---
title: Anthropic
sidebar_position: 2
description: 'Configure Anthropic Claude models in Promptfoo: authentication, tool use, thinking, effort, structured outputs, prompt caching, and model-graded assertions.'
---

# Anthropic

This provider supports the [Anthropic Claude](https://www.anthropic.com/claude) series of models.

> **Note:** Anthropic models can also be accessed through [Azure AI Foundry](/docs/providers/azure/#using-claude-models), [AWS Bedrock](/docs/providers/aws-bedrock/), and [Google Vertex](/docs/providers/vertex/).

:::tip Agentic Evals
For agentic evaluations that need built-in file access and skill plugins on top of the Messages API, see the [Claude Agent SDK provider](/docs/providers/claude-agent-sdk/). The Messages provider documented here speaks directly to MCP servers via the [`mcp` config](#model-context-protocol-mcp) so you can plug in your own tools without changing providers.
:::

## Setup

To use Anthropic, you need to set the `ANTHROPIC_API_KEY` environment variable or specify the `apiKey` in the provider configuration.

Create Anthropic API keys [here](https://console.anthropic.com/settings/keys).

Example of setting the environment variable:

```sh
export ANTHROPIC_API_KEY=your_api_key_here
```

### Authenticating via a Claude Code session

If you already have an active Claude Code session (for example as a Claude Pro or Max subscriber), you can reuse its OAuth credential instead of creating a separate Anthropic Console API key. Set `apiKeyRequired: false` on the provider config:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      apiKeyRequired: false
```

When `apiKeyRequired` is `false` and no `ANTHROPIC_API_KEY` is available, Promptfoo loads the Claude Code OAuth credential from:

1. The macOS keychain entry `Claude Code-credentials` (darwin only), then
2. `$HOME/.claude/.credentials.json` on Linux and macOS, or `%USERPROFILE%\.claude\.credentials.json` on Windows.

Set `CLAUDE_CONFIG_DIR` to read the credential from a different Claude Code profile — the same environment variable the Claude Code CLI itself uses to relocate `~/.claude`. It can be set in your shell, in the config's top-level `env:` block, or in a provider's `env:` block (the provider-scoped value wins). On macOS, where Claude Code stores credentials in the system keychain, Promptfoo mirrors the CLI's profile-specific keychain entry: when `CLAUDE_CONFIG_DIR` is set, the credential is looked up under that profile's keychain service (derived from the configured directory) rather than the default one, so evals authenticate as the profile you selected.

Promptfoo authenticates requests with a Bearer token, sends the `claude-code-20250219,oauth-2025-04-20` beta headers, and prepends the required Claude Code identity system block (`"You are Claude Code, Anthropic's official CLI for Claude."`) to every Messages request. Your own system prompt is still forwarded as the next system block.

If you haven't logged in yet, run `claude /login` to create a credential. Re-run it if Promptfoo warns that the credential has expired. Requests made this way are expected to count against your Claude subscription the same way calls from the Claude Code CLI do — check [Anthropic's documentation](https://docs.claude.com/en/docs/claude-code/overview) for current billing behavior.

This also enables [model-graded assertions](#model-graded-tests) such as `llm-rubric` to run without a separate Anthropic Console key — see [the example below](#model-graded-tests).

## Models

These models currently resolve on the Anthropic Messages API:

| Model ID                                                            | Description       |
| ------------------------------------------------------------------- | ----------------- |
| `anthropic:messages:claude-fable-5-1`                               | Claude Fable 5.1  |
| `anthropic:messages:claude-mythos-5-1`                              | Claude Mythos 5.1 |
| `anthropic:messages:claude-fable-5`                                 | Claude Fable 5    |
| `anthropic:messages:claude-mythos-5`                                | Claude Mythos 5   |
| `anthropic:messages:claude-opus-5-5`                                | Claude Opus 5.5   |
| `anthropic:messages:claude-sonnet-5-5`                              | Claude Sonnet 5.5 |
| `anthropic:messages:claude-opus-5`                                  | Claude Opus 5     |
| `anthropic:messages:claude-opus-4-8`                                | Claude 4.8 Opus   |
| `anthropic:messages:claude-opus-4-7`                                | Claude 4.7 Opus   |
| `anthropic:messages:claude-sonnet-5`                                | Claude Sonnet 5   |
| `anthropic:messages:claude-sonnet-4-6`                              | Claude 4.6 Sonnet |
| `anthropic:messages:claude-opus-4-6`                                | Claude 4.6 Opus   |
| `anthropic:messages:claude-opus-4-5-20251101` (claude-opus-4-5)     | Claude 4.5 Opus   |
| `anthropic:messages:claude-sonnet-4-5-20250929` (claude-sonnet-4-5) | Claude 4.5 Sonnet |
| `anthropic:messages:claude-haiku-4-5-20251001` (claude-haiku-4-5)   | Claude 4.5 Haiku  |

The Mythos rows are limited-access models: the ID is correct, but an org without access gets
the same `not_found_error` a retired model returns.

### Retired on the Anthropic API

These IDs return `404 not_found_error` from Anthropic, so a direct `anthropic:messages:`
call will fail. Promptfoo still keeps their pricing, because cost attribution on historical
evals needs it and because partner platforms set their own lifecycle dates.

Availability elsewhere is per-model, not a blanket rule — check the row in
[Cross-Platform Model Availability](#cross-platform-model-availability) before assuming a
retired ID still works somewhere. Three of these are withdrawn from Bedrock as well
(`claude-3-opus-20240229`, `claude-opus-4-20250514`, `claude-3-5-haiku-20241022`) and are
rejected locally with `Unknown Amazon Bedrock model`.

| Model ID                     | Description            | Suggested replacement |
| ---------------------------- | ---------------------- | --------------------- |
| `claude-opus-4-1-20250805`   | Claude 4.1 Opus        | `claude-opus-5-5`     |
| `claude-opus-4-20250514`     | Claude 4 Opus          | `claude-opus-5-5`     |
| `claude-sonnet-4-20250514`   | Claude 4 Sonnet        | `claude-sonnet-5`     |
| `claude-3-7-sonnet-20250219` | Claude 3.7 Sonnet      | `claude-sonnet-5`     |
| `claude-3-5-sonnet-20241022` | Claude 3.5 Sonnet (v2) | `claude-sonnet-5`     |
| `claude-3-5-sonnet-20240620` | Claude 3.5 Sonnet (v1) | `claude-sonnet-5`     |
| `claude-3-5-haiku-20241022`  | Claude 3.5 Haiku       | `claude-haiku-4-5`    |
| `claude-3-opus-20240229`     | Claude 3 Opus          | `claude-opus-5-5`     |
| `claude-3-haiku-20240307`    | Claude 3 Haiku         | `claude-haiku-4-5`    |

:::note Model aliases

Anthropic does not publish `-latest` aliases — `claude-sonnet-4-5-latest` returns a
`not_found_error`. Where a shorter alias exists it is the bare family ID shown in
parentheses above: `claude-sonnet-4-5` resolves to `claude-sonnet-4-5-20250929`.
Claude 4.6 and newer are already unversioned IDs, so there is nothing to shorten.

Rows without a parenthetical have no alias.
[Retired on the Anthropic API](#retired-on-the-anthropic-api) lists retirements from the
direct Anthropic API. Check the platform guides below for availability through other providers.

:::

### Cross-Platform Model Availability

Claude models are available across multiple platforms. Here's how the model names map across different providers:

| Model             | Anthropic API                                  | Azure AI Foundry ([docs](/docs/providers/azure/#using-claude-models)) | AWS Bedrock ([docs](/docs/providers/aws-bedrock)) | GCP Vertex AI ([docs](/docs/providers/vertex)) |
| ----------------- | ---------------------------------------------- | --------------------------------------------------------------------- | ------------------------------------------------- | ---------------------------------------------- |
| Claude Fable 5.1  | claude-fable-5-1                               | claude-fable-5-1                                                      | anthropic.claude-fable-5-1                        | claude-fable-5-1                               |
| Claude Mythos 5.1 | claude-mythos-5-1                              | claude-mythos-5-1 (limited)                                           | anthropic.claude-mythos-5-1 (limited)             | claude-mythos-5-1 (limited)                    |
| Claude Fable 5    | claude-fable-5                                 | claude-fable-5                                                        | anthropic.claude-fable-5                          | claude-fable-5                                 |
| Claude Mythos 5   | claude-mythos-5                                | Not available                                                         | anthropic.claude-mythos-5 (limited)               | Limited availability; ID not public            |
| Claude Opus 5.5   | claude-opus-5-5                                | claude-opus-5-5                                                       | anthropic.claude-opus-5-5                         | claude-opus-5-5                                |
| Claude Sonnet 5.5 | claude-sonnet-5-5                              | claude-sonnet-5-5                                                     | global.anthropic.claude-sonnet-5-5                | claude-sonnet-5-5                              |
| Claude Opus 5     | claude-opus-5                                  | claude-opus-5                                                         | anthropic.claude-opus-5                           | claude-opus-5                                  |
| Claude 4.8 Opus   | claude-opus-4-8                                | claude-opus-4-8                                                       | anthropic.claude-opus-4-8                         | claude-opus-4-8                                |
