Streaming Guide

This guide covers streaming in the Claude Agent SDK for Elixir, from simple queries to advanced real-time streaming with multi-turn conversations.

Table of Contents

1. Overview
2. Simple Query Streaming with query/2
3. Streaming API
4. Event Types
   • Subagent Events (parent_tool_use_id)
5. Real-Time Typewriter Effect
6. Multi-Turn Conversations
7. Error Handling in Streams
8. Best Practices

Overview

The Claude Agent SDK provides two streaming approaches:

1. Simple Streaming via query/2 - Returns a lazy Elixir stream of parsed messages as they arrive from the CLI.

2. Bidirectional Streaming API - Provides persistent sessions with real-time character-by-character updates. Best for chat interfaces and interactive applications.

Key Differences

Simple Query Streaming with query/2

The ClaudeAgentSDK.query/2 function returns a lazy stream of Message structs. This is the simplest way to interact with Claude, and it yields messages as the CLI emits them.

Basic Usage

alias ClaudeAgentSDK.{Options, ContentExtractor}

# Simple query with default options
ClaudeAgentSDK.query("Write a haiku about Elixir")
|> Enum.each(fn message ->
  case message.type do
    :system ->
      IO.puts("Session started: #{message.data.session_id}")

    :assistant ->
      text = ContentExtractor.extract_text(message)
      IO.puts("Claude: #{text}")

    :result ->
      IO.puts("Cost: $#{message.data.total_cost_usd}")

    _ ->
      :ok
  end
end)

With Options

options = %Options{
  model: "haiku",
  max_turns: 5,
  system_prompt: "You are a helpful coding assistant.",
  output_format: :stream_json
}

messages = ClaudeAgentSDK.query("Explain pattern matching", options)
  |> Enum.to_list()

# Extract just the text responses
text = messages
  |> Enum.filter(&(&1.type == :assistant))
  |> Enum.map(&ContentExtractor.extract_text/1)
  |> Enum.join("\n")

IO.puts(text)

Using OptionBuilder Presets

alias ClaudeAgentSDK.OptionBuilder

# Quick preset with Haiku model
options = OptionBuilder.with_haiku()

# Development preset (verbose, permissive)
options = OptionBuilder.build_development_options()

# Production preset (restrictive, safe)
options = OptionBuilder.build_production_options()

ClaudeAgentSDK.query("Hello!", options) |> Enum.to_list()

Collecting Results

# Collect all messages
messages = ClaudeAgentSDK.query("Hello") |> Enum.to_list()

# Find specific message types
init_message = Enum.find(messages, &(&1.type == :system and &1.subtype == :init))
result = Enum.find(messages, &(&1.type == :result))

# Extract session ID for later resumption
session_id = init_message.data.session_id

# Check for successful completion
success? = match?(%{type: :result, subtype: :success}, result)

Streaming Input (Enumerable Prompts)

You can stream a sequence of user messages into a single query by passing an Enumerable prompt:

prompts = [
  %{"type" => "user", "message" => %{"role" => "user", "content" => "Hello"}},
  %{"type" => "user", "message" => %{"role" => "user", "content" => "How are you?"}}
]

ClaudeAgentSDK.query(prompts, %Options{})
|> Enum.to_list()

If the internal input-stream worker crashes (or EOF signaling fails), the query now emits an explicit :error_during_execution result message instead of waiting indefinitely.

Custom Transport Injection

You can inject a transport for query flows (module or {module, opts} tuple):

ClaudeAgentSDK.query("Hello", %Options{}, {ClaudeAgentSDK.Transport.Port, []})
|> Enum.to_list()

You can also defer subprocess startup with lazy mode:

ClaudeAgentSDK.query(
  "Hello",
  %Options{},
  {ClaudeAgentSDK.Transport.Port, [startup_mode: :lazy]}
)
|> Enum.to_list()

In lazy mode, startup errors can happen after start_link succeeds and are delivered as process exits.

Query Streaming Module Configuration

For advanced use cases, you can override the query streaming module globally:

config :claude_agent_sdk,
  cli_stream_module: ClaudeAgentSDK.Query.CLIStream

process_module remains as a deprecated fallback key for backward compatibility, but new config should use cli_stream_module.

Streaming API

The Streaming API provides persistent sessions with real-time character-level updates. This is ideal for building chat interfaces.

