_live_... application keys, usage metadata per request, and team seats, without changing how your frontend consumes the stream — it's the same event:/data: pattern described above. See the streaming docs for event types and reconnection handling, or the quickstart to get a key running in a few minutes. Plans start at €9/month for solo use, with team and scale tiers for larger setups.

Practical Takeaways

• Claude's API streams exclusively over SSE — there is no WebSocket option at the model layer.
• SSE is simpler to deploy, debug, and scale for one-directional token streaming.
• Use WebSockets only when you need true bidirectional communication elsewhere in your app, and layer it independently from the model call.
• Both protocols can coexist: SSE for the LLM response, WebSockets for live UI features built around it.

Questions

Does Claude's API support WebSockets at all? No. Anthropic's Messages API streams exclusively via Server-Sent Events. If you need a WebSocket interface, you'd build one yourself as a relay layer in front of the SSE stream.

Is SSE slower than WebSockets for streaming tokens? No meaningful difference for this use case. Both deliver data as it's produced; the latency difference between SSE and WebSockets is negligible for one-way token delivery, and SSE's HTTP-based simplicity outweighs any theoretical overhead.

Can I reconnect an SSE stream if it drops mid-response? Yes. The EventSource API supports automatic reconnection with Last-Event-ID, and well-implemented servers can resume from that point. Check your provider's or platform's streaming docs for exact resume behavior.