| Claude 4.7 Opus   | claude-opus-4-7                                | claude-opus-4-7                                                       | anthropic.claude-opus-4-7                         | claude-opus-4-7                                |
| Claude Sonnet 5   | claude-sonnet-5                                | claude-sonnet-5                                                       | anthropic.claude-sonnet-5                         | claude-sonnet-5                                |
| Claude 4.6 Sonnet | claude-sonnet-4-6                              | claude-sonnet-4-6                                                     | anthropic.claude-sonnet-4-6                       | claude-sonnet-4-6                              |
| Claude 4.6 Opus   | claude-opus-4-6                                | claude-opus-4-6-20260205                                              | anthropic.claude-opus-4-6-v1                      | claude-opus-4-6                                |
| Claude 4.5 Opus   | claude-opus-4-5-20251101 (claude-opus-4-5)     | claude-opus-4-5-20251101                                              | anthropic.claude-opus-4-5-20251101-v1:0           | claude-opus-4-5@20251101                       |
| Claude 4.5 Sonnet | claude-sonnet-4-5-20250929 (claude-sonnet-4-5) | claude-sonnet-4-5-20250929                                            | anthropic.claude-sonnet-4-5-20250929-v1:0         | claude-sonnet-4-5@20250929                     |
| Claude 4.5 Haiku  | claude-haiku-4-5-20251001 (claude-haiku-4-5)   | claude-haiku-4-5-20251001                                             | anthropic.claude-haiku-4-5-20251001-v1:0          | claude-haiku-4-5@20251001                      |
| Claude 4.1 Opus   | Retired on the direct API                      | claude-opus-4-1-20250805                                              | anthropic.claude-opus-4-1-20250805-v1:0           | claude-opus-4-1@20250805                       |
| Claude 4 Opus     | Retired on the direct API                      | claude-opus-4-20250514                                                | Withdrawn from Bedrock                            | claude-opus-4@20250514                         |
| Claude 4 Sonnet   | Retired on the direct API                      | claude-sonnet-4-20250514                                              | anthropic.claude-sonnet-4-20250514-v1:0           | claude-sonnet-4@20250514                       |
| Claude 3.7 Sonnet | Retired on the direct API                      | claude-3-7-sonnet-20250219                                            | anthropic.claude-3-7-sonnet-20250219-v1:0         | claude-3-7-sonnet@20250219                     |
| Claude 3.5 Sonnet | Retired on the direct API                      | claude-3-5-sonnet-20241022                                            | anthropic.claude-3-5-sonnet-20241022-v2:0         | claude-3-5-sonnet-v2@20241022                  |
| Claude 3.5 Haiku  | Retired on the direct API                      | claude-3-5-haiku-20241022                                             | Withdrawn from Bedrock                            | claude-3-5-haiku@20241022                      |
| Claude 3 Opus     | Retired on the direct API                      | claude-3-opus-20240229                                                | Withdrawn from Bedrock                            | claude-3-opus@20240229                         |
| Claude 3 Haiku    | Retired on the direct API                      | claude-3-haiku-20240307                                               | anthropic.claude-3-haiku-20240307-v1:0            | claude-3-haiku@20240307                        |

### Supported Parameters

| Config Property | Environment Variable  | Description                                                                                                                                                                           |
| --------------- | --------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| apiKey          | ANTHROPIC_API_KEY     | Your API key from Anthropic                                                                                                                                                           |
| apiKeyRequired  | -                     | Skip the API key preflight and authenticate via a local Claude Code session                                                                                                           |
| apiBaseUrl      | ANTHROPIC_BASE_URL    | The base URL for requests to the Anthropic API                                                                                                                                        |
| temperature     | ANTHROPIC_TEMPERATURE | Controls the randomness of the output (default: 0). Omitted when `top_p` is set.                                                                                                      |
| max_tokens      | ANTHROPIC_MAX_TOKENS  | The maximum length of the generated text (default: 1024, or 2048 whenever thinking will consume output tokens — either because you enabled it or because the model thinks by default) |
| cost            | -                     | Legacy per-token override applied to both input and output pricing                                                                                                                    |
| inputCost       | -                     | Override input token pricing in promptfoo cost estimates                                                                                                                              |
| outputCost      | -                     | Override output token pricing in promptfoo cost estimates                                                                                                                             |
| top_p           | -                     | Controls nucleus sampling. Mutually exclusive with `temperature`.                                                                                                                     |
| top_k           | -                     | Only sample from the top K options for each subsequent token                                                                                                                          |
| stop_sequences  | -                     | Array of strings that will stop generation when encountered                                                                                                                           |
| stream          | -                     | Enable streaming (required when `max_tokens` > 21,333)                                                                                                                                |
| tools           | -                     | An array of tool or function definitions for the model to call                                                                                                                        |
| tool_choice     | -                     | An object specifying the tool to call                                                                                                                                                 |
| effort          | -                     | Output effort level: `low`, `medium`, `high`, `xhigh`, or `max`                                                                                                                       |
| output_format   | -                     | JSON schema configuration for structured outputs                                                                                                                                      |
| thinking        | -                     | Configuration for Claude's extended thinking (`enabled`, `adaptive`, `disabled`, or `between_tools`)                                                                                  |
| showThinking    | -                     | Whether to include thinking content in the output (default: true)                                                                                                                     |
| cache_control   | -                     | Auto-apply cache_control to the last cacheable block in the request                                                                                                                   |
| metadata        | -                     | Request metadata such as `user_id` for tracking purposes                                                                                                                              |
| service_tier    | -                     | Priority tier: `auto` (default) or `standard_only`                                                                                                                                    |
| beta            | -                     | Array of `anthropic-beta` feature flags to send with the request                                                                                                                      |
| headers         | -                     | Additional headers to be sent with the API request                                                                                                                                    |
| extra_body      | -                     | Additional parameters to be included in the API request body                                                                                                                          |

For [MCP tools](#model-context-protocol-mcp), set `mcp.enabled: true`. The
`max_tool_calls` option caps MCP tool executions per request (default: `8`).

Sampling support varies by model. Promptfoo omits unsupported `temperature`,
`top_p`, and `top_k` settings; see the model notes below.

### Prompt Template

To allow for compatibility with the OpenAI prompt template, the following format is supported:

