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The OpenRouter API allows streaming responses from any model. This is useful for building chat interfaces or other applications where the UI should update as the model generates the response. To enable streaming, you can set the stream parameter to true in your request. The model will then stream the response to the client in chunks, rather than returning the entire response at once. Here is an example of how to stream a response, and process it:

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Additional Information

For SSE (Server-Sent Events) streams, OpenRouter occasionally sends comments to prevent connection timeouts. These comments look like: 
: OPENROUTER PROCESSING
Comment payload can be safely ignored per the SSE specs. However, you can leverage it to improve UX as needed, e.g. by showing a dynamic loading indicator. The generation ID is returned in the X-Generation-Id response header for all endpoints (chat completions, completions, responses, and messages), which can be useful for debugging and correlating requests. Some SSE client implementations might not parse the payload according to spec, which leads to an uncaught error when you JSON.stringify the non-JSON payloads. We recommend the following clients:

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Stream Cancellation

Streaming requests can be cancelled by aborting the connection. For supported providers, this immediately stops model processing and billing.
Supported
  • OpenAI, Azure, Anthropic
  • Fireworks, Mancer, Recursal
  • AnyScale, Lepton, OctoAI
  • Novita, DeepInfra, Together
  • Cohere, Hyperbolic, Infermatic
  • Avian, XAI, Cloudflare
  • SFCompute, Nineteen, Liquid
  • Friendli, Chutes, DeepSeek
Not Currently Supported
  • AWS Bedrock, Groq, Modal
  • Google, Google AI Studio, Minimax
  • HuggingFace, Replicate, Perplexity
  • Mistral, AI21, Featherless
  • Lynn, Lambda, Reflection
  • SambaNova, Inflection, ZeroOneAI
  • AionLabs, Alibaba, Nebius
  • Kluster, Targon, InferenceNet
To implement stream cancellation:
Cancellation only works for streaming requests with supported providers. For non-streaming requests or unsupported providers, the model will continue processing and you will be billed for the complete response.

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Handling Errors During Streaming

OpenRouter handles errors differently depending on when they occur during the streaming process:

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Errors Before Any Tokens Are Sent

If an error occurs before any tokens have been streamed to the client, OpenRouter returns a standard JSON error response with the appropriate HTTP status code. This follows the standard error format: 
{
  "error": {
    "code": 400,
    "message": "Invalid model specified"
  }
}
Common HTTP status codes include:
  • 400: Bad Request (invalid parameters)
  • 401: Unauthorized (invalid API key)
  • 402: Payment Required (insufficient credits)
  • 429: Too Many Requests (rate limited)
  • 502: Bad Gateway (provider error)
  • 503: Service Unavailable (no available providers)

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Errors After Tokens Have Been Sent (Mid-Stream)

If an error occurs after some tokens have already been streamed to the client, OpenRouter cannot change the HTTP status code (which is already 200 OK). Instead, the error is sent as a Server-Sent Event (SSE) with a unified structure: 
data: {"id":"cmpl-abc123","object":"chat.completion.chunk","created":1234567890,"model":"openai/gpt-4o","provider":"openai","error":{"code":"server_error","message":"Provider disconnected unexpectedly"},"choices":[{"index":0,"delta":{"content":""},"finish_reason":"error"}]}
Key characteristics of mid-stream errors:
  • The error appears at the top level alongside standard response fields (id, object, created, etc.)
  • A choices array is included with finish_reason: "error" to properly terminate the stream
  • The HTTP status remains 200 OK since headers were already sent
  • The stream is terminated after this unified error event

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Code Examples

Here’s how to properly handle both types of errors in your streaming implementation:

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API-Specific Behavior

Different API endpoints may handle streaming errors slightly differently:
  • OpenAI Chat Completions API: Returns ErrorResponse directly if no chunks were processed, or includes error information in the response if some chunks were processed
  • OpenAI Responses API: May transform certain error codes (like context_length_exceeded) into a successful response with finish_reason: "length" instead of treating them as errors