Starting a Session

alias ClaudeAgentSDK.{Streaming, Options}

options = %Options{
  model: "haiku",
  max_turns: 10,
  allowed_tools: []
}

{:ok, session} = Streaming.start_session(options)

Sending Messages

# Send a message and receive stream of events
Streaming.send_message(session, "Hello! What can you do?")
|> Enum.each(fn event ->
  case event do
    %{type: :text_delta, text: text} ->
      IO.write(text)

    %{type: :message_stop} ->
      IO.puts("")  # Newline after response

    _ ->
      :ok
  end
end)

Closing Sessions

Always close sessions when done to release resources:

# Close session
Streaming.close_session(session)

# Or use try/after pattern
{:ok, session} = Streaming.start_session(options)

try do
  Streaming.send_message(session, "Hello")
  |> Enum.to_list()
after
  Streaming.close_session(session)
end

Getting Session ID

{:ok, session_id} = Streaming.get_session_id(session)
IO.puts("Session ID: #{session_id}")

Event Types

The Streaming API emits various event types for fine-grained control. Each event map also includes streaming metadata: uuid, session_id, parent_tool_use_id, and raw_event (the raw CLI event map with string keys). Stream event wrappers require both uuid and session_id (missing keys raise) to match Python SDK behavior.

Text Streaming Events

# Text delta - partial text as it's generated
%{type: :text_delta, text: "Hello", accumulated: "Hello"}

# Message stop - response complete
%{type: :message_stop, final_text: "Hello, how can I help?"}

Message Lifecycle Events

# Message start - new response beginning
%{type: :message_start, model: "claude-3-haiku", role: "assistant", usage: %{}}

# Content block lifecycle
%{type: :content_block_start}
%{type: :content_block_stop, final_text: "Complete text"}

# Message delta with metadata
%{type: :message_delta, stop_reason: "end_turn", stop_sequence: nil}

Tool Events

When Claude uses tools, you receive lifecycle events:

# Tool use start
%{type: :tool_use_start, name: "Bash", id: "tool_123"}

# Tool input being streamed
%{type: :tool_input_delta, json: "{\"command\": \"ls\"}"}

# Tool execution complete
%{type: :tool_complete, tool_name: "Bash", result: "..."}

Thinking Events (Extended Thinking)

# Thinking start
%{type: :thinking_start}

# Thinking content
%{type: :thinking_delta, thinking: "Let me analyze this..."}

# Thinking end
%{type: :thinking_stop}

Error Events

# Stream error
%{type: :error, error: :timeout}
%{type: :error, error: :connection_closed}
%{type: :error, error: {:api_error, "Rate limit exceeded"}}

If the CLI emits a JSON frame larger than max_buffer_size (default 1MB), the stream terminates with a CLIJSONDecodeError.

Subagent Events (parent_tool_use_id)

When Claude uses the Task tool to spawn subagents, streaming events include a parent_tool_use_id field that identifies which Task tool call produced the event. This is critical for building hierarchical UIs that route subagent output to the correct panel.

# Main agent events have parent_tool_use_id: nil
%{type: :text_delta, text: "Let me search...", parent_tool_use_id: nil}

# Subagent events have parent_tool_use_id set to the Task tool call ID
%{type: :text_delta, text: "Found 3 files", parent_tool_use_id: "toolu_01ABC123"}
%{type: :message_stop, parent_tool_use_id: "toolu_01ABC123"}

Use cases:

• Route main agent output to the primary chat panel
• Display subagent output in nested/collapsible panels
• Track which subagent produced which response
• Build hierarchical streaming UIs for multi-agent workflows

Example: Routing by source

Streaming.send_message(session, "Use the Task tool to find .ex files")
|> Enum.each(fn event ->
  label = case event.parent_tool_use_id do
    nil -> "[MAIN]"
    id -> "[SUB:#{String.slice(id, 0, 8)}]"
  end

  case event do
    %{type: :text_delta, text: text} ->
      IO.puts("#{label} #{text}")
    %{type: :message_stop} ->
      IO.puts("#{label} Complete")
    _ ->
      :ok
  end
end)

See examples/streaming_tools/subagent_streaming.exs for a complete working example.