```json title="prompt.json"
[
  {
    "role": "system",
    "content": "{{ system_message }}"
  },
  {
    "role": "user",
    "content": "{{ question }}"
  }
]
```

Promptfoo extracts `system` messages into the API's system prompt and forwards `user` and
`assistant` messages. Set `system_message` and `question` in your test's `vars`.

:::warning Assistant prefill

Claude 4.6 and later models reject prompts ending with an `assistant` message. End
with a `user` message instead. To constrain the response format, use
[structured outputs](#structured-outputs) or a system instruction. The 4.5 models
still accept assistant prefill.

:::

### Options

Set [supported parameters](#supported-parameters) under `config`:

```yaml title="promptfooconfig.yaml"
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      effort: medium
      max_tokens: 2048 # Includes thinking and the final answer
prompts:
  - file://prompt.json
```

### Stop Sequences

Use `stop_sequences` to halt generation when Claude encounters specific strings:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      stop_sequences:
        - "\n\nHuman:"
        - 'STOP'
```

### Classifier Refusals

An Anthropic safety-classifier refusal can arrive as a successful Messages response with `stop_reason: refusal`. When the response also includes `stop_details`, Promptfoo returns the provider content as `output` and normalizes:

```json
{
  "finishReason": "content_filter",
  "guardrails": {
    "flagged": true,
    "reason": "Content refused by Anthropic safety filters — category: ..."
  }
}
```

Promptfoo currently requires `stop_details` to create the top-level `guardrails` signal. A model-written refusal or `stop_reason: refusal` without details is therefore not the same assertion result. Treat the optional `stop_details.category` and `stop_details.explanation` as diagnostic evidence. API validation errors remain provider errors and skip assertions.

In a stream, the terminal refusal reason can arrive after partial text in the final message delta. Promptfoo merges those details before returning the provider response, and cached structured refusals preserve the signal. Use [`not-guardrails`](/docs/configuration/expected-outputs/guardrails#inverse-assertion-not-guardrails) to require the structured classifier signal. Use [`is-refusal`](/docs/configuration/expected-outputs/deterministic#is-refusal) for model-written refusal text.

Cost estimates follow [Anthropic's refusal billing rules](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#how-refusals-are-billed): refusals before any output cost zero unless the category is `bio`, `frontier_llm`, or `reasoning_extraction`. Those categories and refusals after output begins use normal token pricing.

### Metadata

Pass request metadata to the API for tracking or auditing purposes:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      metadata:
        user_id: 'user-123'
```

### Tool Calling

The Anthropic provider supports tool calling (function calling). Here's an example configuration for defining tools.

```yaml title="promptfooconfig.yaml"
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      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
              unit:
                type: string
                enum:
                  - celsius
                  - fahrenheit
            required:
              - location
```

#### Web Search and Web Fetch Tools

Anthropic provides specialized tools for web search and web fetching capabilities:

##### Web Fetch Tool

The web fetch tool allows Claude to retrieve full content from web pages and PDF documents. This is useful when you want Claude to access and analyze specific web content.

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      tools:
        - type: web_fetch_20250910
          name: web_fetch
          max_uses: 5
          allowed_domains:
            - docs.example.com
            - help.example.com
          citations:
            enabled: true
          max_content_tokens: 50000
```

Use one fetch version per request. `web_fetch_20260209` adds dynamic filtering,
`web_fetch_20260309` adds cache control, and `web_fetch_20260318` adds response inclusion control:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      tools:
        - type: web_fetch_20260318
          name: web_fetch
          max_uses: 3
          use_cache: false
          response_inclusion: excluded
```

`response_inclusion: excluded` omits nested tool-use/result pairs consumed by a
completed code-execution call. Direct fetches and paused calls still return their
full results. See Anthropic's [web fetch documentation](https://platform.claude.com/docs/en/agents-and-tools/tool-use/web-fetch-tool).

**Web Fetch Tool Configuration Options:**

| Parameter            | Type     | Description                                                                                                                                                  |
| -------------------- | -------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `type`               | string   | `web_fetch_20250910` (beta), `web_fetch_20260209`, `web_fetch_20260309` (adds `use_cache`), or `web_fetch_20260318` (adds `response_inclusion`)              |
| `name`               | string   | Must be `web_fetch`                                                                                                                                          |
| `max_uses`           | number   | Maximum number of web fetches per request (optional)                                                                                                         |
| `allowed_callers`    | string[] | Restrict which tool callers may invoke the server tool (optional)                                                                                            |
| `allowed_domains`    | string[] | List of domains to allow fetching from (optional, mutually exclusive with `blocked_domains`)                                                                 |
| `blocked_domains`    | string[] | List of domains to block fetching from (optional, mutually exclusive with `allowed_domains`)                                                                 |
| `defer_loading`      | boolean  | Load the tool lazily instead of including it in the initial system prompt (optional)                                                                         |
| `citations`          | object   | Enable citations with `{ enabled: true }` (optional)                                                                                                         |
| `max_content_tokens` | number   | Maximum tokens for web content (optional)                                                                                                                    |
| `cache_control`      | object   | Apply Anthropic cache control to the tool definition (optional)                                                                                              |
| `strict`             | boolean  | Enable strict schema validation for tool names and inputs (optional)                                                                                         |
| `url_sources`        | object   | Limit fetchable URLs by source: `user_input`, `client_tool_results`, or `server_tool_results`, each with a tagged filter such as `{ type: none }` (optional) |
| `use_cache`          | boolean  | Whether to use cached content (`web_fetch_20260309` and `web_fetch_20260318`, optional)                                                                      |
| `response_inclusion` | string   | `full` (default) or `excluded`; applies to completed nested code-execution calls (`web_fetch_20260318` only)                                                 |

##### Web Search Tool

The web search tool allows Claude to search the internet for information:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      tools:
        - type: web_search_20260209
          name: web_search
          max_uses: 3
```

