Extended Thinking (Ultrathink) Skill

Enable Claude's extended thinking capabilities for complex reasoning tasks that benefit from internal deliberation before responding.

When to Use

• Complex problem solving requiring multi-step reasoning

• Code architecture decisions with multiple trade-offs

• Debugging complex issues needing systematic analysis

• Strategic planning with many variables

• Mathematical or logical proofs

• Security analysis requiring threat modeling

• Performance optimization with multiple factors

Supported Models

Model Extended Thinking Summarized Thinking

Claude Opus 4.5 ✓ Full

Claude Opus 4.1 ✓ Full

Claude Opus 4 ✓ ✓ Summarized

Claude Sonnet 4.5 ✓ Full

Claude Sonnet 4 ✓ ✓ Summarized

Claude Haiku 4.5 ✓ Full

API Configuration

Basic Extended Thinking (Python)

import anthropic

client = anthropic.Anthropic()

response = client.messages.create( model="claude-sonnet-4-20250514", max_tokens=16000, thinking={ "type": "enabled", "budget_tokens": 10000 # Minimum 1,024 }, messages=[{ "role": "user", "content": "Analyze this complex architecture decision..." }] )

Access thinking and response

for block in response.content: if block.type == "thinking": print(f"Thinking: {block.thinking}") elif block.type == "text": print(f"Response: {block.text}")

TypeScript Configuration

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic();

const response = await client.messages.create({ model: "claude-sonnet-4-20250514", max_tokens: 16000, thinking: { type: "enabled", budget_tokens: 10000, }, messages: [ { role: "user", content: "Analyze this complex architecture decision...", }, ], });

Streaming (Required for max_tokens > 21,333)

with client.messages.stream( model="claude-sonnet-4-20250514", max_tokens=32000, thinking={ "type": "enabled", "budget_tokens": 20000 }, messages=[{"role": "user", "content": prompt}] ) as stream: for event in stream: if event.type == "content_block_delta": if hasattr(event.delta, "thinking"): print(event.delta.thinking, end="", flush=True) elif hasattr(event.delta, "text"): print(event.delta.text, end="", flush=True)

Budget Recommendations

Task Complexity Budget Tokens Use Case

Light 1,024 - 4,000 Simple clarifications, basic analysis

Medium 4,000 - 10,000 Code review, debugging, design decisions

Heavy 10,000 - 20,000 Architecture planning, security audits

Complex 20,000 - 32,000 Multi-system analysis, comprehensive reviews

Maximum 32,000+ Use batch API for budgets exceeding 32k

Tool Use with Extended Thinking

CRITICAL: When using tools with extended thinking, you MUST preserve thinking blocks in the conversation history.

Initial request with thinking

response = client.messages.create( model="claude-sonnet-4-20250514", max_tokens=16000, thinking={"type": "enabled", "budget_tokens": 8000}, tools=[{ "name": "analyze_code", "description": "Analyze code for issues", "input_schema": { "type": "object", "properties": {"code": {"type": "string"}}, "required": ["code"] } }], messages=[{"role": "user", "content": "Analyze this code..."}] )

MUST include ALL content blocks including thinking

tool_use_block = next(b for b in response.content if b.type == "tool_use") tool_result = execute_tool(tool_use_block)

Continue with thinking blocks preserved

follow_up = client.messages.create( model="claude-sonnet-4-20250514", max_tokens=16000, thinking={"type": "enabled", "budget_tokens": 8000}, tools=[...], messages=[ {"role": "user", "content": "Analyze this code..."}, {"role": "assistant", "content": response.content}, # Includes thinking! {"role": "user", "content": [{ "type": "tool_result", "tool_use_id": tool_use_block.id, "content": tool_result }]} ] )

Interleaved Thinking (Claude 4 Models)

For Claude 4 models, use interleaved thinking for thinking between tool calls:

response = client.messages.create( model="claude-sonnet-4-20250514", max_tokens=16000, thinking={"type": "enabled", "budget_tokens": 10000}, betas=["interleaved-thinking-2025-05-14"], # Required for Claude 4 messages=[...] )

Constraints

• Minimum budget: 1,024 tokens

• Maximum output: 128k tokens (thinking + response)

• Temperature: Must be 1 (default) - cannot modify

• top_k: Cannot be used with extended thinking

• Streaming required: When max_tokens > 21,333

• System prompts: Fully compatible

Best Practices

• Start conservative: Begin with lower budgets, increase as needed

• Monitor actual usage: Track thinking_tokens in response usage

• Use streaming: For better UX and larger outputs

• Preserve thinking blocks: Critical for multi-turn tool use

• Batch for heavy workloads: Use batch API for budgets > 32k tokens

• Match budget to task: Don't over-allocate for simple tasks

Integration with Claude Code

When using Claude Code CLI with extended thinking models:

The CLI automatically handles extended thinking for supported models

Use opus or sonnet models for complex tasks

claude --model claude-opus-4-5-20250514 "Analyze this codebase architecture"

See Also

• [[complex-reasoning]] - Multi-step reasoning patterns

• [[deep-analysis]] - Analytical thinking templates

• [[llm-integration]] - General LLM API patterns