A streaming event delivered to the caller's stream callback.

Stream chunks arrive incrementally during a streaming request. The :type field indicates what kind of data the chunk carries.

Chunk Types

• :text_delta - data is a string fragment of generated text

• :tool_call_delta - data is %{id: String.t(), name: String.t() | nil, arguments_delta: String.t()}

• :reasoning_delta - data is a string fragment of reasoning text
• :usage - data is a %Sycophant.Usage{} struct with final token counts
• :failed - data is the Splode.Error terminating the stream; emitted immediately before the stream halts with an error (provider failure, refusal, transport error)
• :incomplete - data is the Splode.Error describing why generation stopped short (token limit, content filter, truncation); emitted immediately before the stream halts
• :cancelled - data is the Splode.Error describing the cancellation reason; emitted immediately before the stream halts when the provider cancels the in-flight response
• :done - data is the final accumulator value; always the last chunk emitted on success

Examples

Simple 1-arity callback (no accumulator):

Sycophant.generate_text("openai:gpt-4o-mini", messages,
  stream: fn
    %Sycophant.StreamChunk{type: :text_delta, data: text} -> IO.write(text)
    %Sycophant.StreamChunk{type: :usage, data: usage} -> IO.inspect(usage)
    _other -> :ok
  end
)

2-arity accumulator callback ({initial_acc, fun}):

Sycophant.generate_text("openai:gpt-4o-mini", messages,
  stream: {[], fn
    %Sycophant.StreamChunk{type: :text_delta, data: text}, acc -> [text | acc]
    _chunk, acc -> acc
  end}
)