**Web Search Tool Configuration Options:**

| Parameter            | Type     | Description                                                                                           |
| -------------------- | -------- | ----------------------------------------------------------------------------------------------------- |
| `type`               | string   | `web_search_20250305`, `web_search_20260209`, or `web_search_20260318` (adds `response_inclusion`)    |
| `name`               | string   | Must be `web_search`                                                                                  |
| `max_uses`           | number   | Maximum number of searches per request (optional)                                                     |
| `allowed_callers`    | string[] | Restrict which tool callers may invoke the server tool (optional)                                     |
| `allowed_domains`    | string[] | Restrict results to specific domains (optional, mutually exclusive with `blocked_domains`)            |
| `blocked_domains`    | string[] | Exclude domains from results (optional, mutually exclusive with `allowed_domains`)                    |
| `cache_control`      | object   | Apply Anthropic cache control to the tool definition (optional)                                       |
| `defer_loading`      | boolean  | Load the tool lazily instead of including it in the initial system prompt (optional)                  |
| `strict`             | boolean  | Enable strict schema validation for tool names and inputs (optional)                                  |
| `response_inclusion` | string   | `full` (default) or `excluded` — see the web fetch table above (`web_search_20260318` only, optional) |
| `user_location`      | object   | Approximate user location to improve search relevance (optional)                                      |

##### Combined Web Search and Web Fetch

Use web search to find URLs and web fetch to read their contents:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      tools:
        - type: web_search_20260209
          name: web_search
          max_uses: 3
        - type: web_fetch_20260309
          name: web_fetch
          max_uses: 5
          citations:
            enabled: true
```

This configuration allows the model to first search for relevant information, then fetch full content from the most promising results.

##### Paused Turns

A long server-tool run can stop with `stop_reason: pause_turn` before Claude finishes. Promptfoo resumes it up to 5 times, preserving the code execution container and summing each request's usage and cost. Structured output uses the final response's JSON; generated file references remain in `metadata.fileReferences`. If resuming fails or reaches the limit, the result keeps its partial output with `finishReason: pause_turn`, logs a warning, and is not cached. Cancellation stops further requests.

##### Memory Tool

Anthropic's `memory_20250818` tool can be included in `tools`. Promptfoo passes this native tool definition through unchanged, which is useful for evaluating whether a model requests memory operations. Promptfoo does not manage Anthropic memory stores or run local memory handlers for you.

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      tools:
        - type: memory_20250818
          name: memory
          allowed_callers:
            - direct
```

**Memory Tool Configuration Options:**

| Parameter         | Type     | Description                                                          |
| ----------------- | -------- | -------------------------------------------------------------------- |
| `type`            | string   | Must be `memory_20250818`                                            |
| `name`            | string   | Must be `memory`                                                     |
| `allowed_callers` | string[] | Restrict which tool callers may invoke the memory tool (optional)    |
| `cache_control`   | object   | Apply Anthropic cache control to the tool definition (optional)      |
| `defer_loading`   | boolean  | Load the tool lazily instead of including it in the initial prompt   |
| `input_examples`  | object[] | Example memory commands to include in the tool definition (optional) |
| `strict`          | boolean  | Enable strict schema validation for tool names and inputs (optional) |

**Important Security Notes:**

- The web fetch tool requires trusted environments due to potential data exfiltration risks
- The model cannot dynamically construct URLs - only URLs provided by users or from search results can be fetched
- Use domain filtering to restrict access to specific sites:
  - Use `allowed_domains` to whitelist trusted domains (recommended)
  - Use `blocked_domains` to blacklist specific domains
  - **Note:** Only one of `allowed_domains` or `blocked_domains` can be specified, not both

#### Model Context Protocol (MCP)

The Anthropic Messages provider can connect to any [MCP server](https://modelcontextprotocol.io) — stdio, SSE, or streamable HTTP — and execute the model's `tool_use` blocks against that server, feeding the `tool_result` back into the conversation until Claude produces a final reply.

```yaml title="promptfooconfig.yaml"
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      mcp:
        enabled: true
        # Inline command-based stdio server, or `path` to a local script
        server:
          command: npx
          args: ['-y', '@modelcontextprotocol/server-filesystem', '/tmp/workspace']
        # Or use a remote SSE / streamable HTTP server
        # servers:
        #   - name: deepwiki
        #     url: https://mcp.deepwiki.com/mcp
      # Optional cap on MCP rounds per request (default 8). Enforced locally;
      # not sent to Anthropic.
      max_tool_calls: 5
```

How it works:

- Tools discovered on the MCP server are passed to Claude alongside any inline `tools`.
- When Claude returns a `tool_use` block whose name matches an MCP tool, promptfoo calls the tool with the model's arguments and appends a matching `tool_result` block on the user turn.
- The loop repeats until Claude returns text (no more `tool_use`) or `max_tool_calls` is hit. Tool errors are forwarded as `tool_result` blocks with `is_error: true` so the model can recover.
- Non-MCP `tool_use` blocks (regular function tools, or built-ins like `web_search`) are passed through to the existing output and not auto-executed.

:::note Response caching with MCP
The disk response cache is skipped while `mcp.enabled` is `true`, because tool results can be non-deterministic between runs. Use `max_tool_calls` to bound spend.
:::

See the [MCP integration guide](/docs/integrations/mcp/) for full server configuration options (auth, timeouts, multiple servers, etc.) and the [Anthropic MCP example](https://github.com/promptfoo/promptfoo/tree/main/examples/anthropic/mcp).

See the [Anthropic Tool Use Guide](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview) for more information on how to define tools and the tool use example [here](https://github.com/promptfoo/promptfoo/tree/main/examples/eval-tool-use).

### Images / Vision

All current Claude models accept images in the prompt.

See the [Claude vision example](https://github.com/promptfoo/promptfoo/tree/main/examples/claude-vision).

Claude accepts base64 images and image URLs. Use Anthropic `image` content blocks;
their shape differs from OpenAI's `image_url` blocks. See the
[vision API guide](https://platform.claude.com/docs/en/build-with-claude/vision) for request examples.

### Prompt Caching

Prompt caching reuses unchanged prompt prefixes and is supported by current Claude
models. Mark the prefix to cache with `cache_control`:

```yaml title="promptfooconfig.yaml"
providers:
  - id: anthropic:messages:claude-sonnet-5
prompts:
  - file://prompts.yaml
```

```yaml title="prompts.yaml"
- role: system
  content:
    - type: text
      text: 'System message'
      cache_control:
        type: ephemeral
    - type: text
      text: '{{context}}'
      cache_control:
        type: ephemeral
- role: user
  content: '{{question}}'
```

As a simpler alternative, use the top-level `cache_control` parameter to automatically apply a cache marker to the last cacheable block in the request, without annotating each block individually:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      cache_control:
        type: ephemeral
```

Common use cases for caching:

- System messages and instructions
- Tool/function definitions
- Large context documents
- Frequently used images

Cache read and creation token counts are tracked in the response's token usage details.
Cost estimates use the reported cache duration: five-minute writes cost 1.25 times
the base input rate, and one-hour writes cost twice the base input rate.

See [Anthropic's Prompt Caching Guide](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) for more details on requirements, pricing, and best practices.

### Citations

Claude can provide detailed citations when answering questions about documents. Basic example:

```yaml title="promptfooconfig.yaml"
providers:
  - id: anthropic:messages:claude-sonnet-5
prompts:
  - file://prompts.yaml
```

```yaml title="prompts.yaml"
- role: user
  content:
    - type: document
      source:
        type: text
        media_type: text/plain
        data: 'Your document text here'
      citations:
        enabled: true
    - type: text
      text: 'Your question here'
```

See [Anthropic's Citations Guide](https://platform.claude.com/docs/en/build-with-claude/citations) for more details.

### PDF Documents

Claude can process PDF files using document content blocks. Pass the PDF as base64-encoded data:

```yaml
- role: user
  content:
    - type: document
      source:
        type: base64
        media_type: application/pdf
        data: '{{pdf_base64}}'
    - type: text
      text: 'Summarize this document'
```

Use a test var to supply the base64-encoded PDF content:

```yaml
tests:
  - vars:
      pdf_base64: file://document.pdf
```

### Claude Fable 5.1 and Mythos 5.1

Use the pinned model IDs below. Mythos 5.1 requires Project Glasswing access and
provider approval.

```yaml
providers:
  - id: anthropic:messages:claude-fable-5-1
    config:
      max_tokens: 4096
      effort: high
  - id: anthropic:messages:claude-mythos-5-1
    config:
      max_tokens: 4096
      effort: high
```

Both models have a 1M-token context window and support up to 128K output tokens.
Input and output cost $10 and $50 per million tokens, respectively. Cache reads cost
**$0.25 per million tokens**, down from $1 on Fable 5 and Mythos 5; promptfoo includes
this discount in its cost estimates. See [Anthropic's pricing](https://platform.claude.com/docs/en/about-claude/pricing).

Thinking is always on, and the sampling and thinking normalization described below
also applies to 5.1. Unlike Fable 5, 5.1 rejects forced tool use: promptfoo omits
`tool_choice` with type `any` or `tool` and warns. Use `auto` or `none` instead.
When replaying Fable 5.1 thinking blocks, keep earlier messages, system prompts,
and tools unchanged; edited prefixes can cause API errors. See
[Anthropic's migration notes](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1).

### Claude Fable 5 and Mythos 5 notes

Fable 5 and Mythos 5 use always-on adaptive thinking. Promptfoo omits unsupported
`temperature`, `top_p`, and `top_k` values, converts legacy
`thinking: { type: 'enabled', budget_tokens: N }` configs to adaptive thinking, and
omits `thinking: { type: 'disabled' }` because thinking cannot be disabled.
Set `thinking: { type: 'adaptive', display: 'summarized' }` to include a readable
thinking summary; the default `display: 'omitted'` returns an empty thinking block,
which Promptfoo excludes from the output.

Both models use a 1M-token context window, support up to 128K output tokens, and are
priced at $10 per million input tokens and $50 per million output tokens. Mythos 5
access is limited through Project Glasswing and may require provider approval. Both model IDs are pinned.

### Claude Opus 5.5 notes

Opus 5.5 is priced below Opus 5 and has the same context window, maximum output, and tokenizer.
Its request rules match Fable 5.1's, and promptfoo adjusts requests to follow them:

- **Thinking is always on.** Opus 5.5 rejects `thinking: { type: 'disabled' }` at every effort
  level, so promptfoo removes it and logs a warning once. Manual
  `thinking: { type: 'enabled', budget_tokens: N }` configs become `thinking: { type: 'adaptive' }`.
- **`effort` defaults to `medium`**, one level below Opus 5's `high`. Set `effort` explicitly
  when you compare the two. It is the only way to control how much the model thinks.
- **Forced tool use and sampling controls are rejected.** Promptfoo omits `tool_choice` values
  of type `any` or `tool` (use `auto` or `none`) and all of `temperature`, `top_p`, and `top_k`.

Opus 5.5 costs a flat **$4 per million input tokens and $20 per million output tokens** across
its 1M-token context window. Cache reads cost **$0.20 per million tokens**, and promptfoo's cost
estimates include that rate. To track Anthropic's fast mode, set `inputCost: 8 / 1e6` and
`outputCost: 40 / 1e6`.

```yaml title="promptfooconfig.yaml"
providers:
  - id: anthropic:messages:claude-opus-5-5
    config:
      effort: high
      max_tokens: 16000
```

### Claude Opus 5 notes

Opus 5 is the Opus-tier Claude 5 model, aimed at complex agentic coding and long-horizon
work. It keeps Opus 4.8's request surface and pricing, with two behavior changes promptfoo
handles for you:

- **Thinking is on by default.** Unlike Opus 4.7/4.8 — where omitting `thinking` meant no
  extended thinking — an omitted `thinking` block on Opus 5 runs adaptive thinking. Because
  `max_tokens` caps thinking _plus_ response text, promptfoo sizes its default `max_tokens`
  with thinking headroom (2048 instead of 1024) so responses aren't truncated mid-answer.
  Set `max_tokens` explicitly for anything longer.
- **Disabling thinking is effort-gated.** `thinking: { type: 'disabled' }` is only accepted
  at `effort` `high` or below; pairing it with `xhigh` or `max` returns a 400. Promptfoo
  drops the rejected `thinking: { type: 'disabled' }` (keeping your `effort`) and logs a
  one-time warning. Lower `effort` to `high` if you actually need thinking off.
- **Sampling controls are managed for you.** Like Opus 4.7/4.8, Opus 5 rejects
  `temperature`, `top_p`, and `top_k` with a 400; promptfoo omits all three from every
  request, including its built-in `temperature: 0` default. A legacy
  `thinking: { type: 'enabled', budget_tokens: N }` config is converted to
  `thinking: { type: 'adaptive' }`.
- **The full `low` → `max` effort ladder is available.** Start at `xhigh` for coding and
  agentic work, then sweep downward — `low` and `medium` are unusually strong on this model
  and are the main cost and latency lever. See the [Effort Level](#effort-level) section.

Opus 5 uses a 1M-token context window (both the default and the maximum) billed at a flat
**$5 per million input / $25 per million output** — the same list rates as Opus 4.8, with no
long-context surcharge above 200K tokens. Anthropic's fast mode ($10 / $50, Claude API only)
is a separate research-preview rate that promptfoo does not encode. To track it, set
`inputCost: 10 / 1e6` and `outputCost: 50 / 1e6` — a single `cost` cannot express asymmetric
rates, because it is applied as both the input and the output per-token price.

