A data stream follows a special protocol that the AI SDK provides to send information to the frontend.

The data stream protocol uses Server-Sent Events (SSE) format for improved standardization, keep-alive through ping, reconnect capabilities, and better cache handling.

The following stream parts are currently supported:

Indicates the beginning of a new message with metadata.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"start","messageId":"..."}

Text content is streamed using a start/delta/end pattern with unique IDs for each text block.

Indicates the beginning of a text block.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"text-start","id":"msg_68679a454370819ca74c8eb3d04379630dd1afb72306ca5d"}

Contains incremental text content for the text block.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"text-delta","id":"msg_68679a454370819ca74c8eb3d04379630dd1afb72306ca5d","delta":"Hello"}

Indicates the completion of a text block.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"text-end","id":"msg_68679a454370819ca74c8eb3d04379630dd1afb72306ca5d"}

Reasoning content is streamed using a start/delta/end pattern with unique IDs for each reasoning block.

Indicates the beginning of a reasoning block.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"reasoning-start","id":"reasoning_123"}

Contains incremental reasoning content for the reasoning block.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"reasoning-delta","id":"reasoning_123","delta":"This is some reasoning"}

Indicates the completion of a reasoning block.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"reasoning-end","id":"reasoning_123"}

Source parts provide references to external content sources.

References to external URLs.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"source-url","sourceId":"https://example.com","url":"https://example.com"}

References to documents or files.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"source-document","sourceId":"https://example.com","mediaType":"file","title":"Title"}

The file parts contain references to files with their media type.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"file","url":"https://example.com/file.png","mediaType":"image/png"}

Custom data parts allow streaming of arbitrary structured data with type-specific handling.

Format: Server-Sent Event with JSON object where the type includes a custom suffix

Example:

data: {"type":"data-weather","data":{"location":"SF","temperature":100}}

The data-* type pattern allows you to define custom data types that your frontend can handle specifically.

The error parts are appended to the message as they are received.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"error","errorText":"error message"}

Indicates the beginning of tool input streaming.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"tool-input-start","toolCallId":"call_fJdQDqnXeGxTmr4E3YPSR7Ar","toolName":"getWeatherInformation"}

Incremental chunks of tool input as it's being generated.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"tool-input-delta","toolCallId":"call_fJdQDqnXeGxTmr4E3YPSR7Ar","inputTextDelta":"San Francisco"}

Indicates that tool input is complete and ready for execution.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"tool-input-available","toolCallId":"call_fJdQDqnXeGxTmr4E3YPSR7Ar","toolName":"getWeatherInformation","input":{"city":"San Francisco"}}

Contains the result of tool execution.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"tool-output-available","toolCallId":"call_fJdQDqnXeGxTmr4E3YPSR7Ar","output":{"city":"San Francisco","weather":"sunny"}}

A part indicating the start of a step.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"start-step"}

A part indicating that a step (i.e., one LLM API call in the backend) has been completed.

This part is necessary to correctly process multiple stitched assistant calls, e.g. when calling tools in the backend, and using steps in useChat at the same time.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"finish-step"}

A part indicating the completion of a message.

Format: Server-Sent Event with JSON object

Example:

data: {"type":"finish"}

The stream ends with a special [DONE] marker.

Format: Server-Sent Event with literal [DONE]

Example:

data: [DONE]