Complete Event Handling Pattern

Streaming.send_message(session, prompt)
|> Enum.reduce_while(%{text: "", tools: [], error: nil}, fn event, acc ->
  case event do
    %{type: :text_delta, text: chunk} ->
      IO.write(chunk)
      {:cont, %{acc | text: acc.text <> chunk}}

    %{type: :tool_use_start, name: name, id: id} ->
      IO.puts("\n[Using tool: #{name}]")
      {:cont, %{acc | tools: [{name, id} | acc.tools]}}

    %{type: :tool_complete, tool_name: name} ->
      IO.puts("[Tool #{name} complete]")
      {:cont, acc}

    %{type: :message_stop} ->
      IO.puts("")
      {:halt, acc}

    %{type: :error, error: reason} ->
      {:halt, %{acc | error: reason}}

    _ ->
      {:cont, acc}
  end
end)

Real-Time Typewriter Effect

Creating a typewriter effect for chat interfaces:

Basic Typewriter

alias ClaudeAgentSDK.{Streaming, Options}

defmodule Typewriter do
  def chat(prompt) do
    options = %Options{model: "haiku", max_turns: 1, allowed_tools: []}

    {:ok, session} = Streaming.start_session(options)

    try do
      IO.write("Claude: ")

      Streaming.send_message(session, prompt)
      |> Enum.each(fn
        %{type: :text_delta, text: text} ->
          IO.write(text)
          # Optional: add delay for visible effect
          Process.sleep(10)

        %{type: :message_stop} ->
          IO.puts("\n")

        _ ->
          :ok
      end)
    after
      Streaming.close_session(session)
    end
  end
end

Typewriter.chat("Tell me a short story")

With Character Count and Progress

defmodule TypewriterWithStats do
  def stream_response(session, prompt) do
    IO.write("Claude: ")

    result = Streaming.send_message(session, prompt)
    |> Enum.reduce(%{chars: 0, words: 0}, fn event, acc ->
      case event do
        %{type: :text_delta, text: text} ->
          IO.write(text)
          words = text |> String.split() |> length()
          %{acc | chars: acc.chars + String.length(text), words: acc.words + words}

        %{type: :message_stop} ->
          IO.puts("")
          acc

        _ ->
          acc
      end
    end)

    IO.puts("Stats: #{result.chars} characters, #{result.words} words")
    result
  end
end

LiveView Integration Pattern

For Phoenix LiveView applications:

defmodule MyAppWeb.ChatLive do
  use Phoenix.LiveView

  alias ClaudeAgentSDK.{Streaming, Options}

  def mount(_params, _session, socket) do
    {:ok, assign(socket, messages: [], streaming: false, current_text: "")}
  end

  def handle_event("send_message", %{"message" => text}, socket) do
    # Start streaming in background task
    parent = self()

    Task.start(fn ->
      options = %Options{model: "haiku", max_turns: 1}
      {:ok, session} = Streaming.start_session(options)

      try do
        Streaming.send_message(session, text)
        |> Enum.each(fn event ->
          send(parent, {:stream_event, event})
        end)
      after
        Streaming.close_session(session)
      end
    end)

    {:noreply, assign(socket, streaming: true, current_text: "")}
  end

  def handle_info({:stream_event, event}, socket) do
    case event do
      %{type: :text_delta, text: chunk} ->
        new_text = socket.assigns.current_text <> chunk
        {:noreply, assign(socket, current_text: new_text)}

      %{type: :message_stop} ->
        messages = socket.assigns.messages ++ [socket.assigns.current_text]
        {:noreply, assign(socket, messages: messages, streaming: false, current_text: "")}

      _ ->
        {:noreply, socket}
    end
  end
end

Multi-Turn Conversations

The Streaming API maintains context across messages within a session.

Basic Multi-Turn

alias ClaudeAgentSDK.{Streaming, Options}

options = %Options{model: "haiku", max_turns: 5}
{:ok, session} = Streaming.start_session(options)

try do
  # First message
  IO.puts("You: My name is Alice")
  IO.write("Claude: ")
  Streaming.send_message(session, "My name is Alice")
  |> Enum.each(fn
    %{type: :text_delta, text: t} -> IO.write(t)
    %{type: :message_stop} -> IO.puts("\n")
    _ -> :ok
  end)

  # Follow-up - Claude remembers context
  IO.puts("You: What's my name?")
  IO.write("Claude: ")
  Streaming.send_message(session, "What's my name?")
  |> Enum.each(fn
    %{type: :text_delta, text: t} -> IO.write(t)
    %{type: :message_stop} -> IO.puts("\n")
    _ -> :ok
  end)
after
  Streaming.close_session(session)
end

Interactive Chat Loop

defmodule InteractiveChat do
  alias ClaudeAgentSDK.{Streaming, Options}

  def start do
    options = %Options{
      model: "haiku",
      max_turns: 50,
      system_prompt: "You are a helpful assistant. Be concise."
    }

    {:ok, session} = Streaming.start_session(options)
    IO.puts("Chat started. Type 'quit' to exit.\n")
    chat_loop(session)
  end

  defp chat_loop(session) do
    IO.write("You: ")
    input = IO.gets("") |> String.trim()

    case input do
      "quit" ->
        Streaming.close_session(session)
        IO.puts("Goodbye!")

