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fetch-mcp

by YawLabs
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TypeScript
Remote

http_options

Read-onlyIdempotent

Discover a web resource's supported HTTP methods and CORS policy by sending an OPTIONS request. Use for API discovery and preflight checks.

Instructions

Perform an HTTP OPTIONS. Returns the server's supported methods and CORS policy for a resource. Helpful for API discovery.

Input Schema

NameRequiredDescriptionDefault
urlYesTarget URL (http:// or https://)
headersNoExtra request headers as a key/value object
retriesNoRetries on 408/425/429/500/502/503/504 with exponential backoff (default 0)
max_bytesNoMax response body size in bytes before the body is truncated (default 5MiB, ceiling 100MiB)
basic_authNoHTTP Basic auth credentials
timeout_msNoTimeout in ms for each attempt, covering DNS, every redirect hop and the body (default 10000, max 120000). Retries get a fresh budget; a whole call is capped at 5 minutes.
user_agentNoUser-Agent override (default identifies as @yawlabs/fetch-mcp)
decode_textNoForce text decoding (true) or binary base64 (false). Defaults to auto — text for text/*, json, xml, etc; binary otherwise.
bearer_tokenNoBearer token sent as Authorization: Bearer <token>
max_redirectsNoMax redirect hops to follow (default 5)
allow_private_hostsNoAllow loopback / private / link-local targets for this call (default false). Refused unless the server operator launched fetch-mcp with FETCH_MCP_ALLOW_PRIVATE_HOSTS=1 -- SSRF protection stays on by default either way.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed2 schema fields changedv0.8.0
    • changedInput schema / properties / allow_private_hosts / description
      Previous value: -"Allow requests to loopback / private / link-local addresses. SSRF protection is on by default — only flip this when intentionally talking to localhost."New value: +"Allow loopback / private / link-local targets for this call (default false). Refused unless the server operator launched fetch-mcp with FETCH_MCP_ALLOW_PRIVATE_HOSTS=1 -- SSRF protection stays on by default either way."
    • changedInput schema / properties / timeout_ms / description
      Previous value: -"Request timeout in ms (default 10000, max 120000)"New value: +"Timeout in ms for each attempt, covering DNS, every redirect hop and the body (default 10000, max 120000). Retries get a fresh budget; a whole call is capped at 5 minutes."
  2. First observedv0.6.1

TDQS

A4/5.0
Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, covering the safety profile. The description adds the output behavior (supported methods and CORS policy) but does not disclose additional traits such as behavior when OPTIONS is unsupported or response variability. This is adequate but not rich.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two short sentences with no filler. The action is front-loaded, followed by return value and use case. Every sentence contributes useful information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the rich schema and safety annotations, the description is mostly complete. It explains the return value in the absence of an output schema, though it could mention edge cases like servers that omit CORS headers. Overall it is sufficient for correct invocation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the schema fully documents all 11 parameters. The description adds no parameter-level meaning beyond the schema, which aligns with the baseline score of 3.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description uses a specific verb and resource ('Perform an HTTP OPTIONS'), states the return value (supported methods and CORS policy), and names a clear use case (API discovery). This makes it readily distinguishable from sibling HTTP method tools like http_get or http_head.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides clear context for when to use the tool ('Helpful for API discovery') and explains what it returns. It does not explicitly name alternatives or exclusions, but the use case is clear enough for an agent to select it over siblings.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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