AssistantMessage objects after Claude finishes generating each response. To receive incremental updates as text and tool calls are generated, enable partial message streaming by setting include_partial_messages (Python) or includePartialMessages (TypeScript) to true in your options.
This page covers output streaming (receiving tokens in real-time). For input modes (how you send messages), see Send messages to agents. You can also stream responses using the Agent SDK via the CLI.
Enable streaming output
To enable streaming, setinclude_partial_messages (Python) or includePartialMessages (TypeScript) to true in your options. This causes the SDK to yield StreamEvent messages containing raw API events as they arrive, in addition to the usual AssistantMessage and ResultMessage.
Your code then needs to:
- Check each message’s type to distinguish
StreamEventfrom other message types - For
StreamEvent, extract theeventfield and check itstype - Look for
content_block_deltaevents wheredelta.typeistext_delta, which contain the actual text chunks
StreamEvent, then for content_block_delta, then for text_delta:
from claude_agent_sdk import query, ClaudeAgentOptions
from claude_agent_sdk.types import StreamEvent
import asyncio
async def stream_response():
options = ClaudeAgentOptions(
include_partial_messages=True,
allowed_tools=["Bash", "Read"],
)
async for message in query(prompt="List the files in my project", options=options):
if isinstance(message, StreamEvent):
event = message.event
if event.get("type") == "content_block_delta":
delta = event.get("delta", {})
if delta.get("type") == "text_delta":
print(delta.get("text", ""), end="", flush=True)
asyncio.run(stream_response())
StreamEvent reference
When partial messages are enabled, you receive raw Claude API streaming events wrapped in an object. The type has different names in each SDK:- Python:
StreamEvent(import fromclaude_agent_sdk.types) - TypeScript:
SDKPartialAssistantMessagewithtype: 'stream_event'
@dataclass
class StreamEvent:
uuid: str # Unique identifier for this event
session_id: str # Session identifier
event: dict[str, Any] # The raw Claude API stream event
parent_tool_use_id: str | None # Parent tool ID if from a subagent
event field contains the raw streaming event from the Claude API. Common event types include:
| Event Type | Description |
|---|---|
message_start | Start of a new message |
content_block_start | Start of a new content block (text or tool use) |
content_block_delta | Incremental update to content |
content_block_stop | End of a content block |
message_delta | Message-level updates (stop reason, usage) |
message_stop | End of the message |
Message flow
With partial messages enabled, you receive messages in this order:StreamEvent (message_start)
StreamEvent (content_block_start) - text block
StreamEvent (content_block_delta) - text chunks...
StreamEvent (content_block_stop)
StreamEvent (content_block_start) - tool_use block
StreamEvent (content_block_delta) - tool input chunks...
StreamEvent (content_block_stop)
StreamEvent (message_delta)
StreamEvent (message_stop)
AssistantMessage - complete message with all content
... tool executes ...
... more streaming events for next turn ...
ResultMessage - final result
include_partial_messages in Python, includePartialMessages in TypeScript), you receive all message types except StreamEvent. Common types include SystemMessage (session initialization), AssistantMessage (complete responses), ResultMessage (final result), and a compact boundary message indicating when conversation history was compacted (SDKCompactBoundaryMessage in TypeScript; SystemMessage with subtype "compact_boundary" in Python).
Stream text responses
To display text as it’s generated, look forcontent_block_delta events where delta.type is text_delta. These contain the incremental text chunks. The example below prints each chunk as it arrives:
from claude_agent_sdk import query, ClaudeAgentOptions
from claude_agent_sdk.types import StreamEvent
import asyncio
async def stream_text():
options = ClaudeAgentOptions(include_partial_messages=True)
async for message in query(prompt="Explain how databases work", options=options):
if isinstance(message, StreamEvent):
event = message.event
if event.get("type") == "content_block_delta":
delta = event.get("delta", {})
if delta.get("type") == "text_delta":
# Print each text chunk as it arrives
print(delta.get("text", ""), end="", flush=True)
print() # Final newline
asyncio.run(stream_text())
Stream tool calls
Tool calls also stream incrementally. You can track when tools start, receive their input as it’s generated, and see when they complete. The example below tracks the current tool being called and accumulates the JSON input as it streams in. It uses three event types:content_block_start: tool beginscontent_block_deltawithinput_json_delta: input chunks arrivecontent_block_stop: tool call complete
from claude_agent_sdk import query, ClaudeAgentOptions
from claude_agent_sdk.types import StreamEvent
import asyncio
async def stream_tool_calls():
options = ClaudeAgentOptions(
include_partial_messages=True,
allowed_tools=["Read", "Bash"],
)
# Track the current tool and accumulate its input JSON
current_tool = None
tool_input = ""
async for message in query(prompt="Read the README.md file", options=options):
if isinstance(message, StreamEvent):
event = message.event
event_type = event.get("type")
if event_type == "content_block_start":
# New tool call is starting
content_block = event.get("content_block", {})
if content_block.get("type") == "tool_use":
current_tool = content_block.get("name")
tool_input = ""
print(f"Starting tool: {current_tool}")
elif event_type == "content_block_delta":
delta = event.get("delta", {})
if delta.get("type") == "input_json_delta":
# Accumulate JSON input as it streams in
chunk = delta.get("partial_json", "")
tool_input += chunk
print(f" Input chunk: {chunk}")
elif event_type == "content_block_stop":
# Tool call complete - show final input
if current_tool:
print(f"Tool {current_tool} called with: {tool_input}")
current_tool = None
asyncio.run(stream_tool_calls())
Build a streaming UI
This example combines text and tool streaming into a cohesive UI. It tracks whether the agent is currently executing a tool (using anin_tool flag) to show status indicators like [Using Read...] while tools run. Text streams normally when not in a tool, and tool completion triggers a “done” message. This pattern is useful for chat interfaces that need to show progress during multi-step agent tasks.
from claude_agent_sdk import query, ClaudeAgentOptions, ResultMessage
from claude_agent_sdk.types import StreamEvent
import asyncio
import sys
async def streaming_ui():
options = ClaudeAgentOptions(
include_partial_messages=True,
allowed_tools=["Read", "Bash", "Grep"],
)
# Track whether we're currently in a tool call
in_tool = False
async for message in query(
prompt="Find all TODO comments in the codebase", options=options
):
if isinstance(message, StreamEvent):
event = message.event
event_type = event.get("type")
if event_type == "content_block_start":
content_block = event.get("content_block", {})
if content_block.get("type") == "tool_use":
# Tool call is starting - show status indicator
tool_name = content_block.get("name")
print(f"\n[Using {tool_name}...]", end="", flush=True)
in_tool = True
elif event_type == "content_block_delta":
delta = event.get("delta", {})
# Only stream text when not executing a tool
if delta.get("type") == "text_delta" and not in_tool:
sys.stdout.write(delta.get("text", ""))
sys.stdout.flush()
elif event_type == "content_block_stop":
if in_tool:
# Tool call finished
print(" done", flush=True)
in_tool = False
elif isinstance(message, ResultMessage):
# Agent finished all work
print(f"\n\n--- Complete ---")
asyncio.run(streaming_ui())
Known limitations
- Structured output: the JSON result appears only in the final
ResultMessage.structured_output, not as streaming deltas. See structured outputs for details.
Next steps
Now that you can stream text and tool calls in real-time, explore these related topics:- Interactive vs one-shot queries: choose between input modes for your use case
- Structured outputs: get typed JSON responses from the agent
- Permissions: control which tools the agent can use