```yaml title="promptfooconfig.yaml"
providers:
  - id: anthropic:messages:claude-opus-5
    config:
      effort: xhigh
      max_tokens: 8192
```

### Claude Sonnet 5.5 notes

[Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/whats-new-sonnet-5-5)
has a 1M-token context window and a 128K-token output limit. Promptfoo adjusts requests
to its API requirements:

- **Thinking is on by default, and `disabled` is rejected.** Sonnet 5.5's lowest thinking
  setting is `thinking: { type: 'between_tools' }`, which turns off up-front thinking.
  Promptfoo sends `between_tools` in place of `disabled` and logs a warning once.
  `between_tools` is only accepted at `effort` `high` or below, so with `xhigh` or `max`
  promptfoo omits `disabled` or `between_tools`, warns, and the model thinks adaptively.
- **`effort` defaults to `high`.** Compare effort levels for your workload when migrating.
- **Forced tool use is rejected.** Promptfoo omits `tool_choice` values of type `any` or
  `tool`. Use `auto` or `none`, and say in the prompt when a tool applies.
- **Sampling controls and manual budgets are rejected**, as on Sonnet 5. Promptfoo omits
  `temperature`, `top_p`, and `top_k`, and converts
  `thinking: { type: 'enabled', budget_tokens: N }` to `thinking: { type: 'adaptive' }`.
- **Text between tool calls comes back in `thinking` blocks.** With the default
  `display: 'omitted'` those blocks are empty. Set `thinking: { type: 'adaptive', display: 'summarized' }`
  or use `between_tools` to keep that text in the output.

Sonnet 5.5 costs **$2 per million input tokens and $10 per million output tokens** across its
1M-token context window, with cache reads at $0.20 per million tokens.

```yaml title="promptfooconfig.yaml"
providers:
  - id: anthropic:messages:claude-sonnet-5-5
    config:
      effort: medium
      thinking:
        type: between_tools # no up-front thinking; accepted at effort high or below
      max_tokens: 4096
```

### Claude Sonnet 5 notes

Sonnet 5 has a 1M-token context window and supports [effort levels](#effort-level)
from `low` through `max`. It does not support manual sampling controls:

- **Sampling controls:** Promptfoo omits `temperature`, `top_p`, and `top_k`,
  including its default temperature. Explicit settings trigger a one-time warning.
  The parameters are also omitted through AWS Bedrock, GCP Vertex, and Azure AI Foundry.
- **Manual thinking budgets convert to adaptive.** A legacy
  `thinking: { type: 'enabled', budget_tokens: N }` config is converted to
  `thinking: { type: 'adaptive' }`; use `effort` to control reasoning depth.

- **Adaptive thinking is on by default.** Requests without a `thinking` field still use
  adaptive thinking. Pass `thinking: { type: 'disabled' }` to turn it off, and leave enough
  `max_tokens` headroom for thinking plus the visible response.

Sonnet 5 uses a 1M-token context window billed at **$2 per million input / $10 per million output**, with no long-context surcharge above 200K tokens. Anthropic made these rates permanent on August 10, 2026, canceling the planned September increase. See [Anthropic's pricing documentation](https://platform.claude.com/docs/en/about-claude/pricing). The newer tokenizer can produce more tokens for the same text, so compare total request costs when migrating from Sonnet 4.6.

### Claude Opus 4.8 notes

Opus 4.8 uses the same sampling and thinking settings as Opus 4.7:

- **Sampling controls are managed for you.** Like Opus 4.7, Opus 4.8 samples adaptively and rejects `temperature`, `top_p`, and `top_k` (any of them returns a 400); promptfoo omits all three from every request. Setting any of them in config or `ANTHROPIC_TEMPERATURE` logs a one-time heads-up so you can clean the values out of your eval.
- **Adaptive thinking is opt-in.** Set `thinking: { type: 'adaptive' }` to let the model decide how much to reason per request. Without an explicit `thinking` block the model runs **without** extended thinking, even at high effort. Manual budget-based thinking (`thinking: { type: 'enabled', budget_tokens: N }`) is rejected with a 400.
- **`effort` defaults to `high` and `xhigh` is available.** Setting `effort: high` behaves the same as omitting it. Start with `xhigh` for coding and agentic work. See the [Effort Level](#effort-level) section.

The same suppression applies when you reach Opus 4.8 through AWS Bedrock, GCP Vertex, or Azure AI Foundry — promptfoo omits the unsupported sampling parameters on each of those paths too (silently; the one-time warning above is specific to the Anthropic Messages provider).

### Claude Opus 4.7 notes

Opus 4.7 supports adaptive thinking when explicitly enabled:

- **Temperature is managed for you.** Opus 4.7 samples adaptively and does not accept `temperature`; promptfoo omits the field from every request. Passing `temperature` in config or `ANTHROPIC_TEMPERATURE` logs a one-time heads-up so you can clean the value out of your eval.
- **Adaptive thinking is opt-in.** Set `thinking: { type: 'adaptive' }` to let the model choose how much to reason per request; leaving `thinking` unset runs Opus 4.7 **without** extended thinking, even at high effort. (Opus 5 is the model where an omitted block means adaptive.) Budget-based modes from older models aren't used on 4.7.
- **`xhigh` effort level is available.** It sits between `high` and `max` and is a good starting point for coding and agentic tasks. See the [Effort Level](#effort-level) section.
- **Updated tokenizer.** The same input can map to 1.0–1.35× more tokens than Opus 4.6, so measure real traffic if you're comparing costs.

The same guidance applies when you reach Opus 4.7 through AWS Bedrock, GCP Vertex, or Azure AI Foundry — promptfoo suppresses `temperature` on each of those paths as well.

### Extended Thinking

Use `thinking` to configure reasoning before the final answer. When available,
`display: summarized` returns a summary of that reasoning:

```yaml title="promptfooconfig.yaml"
providers:
  # Adaptive thinking
  - id: anthropic:messages:claude-opus-5
    config:
      max_tokens: 20000
      thinking:
        type: 'adaptive'
        display: 'summarized' # Opt in to readable reasoning; default is 'omitted'
      effort: xhigh # Controls reasoning depth instead of a token budget

