Claude Sonnet 4.5 API

What is Claude Sonnet 4.5?

Claude Sonnet 4.5 is Anthropic's balanced flagship model, released in September 2025 with a knowledge cutoff of January 2025. It sits in the middle of Anthropic's model lineup, offering a carefully tuned balance between reasoning capability, speed, and cost. The model powers Claude Code by default and is designed for production workloads that demand strong performance across chat, vision, function calling, and extended reasoning tasks without the latency or expense of the largest models.

Sonnet 4.5 supports multimodal input—text and images—and produces text output. It ships with a 200,000-token context window and can generate up to 64,000 tokens in a single response, making it suitable for long-document analysis, complex code generation, and multi-turn conversations that accumulate substantial history. The model also includes native support for structured output, tool use, and extended thinking, a feature that allows the model to produce intermediate reasoning traces before delivering a final answer.

Who should use Claude Sonnet 4.5?

Claude Sonnet 4.5 is the right choice when you need a model that can handle complex reasoning tasks—code review, technical writing, data extraction, multi-step planning—without the cost or latency overhead of Opus-class models. It excels in scenarios where you need vision capabilities alongside strong text understanding: parsing screenshots, analyzing charts, extracting structured data from invoices or forms, or building agents that interact with visual interfaces.

If you're building a product that requires function calling or structured output—such as a conversational assistant that needs to trigger backend actions, a workflow orchestrator, or a data pipeline that extracts JSON from unstructured documents—Sonnet 4.5 offers reliable tool-use performance at a moderate price point. The extended thinking capability is particularly valuable for tasks where you want the model to show its work: mathematical problem-solving, debugging, legal or medical reasoning, or any domain where transparency and step-by-step logic improve trust and correctness.

Teams migrating from GPT-4 or GPT-4 Turbo often find Sonnet 4.5 to be a strong lateral move or upgrade, especially for tasks that benefit from longer context windows or where prompt caching can significantly reduce costs on repeated prefixes.

How to call Claude Sonnet 4.5 via Aiduct

Aiduct exposes Claude Sonnet 4.5 through an OpenAI-compatible endpoint at https://api.aiduct.ai/v1. You can use the OpenAI Python SDK, the Anthropic SDK, or raw HTTP with curl. The model identifier is claude-sonnet-4-5.

Here's a minimal example using the OpenAI Python SDK:

from openai import OpenAI

client = OpenAI(
    api_key="your-aiduct-api-key",
    base_url="https://api.aiduct.ai/v1"
)

response = client.chat.completions.create(
    model="claude-sonnet-4-5",
    messages=[
        {"role": "user", "content": "Explain how prompt caching works in Claude models."}
    ],
    max_tokens=1024
)

print(response.choices[0].message.content)

For vision tasks, include an image URL or base64-encoded image in the message content array:

response = client.chat.completions.create(
    model="claude-sonnet-4-5",
    messages=[
        {
            "role": "user",
            "content": [
                {"type": "text", "text": "What is shown in this image?"},
                {"type": "image_url", "image_url": {"url": "https://example.com/chart.png"}}
            ]
        }
    ],
    max_tokens=512
)

To invoke extended thinking, set the thinking parameter in your request. This will return both the reasoning trace and the final answer. Consult the Aiduct documentation for the exact parameter format, as this may vary depending on whether you're using the OpenAI or Anthropic protocol.

For structured output, use the response_format parameter (OpenAI protocol) or the equivalent Anthropic Messages API fields. Aiduct normalizes these across both protocols, so you can choose the SDK that fits your stack.

Pricing and cost optimization

Claude Sonnet 4.5 is priced per token, with separate rates for prompt tokens, completion tokens, and prompt caching. Prompt caching allows you to mark portions of your prompt—such as system instructions, few-shot examples, or large documents—as cacheable. On subsequent requests, cached tokens are billed at a significantly lower rate, often reducing costs by 90% or more for workloads with repeated prefixes.

For current per-token rates and detailed caching mechanics, visit the Aiduct pricing page. Pricing is subject to change as upstream providers adjust their rates.

To minimize costs:

• Use prompt caching for any repeated context. System prompts, large documents, and few-shot examples are ideal candidates.
• Set max_tokens appropriately. Sonnet 4.5 can generate up to 64,000 tokens, but most tasks require far fewer. Capping output length prevents runaway costs.
• Batch requests where possible. If you're processing many independent tasks, consider batching them into a single prompt with structured output to amortize the per-request overhead.
• Monitor token usage. Aiduct returns token counts in the response metadata. Log these to identify high-cost queries and optimize prompts.