      "" ->
        chat_loop(session)

      message ->
        IO.write("Claude: ")

        Streaming.send_message(session, message)
        |> Enum.each(fn
          %{type: :text_delta, text: text} -> IO.write(text)
          %{type: :message_stop} -> IO.puts("\n")
          %{type: :error, error: reason} -> IO.puts("\n[Error: #{inspect(reason)}]")
          _ -> :ok
        end)

        chat_loop(session)
    end
  end
end

InteractiveChat.start()

Resuming Sessions with query/2

For non-streaming session resumption:

alias ClaudeAgentSDK.{Options, Session}

# Initial query
messages = ClaudeAgentSDK.query("My name is Bob", options) |> Enum.to_list()
session_id = Session.extract_session_id(messages)

# Later: resume the conversation
resumed = ClaudeAgentSDK.resume(session_id, "What's my name?", options)
  |> Enum.to_list()

Error Handling in Streams

Control-client streaming surfaces startup/send failures immediately as stream events (%{type: :error, error: reason}), so callers no longer need to wait for the generic 5-minute stream timeout to detect initialization failures.

Handling Stream Errors

alias ClaudeAgentSDK.{Streaming, Options}

{:ok, session} = Streaming.start_session(%Options{})

result = Streaming.send_message(session, prompt)
|> Enum.reduce_while({:ok, ""}, fn event, {status, text} ->
  case event do
    %{type: :text_delta, text: chunk} ->
      {:cont, {status, text <> chunk}}

    %{type: :message_stop} ->
      {:halt, {:ok, text}}

    %{type: :error, error: reason} ->
      {:halt, {:error, reason}}

    _ ->
      {:cont, {status, text}}
  end
end)

case result do
  {:ok, response} ->
    IO.puts("Response: #{response}")

  {:error, :timeout} ->
    IO.puts("Request timed out")

  {:error, :connection_closed} ->
    IO.puts("Connection was closed")

  {:error, :not_connected} ->
    IO.puts("Transport not connected")

  {:error, reason} ->
    IO.puts("Error: #{inspect(reason)}")
end

Detecting Assistant Errors

# In aggregated message mode
messages = ClaudeAgentSDK.query(prompt, options) |> Enum.to_list()

# Check for assistant errors
assistant_error = Enum.find_value(messages, fn
  %{type: :assistant, data: %{error: err}} when not is_nil(err) -> err
  _ -> nil
end)

case assistant_error do
  nil -> :ok
  :rate_limit -> IO.puts("Rate limited, please retry")
  :authentication_failed -> IO.puts("Authentication issue")
  error -> IO.puts("Error: #{inspect(error)}")
end

Handling Connection Issues

defmodule ResilientChat do
  alias ClaudeAgentSDK.{Streaming, Options}

  @max_retries 3

  def send_with_retry(session, message, retries \\ 0) do
    result = collect_response(session, message)

    case result do
      {:ok, text} ->
        {:ok, text}

      {:error, reason} when reason in [:connection_closed, :not_connected] and retries < @max_retries ->
        IO.puts("[Retrying... attempt #{retries + 1}]")
        Process.sleep(1000 * (retries + 1))  # Exponential backoff
        send_with_retry(session, message, retries + 1)

      {:error, reason} ->
        {:error, reason}
    end
  end

  defp collect_response(session, message) do
    Streaming.send_message(session, message)
    |> Enum.reduce_while({:ok, ""}, fn event, {_, text} ->
      case event do
        %{type: :text_delta, text: chunk} ->
          {:cont, {:ok, text <> chunk}}
        %{type: :message_stop} ->
          {:halt, {:ok, text}}
        %{type: :error, error: reason} ->
          {:halt, {:error, reason}}
        _ ->
          {:cont, {:ok, text}}
      end
    end)
  end
end