  # Manual thinking budget — Opus 4.6 / Sonnet 4.6 and older (deprecated on 4.6)
  - id: anthropic:messages:claude-haiku-4-5
    config:
      max_tokens: 20000
      thinking:
        type: 'enabled'
        budget_tokens: 16000 # Must be ≥1024 and less than max_tokens
```

The thinking configuration has three possible values:

1. Adaptive thinking (supported on Claude 4.6 and later):

```yaml
thinking:
  type: 'adaptive'
```

In adaptive mode, Claude decides when and how much to think based on the complexity of the request. Control depth with [`effort`](#effort-level) rather than a token budget.

2. Manual thinking budgets (Claude 4.5 and 4.6):

```yaml
thinking:
  type: 'enabled'
  budget_tokens: 16000 # Must be at least 1024 and less than max_tokens
```

Claude 4.5 and 4.6 accept manual budgets; this mode is deprecated on 4.6. On
adaptive-only models, Promptfoo converts `enabled` to `adaptive`, removes the
budget, and logs a warning.

3. Disabled thinking:

```yaml
thinking:
  type: 'disabled'
```

Not accepted on Opus 5.5 or Fable 5 / Mythos 5 / Fable 5.1 / Mythos 5.1, where thinking is always on,
nor on Opus 5 above `effort: high`. Promptfoo omits it in both cases and warns.
On Sonnet 5.5, it becomes `between_tools` at `high` effort or below and is omitted at higher effort.

4. Between tools (Claude Sonnet 5.5 only):

```yaml
thinking:
  type: between_tools
```

This mode turns off up-front thinking and requires `effort: high` or below. Progress updates
between tool calls still return as thinking blocks. It takes no other fields, including `display`
or `budget_tokens`. At `xhigh` or `max`, Promptfoo omits it so the model uses adaptive thinking.

#### Thinking `display`

`display` controls the reasoning text returned by the API. Omitting that text does
not disable thinking or remove its token cost:

| Value          | Behavior                                                                                                                                                                             |
| -------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `'omitted'`    | **Default** on Fable 5/5.1, Mythos 5/5.1, Opus 5.5, Opus 5, Opus 4.7/4.8, and Sonnet 5. The `thinking` block is returned with empty text plus a signature for multi-turn continuity. |
| `'summarized'` | Returns a readable summary of the reasoning. Default on Claude 4.6 and earlier.                                                                                                      |

Set `display: summarized` to request a readable summary:

```yaml
thinking:
  type: adaptive
  display: summarized
```

Thinking tokens count toward `max_tokens` in both modes. Only manual thinking
requires `budget_tokens`, which must be at least 1,024 and below `max_tokens`.

Promptfoo omits `temperature` and `top_k` when thinking is enabled, and clamps
`top_p` to `[0.95, 1.0]` on models that support it. On models without sampling
controls, all three parameters are omitted.

Forced tool use (`tool_choice` type `any` or `tool`) is incompatible with manual
thinking and with Opus 5.5, Sonnet 5.5, Fable 5.1, and Mythos 5.1. Promptfoo omits it with a
warning in those cases; use `auto` or `none`. Other adaptive models accept forced
tool use.

Example response with thinking enabled:

```json
{
  "content": [
    {
      "type": "thinking",
      "thinking": "Let me analyze this step by step...",
      "signature": "WaUjzkypQ2mUEVM36O2TxuC06KN8xyfbJwyem2dw3URve/op91XWHOEBLLqIOMfFG/UvLEczmEsUjavL...."
    },
    {
      "type": "text",
      "text": "Based on my analysis, here is the answer..."
    }
  ]
}
```

#### Controlling Thinking Output

`thinking.display` controls what the API returns; `showThinking` controls whether
Promptfoo includes that text in the output (default: `true`). It cannot reveal
reasoning omitted by the API. For example, request a summary but exclude it from
the output being graded:

```yaml title="promptfooconfig.yaml"
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      thinking:
        type: 'adaptive'
        display: 'summarized'
      showThinking: false # Exclude thinking content from the output
```

#### Redacted Thinking

Sometimes Claude's internal reasoning may be flagged by safety systems. When this occurs, the thinking block will be encrypted and returned as a `redacted_thinking` block:

```json
{
  "content": [
    {
      "type": "redacted_thinking",
      "data": "EmwKAhgBEgy3va3pzix/LafPsn4aDFIT2Xlxh0L5L8rLVyIwxtE3rAFBa8cr3qpP..."
    },
    {
      "type": "text",
      "text": "Based on my analysis..."
    }
  ]
}
```

Redacted thinking blocks are automatically decrypted when passed back to the API, allowing Claude to maintain context without compromising safety guardrails.

#### Extended Output with Thinking

For longer responses, raise `max_tokens` and enable streaming:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      max_tokens: 64000 # Sonnet 5 supports up to 128K output tokens
      stream: true # Required above 21,333 output tokens
      thinking:
        type: 'adaptive'
      effort: high
```

Fable 5/5.1, Opus 5.5, Opus 5, Opus 4.6–4.8, Sonnet 5, and Sonnet 4.6 all support up to **128K** output
tokens; the 4.5 generation caps at 64K. The `output-128k-2025-02-19` beta feature is specific to
Claude 3.7 Sonnet and is not needed on any current model.

When using extended output:

- Streaming is required when `max_tokens` is greater than 21,333
- Thinking shares the `max_tokens` budget with the answer, so leave headroom for both
- The model may not use the entire allocated budget

See [Anthropic's Extended Thinking Guide](https://platform.claude.com/docs/en/build-with-claude/extended-thinking) for more details on requirements and best practices.

### Effort Level

`effort` controls how many tokens Claude spends on a response, including thinking
and tool calls. Higher settings can increase quality, cost, and latency. It is not
a sampling-temperature control or a hard token limit.

```yaml
providers:
  - id: anthropic:messages:claude-opus-5
    config:
      effort: xhigh # Options: low, medium, high, xhigh, max
```

Support varies by model — sending an unsupported level returns a 400:

| Model                                                            | Supported levels                            |
| ---------------------------------------------------------------- | ------------------------------------------- |
| Fable/Mythos 5 and 5.1, Opus 5.5, Opus 5, Sonnet 5, Opus 4.7/4.8 | `low`, `medium`, `high`, `xhigh`, `max`     |
| Opus 4.6, Sonnet 4.6                                             | `low`, `medium`, `high`, `max` (no `xhigh`) |
| Opus 4.5                                                         | `low`, `medium`, `high`                     |
| Sonnet 4.5, Haiku 4.5                                            | Not supported — omit `effort`               |

The API defaults to `medium` on Opus 5.5 and `high` on the other models that support
effort. Set it explicitly when comparing models. See Anthropic's
[effort guide](https://platform.claude.com/docs/en/build-with-claude/effort) for model-specific guidance.

