Parallel tool use

This page covers parallel tool calls: when Claude calls multiple tools in one turn, how to format the message history so parallelism keeps working, and how to disable it. For the single-call flow, see Handle tool calls.

By default, Claude may use multiple tools to answer a user query. You can disable this behavior by:

Setting disable_parallel_tool_use=true when tool_choice type is auto, which ensures that Claude uses at most one tool
Setting disable_parallel_tool_use=true when tool_choice type is any or tool, which ensures that Claude uses exactly one tool

Worked example

Simpler with Tool Runner: The example below shows manual parallel tool handling. For most use cases, Tool Runner automatically handles parallel tool execution with much less code.

Here's a complete, runnable script to test and verify parallel tool calls are working correctly:

# Define tools
tools = [
    {
        "name": "get_weather",
        "description": "Get the current weather in a given location",
        "input_schema": {
            "type": "object",
            "properties": {
                "location": {
                    "type": "string",
                    "description": "The city and state, e.g. San Francisco, CA",
                }
            },
            "required": ["location"],
        },
    },
    {
        "name": "get_time",
        "description": "Get the current time in a given timezone",
        "input_schema": {
            "type": "object",
            "properties": {
                "timezone": {
                    "type": "string",
                    "description": "The timezone, e.g. America/New_York",
                }
            },
            "required": ["timezone"],
        },
    },
]

# Test conversation with parallel tool calls
messages = [
    {
        "role": "user",
        "content": "What's the weather in SF and NYC, and what time is it there?",
    }
]

# Make initial request
print("Requesting parallel tool calls...")
response = client.messages.create(
    model="claude-opus-4-6", max_tokens=1024, messages=messages, tools=tools
)

# Check for parallel tool calls
tool_uses = [block for block in response.content if block.type == "tool_use"]
print(f"\n✓ Claude made {len(tool_uses)} tool calls")

if len(tool_uses) > 1:
    print("✓ Parallel tool calls detected!")
    for tool in tool_uses:
        print(f"  - {tool.name}: {tool.input}")
else:
    print("✗ No parallel tool calls detected")

# Simulate tool execution and format results correctly
tool_results = []
for tool_use in tool_uses:
    if tool_use.name == "get_weather":
        if "San Francisco" in str(tool_use.input):
            result = "San Francisco: 68°F, partly cloudy"
        else:
            result = "New York: 45°F, clear skies"
    else:  # get_time
        if "Los_Angeles" in str(tool_use.input):
            result = "2:30 PM PST"
        else:
            result = "5:30 PM EST"

    tool_results.append(
        {"type": "tool_result", "tool_use_id": tool_use.id, "content": result}
    )

# Continue conversation with tool results
messages.extend(
    [
        {"role": "assistant", "content": response.content},
        {"role": "user", "content": tool_results},  # All results in one message!
    ]
)

# Get final response
print("\nGetting final response...")
final_response = client.messages.create(
    model="claude-opus-4-6", max_tokens=1024, messages=messages, tools=tools
)

print(f"\nClaude's response:\n{final_response.content[0].text}")

# Verify formatting
print("\n--- Verification ---")
print(f"✓ Tool results sent in single user message: {len(tool_results)} results")
print("✓ No text before tool results in content array")
print("✓ Conversation formatted correctly for future parallel tool use")

This script demonstrates:

How to properly format parallel tool calls and results
How to verify that parallel calls are being made
The correct message structure that encourages future parallel tool use
Common mistakes to avoid (like text before tool results)

Run this script to test your implementation and ensure Claude is making parallel tool calls effectively.

Maximizing parallel tool use

While Claude 4 models have excellent parallel tool use capabilities by default, you can increase the likelihood of parallel tool execution across all models with targeted prompting:

Troubleshooting

If Claude isn't making parallel tool calls when expected, check these common issues:

1. Incorrect tool result formatting

The most common issue is formatting tool results incorrectly in the conversation history. This "teaches" Claude to avoid parallel calls.

Specifically for parallel tool use:

❌ Wrong: Sending separate user messages for each tool result
✅ Correct: All tool results must be in a single user message

// ❌ This reduces parallel tool use
[
  {"role": "assistant", "content": [tool_use_1, tool_use_2]},
  {"role": "user", "content": [tool_result_1]},
  {"role": "user", "content": [tool_result_2]}  // Separate message
]

// ✅ This maintains parallel tool use
[
  {"role": "assistant", "content": [tool_use_1, tool_use_2]},
  {"role": "user", "content": [tool_result_1, tool_result_2]}  // Single message
]

See Handle tool calls for other formatting rules.

2. Weak prompting

Default prompting may not be sufficient. Use the stronger system prompt from the Maximizing parallel tool use section above.

3. Measuring parallel tool usage

To verify parallel tool calls are working:

# Calculate average tools per tool-calling message
tool_call_messages = [
    msg for msg in messages if any(block.type == "tool_use" for block in msg.content)
]
total_tool_calls = sum(
    len([b for b in msg.content if b.type == "tool_use"]) for msg in tool_call_messages
)
avg_tools_per_message = (
    total_tool_calls / len(tool_call_messages) if tool_call_messages else 0.0
)
print(f"Average tools per message: {avg_tools_per_message}")
# Should be > 1.0 if parallel calls are working

Next steps

For the single-tool-call flow and tool_result formatting rules, see Handle tool calls.
For the SDK abstraction that handles parallel execution automatically, see Tool Runner.
For the full tool-use workflow, see Define tools.

Worked example

Maximizing parallel tool use

System prompts for parallel tool use

User message prompting

Troubleshooting

Next steps