Comparing Aiduct access to direct provider access

Accessing Claude Sonnet 4.5 through Aiduct offers several advantages over calling Anthropic directly:

Unified API surface. Aiduct supports both OpenAI and Anthropic protocols on the same API key. You can switch models—across providers—without changing SDKs or authentication logic. This is especially valuable for teams running experiments or building fallback chains.

Simplified billing. One invoice, one set of rate limits, one dashboard. You don't need separate accounts with Anthropic, OpenAI, Google, and others.

Consistent error handling and retry logic. Aiduct normalizes error codes and retry behavior across providers, reducing the surface area of edge cases your application needs to handle.

No vendor lock-in. Because Aiduct abstracts the underlying provider, you can swap claude-sonnet-4-5 for gpt-4o or another model by changing a single string. This makes A/B testing and cost optimization straightforward.

The trade-off is an additional layer of indirection. If you require the absolute lowest latency or need access to beta features the moment Anthropic releases them, direct API access may be preferable. For most production workloads, the operational simplicity and flexibility of Aiduct outweigh the minimal latency overhead.

Common gotchas and best practices

Context window limits. Sonnet 4.5 supports 200,000 tokens of context, but that includes both input and output. If you're analyzing a 150,000-token document, you have 50,000 tokens remaining for your prompt and the model's response. Always account for the full conversation history in multi-turn chats.

Prompt caching requires explicit markers. Caching is not automatic. You must structure your request to indicate which portions of the prompt should be cached. Consult the Aiduct documentation for the correct syntax in your chosen protocol.

Extended thinking increases latency and cost. The thinking feature produces intermediate reasoning tokens that count toward your completion token usage. Use it when transparency or correctness justifies the additional cost, not as a default for every request.

Vision input format matters. Images can be passed as URLs or base64-encoded strings. Large images may be resized or compressed by the model. For best results, preprocess images to reasonable dimensions (e.g., 2048px max width) and use JPEG or PNG formats.

Structured output is not guaranteed. While Sonnet 4.5 has strong structured-output capabilities, the model may occasionally produce malformed JSON or deviate from the schema. Always validate and handle parse errors gracefully.

Rate limits apply. Aiduct enforces rate limits on a per-key basis. If you're building a high-throughput application, contact Aiduct support to discuss custom limits.

Tool use requires careful prompt design. When using function calling, provide clear, concise tool descriptions. Ambiguous or overly complex tool schemas can confuse the model and lead to incorrect invocations.

When to choose Sonnet 4.5 over other models

Choose Claude Sonnet 4.5 when you need a model that balances capability and cost. It's faster and cheaper than Claude Opus 4, but significantly more capable than Haiku. For many tasks—code generation, technical Q&A, document analysis, data extraction—Sonnet 4.5 delivers Opus-level quality at a fraction of the cost.

Prefer Sonnet 4.5 over GPT-4 Turbo or GPT-4o when:

• You need a longer context window (200K vs. 128K).
• Prompt caching can substantially reduce your costs (Anthropic's caching is mature and well-documented).
• You value extended thinking or want the model to show its reasoning.
• You're building agents or tools and want strong function-calling reliability.

Prefer GPT-4o or GPT-4 Turbo when:

• You need the absolute lowest latency (OpenAI's infrastructure is highly optimized).
• You're already deeply integrated with OpenAI-specific features (e.g., DALL·E, Whisper, GPT-4 Vision's specific behavior).
• You need real-time or streaming responses with minimal overhead.

Prefer Claude Opus 4 when:

• You need the highest possible reasoning capability and cost is secondary.
• You're tackling extremely difficult tasks—advanced mathematics, novel research, complex legal reasoning—where Sonnet 4.5's performance is insufficient.

Prefer Claude Haiku when:

• You need the fastest possible response time and the task is relatively simple.
• Cost is the primary constraint and you can tolerate lower quality.

Getting started

To start using Claude Sonnet 4.5 via Aiduct:

1. Sign up for an Aiduct account and generate an API key.
2. Install the OpenAI or Anthropic Python SDK, or use any HTTP client.
3. Point your base_url to https://api.aiduct.ai/v1 and set model to claude-sonnet-4-5.
4. Run your first request. The API is OpenAI-compatible, so existing code should work with minimal changes.

For detailed examples, authentication instructions, and protocol-specific guidance, consult the Aiduct documentation. If you encounter issues or need help optimizing your integration, Aiduct support is available via the dashboard.

Claude Sonnet 4.5 is a versatile, production-ready model that handles a wide range of tasks with strong performance and reasonable cost. Whether you're building conversational agents, automating document workflows, or orchestrating complex multi-step reasoning, Sonnet 4.5 delivers the balance of speed, capability, and price that most engineering teams need.