Timeout Handling

alias ClaudeAgentSDK.Options

# Set custom timeout in options
options = %Options{
  model: "haiku",
  timeout_ms: 120_000  # 2 minutes
}

# Or use Task.yield for client-side timeout
task = Task.async(fn ->
  Streaming.send_message(session, prompt) |> Enum.to_list()
end)

case Task.yield(task, 30_000) || Task.shutdown(task) do
  {:ok, events} ->
    process_events(events)

  nil ->
    IO.puts("Request timed out after 30 seconds")
end

Best Practices

1. Always Close Sessions

# Use try/after pattern
{:ok, session} = Streaming.start_session(options)

try do
  Streaming.send_message(session, prompt)
  |> Enum.to_list()
after
  Streaming.close_session(session)
end

2. Use Appropriate Streaming Mode

# For simple queries without real-time needs
messages = ClaudeAgentSDK.query(prompt, options) |> Enum.to_list()

# For chat UIs needing real-time updates
{:ok, session} = Streaming.start_session(options)
Streaming.send_message(session, prompt) |> ...

3. Handle All Event Types

# Be explicit about handling events
Streaming.send_message(session, prompt)
|> Enum.each(fn
  %{type: :text_delta, text: t} -> handle_text(t)
  %{type: :tool_use_start, name: n} -> handle_tool_start(n)
  %{type: :message_stop} -> handle_complete()
  %{type: :error, error: e} -> handle_error(e)
  event -> Logger.debug("Unhandled event: #{inspect(event)}")
end)

4. Limit Memory Usage with Large Streams

# Process events without collecting all in memory
Streaming.send_message(session, prompt)
|> Stream.filter(&match?(%{type: :text_delta}, &1))
|> Stream.map(& &1.text)
|> Enum.reduce("", &(&2 <> &1))

5. Use Appropriate Options for Your Use Case

# For quick responses
options = %Options{
  model: "haiku",
  max_turns: 1,
  allowed_tools: []  # Disable tools for faster response
}

# For complex tasks
options = %Options{
  model: "sonnet",
  max_turns: 10,
  permission_mode: :accept_edits,
  timeout_ms: 300_000  # 5 minutes
}

6. Monitor Session State

# Get session ID for logging/debugging
{:ok, session_id} = Streaming.get_session_id(session)
Logger.info("Starting chat in session #{session_id}")

7. Consider Using OptionBuilder

alias ClaudeAgentSDK.OptionBuilder

# Environment-appropriate defaults
options = OptionBuilder.for_environment()

# Add specific overrides
options = OptionBuilder.merge(:development, %{max_turns: 10})

8. Handle Partial Message Mode

# Enable partial messages for streaming events
options = %Options{
  include_partial_messages: true,
  preferred_transport: :auto
}

# Now you'll receive text_delta events

9. Use Streaming with Hooks

alias ClaudeAgentSDK.Hooks.{Matcher, Output}

callback = fn input, _id, _ctx ->
  IO.puts("[Tool: #{input["tool_name"]}]")
  Output.allow()
end

options = %Options{
  hooks: %{
    pre_tool_use: [Matcher.new("*", [callback])]
  }
}

{:ok, session} = Streaming.start_session(options)

10. Subscriber Lifecycle Monitoring

Client and Streaming.Session automatically monitor subscriber processes and prune dead subscribers. You do not need to manually unsubscribe when a subscriber process terminates — the SDK handles cleanup via Process.monitor/1. This prevents message sends to terminated processes in production.

11. Clean Shutdown Pattern

defmodule ChatManager do
  use GenServer

  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end

  def init(_opts) do
    Process.flag(:trap_exit, true)
    {:ok, %{session: nil}}
  end

  def terminate(_reason, %{session: session}) when not is_nil(session) do
    Streaming.close_session(session)
  end
  def terminate(_reason, _state), do: :ok
end

Summary

The Claude Agent SDK provides flexible streaming options:

• query/2 for simple, aggregated responses
• Streaming API for real-time, interactive applications

Key patterns:

1. Use Streaming.start_session/1 for persistent sessions
2. Handle events with pattern matching for clean code
3. Always close sessions in after blocks
4. Use Enum.reduce_while/3 for early termination on errors
5. Consider include_partial_messages: true for streaming events

For more examples, see the examples/streaming_tools/ directory in the SDK source.