This can be combined with other features like structured outputs:

```yaml
providers:
  - id: anthropic:messages:claude-opus-5
    config:
      effort: high
      output_format:
        type: json_schema
        schema:
          type: object
          properties:
            analysis:
              type: string
          required:
            - analysis
          additionalProperties: false
```

### Structured Outputs

Structured outputs constrain responses to a JSON schema and are supported by
current Claude models. Promptfoo maps `config.output_format` to the API's
`output_config.format` and adds the `structured-outputs-2025-11-13` beta flag to
the `anthropic-beta` header.

#### JSON Outputs

Add `output_format` to get structured responses:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      output_format:
        type: json_schema
        schema:
          type: object
          properties:
            name:
              type: string
            email:
              type: string
          required:
            - name
            - email
          additionalProperties: false
```

You can also load the entire `output_format` from an external file:

```yaml
config:
  output_format: file://./schemas/analysis-format.json
```

Nested file references are supported for the schema:

```json title="analysis-format.json"
{
  "type": "json_schema",
  "schema": "file://./schemas/analysis-schema.json"
}
```

Variable rendering is supported in file paths:

```yaml
config:
  output_format: file://./schemas/{{ schema_name }}.json
```

#### Strict Tool Use

Add `strict: true` to tool definitions for schema-validated parameters:

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    config:
      tools:
        - name: get_weather
          strict: true
          input_schema:
            type: object
            properties:
              location:
                type: string
            required:
              - location
            additionalProperties: false
```

#### Limitations

**Supported:** object, array, string, integer, number, boolean, null, `enum`, `required`, `additionalProperties: false`

**Not supported:** recursive schemas, `minimum`/`maximum`, `minLength`/`maxLength`

**Incompatible with:** citations, message prefilling

See [Anthropic's guide](https://platform.claude.com/docs/en/build-with-claude/structured-outputs) and the [structured outputs example](https://github.com/promptfoo/promptfoo/tree/main/examples/anthropic/structured-outputs).

## Model-Graded Tests

[Model-graded assertions](/docs/configuration/expected-outputs/model-graded/) such as `factuality` or `llm-rubric` will automatically use Anthropic as the grading provider if `ANTHROPIC_API_KEY` is set and `OPENAI_API_KEY` is not set.

If both API keys are present, OpenAI will be used by default. You can explicitly override the grading provider in your configuration.

Claude Pro/Max subscribers without a separate Anthropic Console key can wire up `llm-rubric` through a local Claude Code session by pointing the grader at `anthropic:messages:<model>` with `apiKeyRequired: false`:

```yaml
defaultTest:
  options:
    provider:
      id: anthropic:messages:claude-sonnet-5
      config:
        apiKeyRequired: false
```

See [Authenticating via a Claude Code session](#authenticating-via-a-claude-code-session) above for how the credential is loaded and what beta headers Promptfoo sets.

Because of how model-graded evals are implemented, **the model must support chat-formatted prompts** (except for embedding or classification models).

You can override the grading provider in several ways:

1. For all test cases using `defaultTest`:

```yaml title="promptfooconfig.yaml"
defaultTest:
  options:
    provider: anthropic:messages:claude-sonnet-5
```

2. For individual assertions:

```yaml
assert:
  - type: llm-rubric
    value: Do not mention that you are an AI or chat assistant
    provider:
      id: anthropic:messages:claude-sonnet-5
      config:
        effort: low
```

3. For specific tests:

```yaml
tests:
  - vars:
      question: What is the capital of France?
    options:
      provider:
        id: anthropic:messages:claude-sonnet-5
    assert:
      - type: llm-rubric
        value: Answer should mention Paris
```

### Additional Capabilities

- **Caching**: Promptfoo caches previous LLM requests by default.
- **Token Usage Tracking**: Provides detailed information on the number of tokens used in each request, aiding in usage monitoring and optimization.
- **Cost Calculation**: Estimates each request from its model and reported token usage.
  For Claude 4.6 and later, U.S. inference adds 10% to token costs, including cache
  reads and writes. The response's `usage.inference_geo` takes precedence over
  `config.extra_body.inference_geo`. Explicit `cost`, `inputCost`, or `outputCost`
  overrides take precedence over this surcharge. See
  [Anthropic pricing](https://platform.claude.com/docs/en/about-claude/pricing#data-residency-pricing).

When using the Anthropic disk response cache across runs, assign each provider a distinct, non-secret `label`. Promptfoo uses that stable label to isolate cached responses without persisting API-key or OAuth-token fingerprints. Unlabeled providers receive an ephemeral per-instance namespace, which safely preserves repeated calls within a run without reusing responses across tenants or processes. Requests with custom headers bypass the disk response cache because those headers may contain tenant credentials.

```yaml
providers:
  - id: anthropic:messages:claude-sonnet-5
    label: tenant-a
    config:
      apiKey: '{{env.ANTHROPIC_API_KEY_A}}'
  - id: anthropic:messages:claude-sonnet-5
    label: tenant-b
    config:
      apiKey: '{{env.ANTHROPIC_API_KEY_B}}'
```

## See Also

### Examples

We provide several example implementations demonstrating Claude's capabilities:

#### Core Features

- [Tool Use Example](https://github.com/promptfoo/promptfoo/tree/main/examples/eval-tool-use) - Shows how to use Claude's tool calling capabilities
- [MCP Example](https://github.com/promptfoo/promptfoo/tree/main/examples/anthropic/mcp) - Connect Claude to a Model Context Protocol server and let it execute the discovered tools
- [Structured Outputs Example](https://github.com/promptfoo/promptfoo/tree/main/examples/anthropic/structured-outputs) - Demonstrates JSON outputs and strict tool use for guaranteed schema compliance
- [Vision Example](https://github.com/promptfoo/promptfoo/tree/main/examples/claude-vision) - Demonstrates using Claude's vision capabilities

#### Model Comparisons & Evaluations

- [Claude vs GPT](https://github.com/promptfoo/promptfoo/tree/main/examples/compare-claude-vs-gpt) - Compares Claude with GPT-5.4 on various tasks
- [Claude vs GPT Image Analysis](https://github.com/promptfoo/promptfoo/tree/main/examples/compare-claude-vs-gpt-image) - Compares Claude's and GPT's image analysis capabilities

#### Cloud Platform Integrations

- [Azure AI Foundry](https://github.com/promptfoo/promptfoo/tree/main/examples/azure/claude) - Using Claude through Azure AI Foundry
- [AWS Bedrock](https://github.com/promptfoo/promptfoo/tree/main/examples/amazon-bedrock) - Using Claude through AWS Bedrock
- [Google Vertex AI](https://github.com/promptfoo/promptfoo/tree/main/examples/google-vertex) - Using Claude through Google Vertex AI

#### Agentic Evaluations

- [Claude Agent SDK](/docs/providers/claude-agent-sdk/) - For agentic evals with file access, tool use, and MCP servers

For more examples and general usage patterns, visit our [examples directory](https://github.com/promptfoo/promptfoo/tree/main/examples) on GitHub.