Anthropic Provider

Anthropic models support Text, Vision and PDF content. Caching is enabled by default, and Remote MCP is supported.

YAML Configuration:

anthropic:
  api_key: "your_anthropic_key" # Required
  base_url: "https://api.anthropic.com/v1" # Default, only include if required
  cache_mode: "auto" # Options: off, prompt, auto (default: auto)
  cache_ttl: "5m" # Options: 5m, 1h (default: 5m)
  web_search:
    enabled: false
    # max_uses: 3
    # allowed_domains: ["example.com", "*.docs.example.com"]
    # blocked_domains: ["social.example"]  # mutually exclusive with allowed_domains
    # user_location:
    #   type: approximate
    #   city: "London"
    #   country: "UK"
  web_fetch:
    enabled: false
    citations_enabled: false
    # max_uses: 3
    # max_content_tokens: 4096
    # allowed_domains: ["example.com"]
    # blocked_domains: ["tracking.example"]  # mutually exclusive with allowed_domains

Environment Variables:

• ANTHROPIC_API_KEY: Your Anthropic API key
• ANTHROPIC_BASE_URL: Override the API endpoint

Caching Options:

The cache_mode setting controls how prompt caching is applied:

• off: No caching, even if global prompt_caching is enabled
• prompt: Caches tools, system prompt, and template content
• auto: Same as prompt (default)

The cache_ttl setting controls how long cached content persists:

• 5m: Standard 5-minute cache (default)
• 1h: Extended 1-hour cache (additional cost)

Reasoning + Structured Outputs:

Claude reasoning support depends on the model family:

Adaptive models use thinking: {"type": "adaptive"} under the hood. Use effort levels (low, medium, high, xhigh where supported, max) or auto with anthropic.reasoning:

anthropic:
  reasoning: "high"

Adaptive models default to auto (provider-chosen). Do not configure fixed thinking budgets for these models; use effort levels instead.

task_budget is available for Claude Opus 4.7+ in fast-agent. It gives the model a visible token budget for a full agentic loop, so the model can self-moderate. It is different from max_tokens, which is still the enforced ceiling for one response:

anthropic:
  reasoning: "xhigh"
  task_budget: 128k

Anthropic models using budget-based thinking default to reasoning on with a 1024 token budget. Use anthropic.reasoning to set a budget, map from effort aliases, or disable reasoning entirely:

anthropic:
  reasoning: 16000 # Reasoning budget tokens (minimum: 1024)

• Disable reasoning with reasoning: "0", reasoning: "off", or reasoning: false.
• Budget models also accept low/medium/high/max to map to preset budgets.
• The reasoning budget must be less than max_tokens. If you set a budget that meets/exceeds max_tokens, fast-agent raises max_tokens so the budget fits.

You can also set reasoning per run using the model string:

• sonnet?reasoning=4096
• opus?reasoning=xhigh&task_budget=128k
• opus47?reasoning=auto&task_budget=64k
• claude-opus-4-6?reasoning=auto

Structured output selection (Anthropic JSON schema vs tool_use):

• Models that support the structured-outputs-2025-11-13 feature default to JSON schema output (structured_output_mode: json). This mode is compatible with reasoning.
• Older models default to the legacy tool_use structured output flow. tool_use is not compatible with reasoning — fast-agent disables reasoning when tool-forced structured output is selected.
• Anthropic on Vertex does not support modern structured outputs in fast-agent; choose structured_output_mode: tool_use / ?structured=tool_use there.

You can override the structured output mode explicitly:

anthropic:
  structured_output_mode: auto # auto (default), json, or tool_use

Deprecated: thinking_enabled and thinking_budget_tokens are ignored. Use reasoning.

Built-in Anthropic web tools (web_search + web_fetch):

fast-agent can enable Anthropic server-side web tools directly (these are not MCP tool calls):

• anthropic.web_search.enabled: true
• anthropic.web_fetch.enabled: true

Optional controls:

• max_uses
• allowed_domains / blocked_domains (mutually exclusive)
• web_search.user_location (approximate city/region/country/timezone)
• web_fetch.max_content_tokens
• web_fetch.citations_enabled

You can override per run in the model string:

• claude-opus-4-6?web_search=on&web_fetch=on
• sonnet?web_search=off

Supported values are on/off (also accepts true/false, 1/0).

Version policy is model-aware:

• Claude 4.6 models use web_search_20260209 and web_fetch_20260209 (with required beta header code-execution-web-tools-2026-02-09).
• Other supported Anthropic models use legacy versions (web_search_20250305, web_fetch_20250910).

Provider-managed remote MCP:

The direct anthropic provider supports provider-managed remote MCP servers declared with management: provider under mcp.servers or card mcp_connect entries.

• Supported on anthropic
• Not supported on anthropic-vertex
• Server must be a remote http/sse URL
• Use access_token for bearer auth if required

See Configuration Reference for the MCP server schema and Agent Cards for card-scoped runtime targets.

Model Name Aliases: