Building with the Claude API · lesson 10 of 17

Streaming

Server-Sent Events (SSE) stream of incremental content deltas. Use for any UI where time-to-first-token matters.

Python

with client.messages.stream(
    model="claude-sonnet-4-6",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Tell me a story"}],
) as stream:
    for text in stream.text_stream:
        print(text, end="", flush=True)

final = stream.get_final_message()  # full Message after stream ends

Event types

message_start — initial message metadata.
content_block_start / content_block_delta / content_block_stop — per content block.
message_delta — top-level updates (e.g., stop_reason arrives here).
message_stop — done.
ping — keepalive.
error — terminal.

For each content block you'll see deltas of type:

text_delta — text content.
input_json_delta — partial JSON for a tool_use (streamed as a string).
thinking_delta — extended thinking content.
signature_delta — extended thinking signature (for redaction proofing).

Streaming + tools

Tool use streams too. You'll see tool_use blocks start with name and id, then input_json_delta events that you concatenate into the JSON string, then a stop. Don't try to parse until the block is fully streamed unless your client does incremental JSON parsing.

Streaming + thinking

When extended thinking is enabled, you'll see thinking blocks streamed before the actual answer. Display these in a collapsed "Reasoning" section.

When to skip streaming