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fetch-mcp

by YawLabs
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TypeScript
Remote

http_head

Read-onlyIdempotent

Check if a resource exists, inspect its headers, or detect changes by sending an HTTP HEAD request that returns status and headers without downloading the body.

Instructions

Perform an HTTP HEAD. Returns status + headers with no body. Useful for checking a resource exists, getting its size (Content-Length), or polling for changes cheaply.

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

A4.2/5.0
Behavior4/5

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

Annotations already declare readOnlyHint, idempotentHint, and non-destructive behavior, so the bar is lower. The description adds meaningful behavioral context beyond annotations: the response includes status and headers with no body, and the operation is cheap because it avoids transferring content.

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 sentences with no filler. The core semantics (HTTP HEAD, no body, returns status/headers) are front-loaded, followed by practical use cases.

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?

For a read-only HTTP HEAD tool, the description covers what it returns and why an agent would use it. There is no output schema, but 'status + headers with no body' adequately communicates the response shape, and the parameter schema covers invocation details.

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 all 11 parameters are already documented structurally. The description does not repeat or add parameter-level meaning, but it does not need to; the baseline of 3 applies.

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 states a specific verb and resource ('Perform an HTTP HEAD') and clearly distinguishes it from sibling methods by noting it returns status and headers with no body. This immediately differentiates it from http_get, http_post, and the fetch_* family.

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?

It gives concrete use cases: checking resource existence, reading Content-Length, and cheap polling. It does not explicitly name alternatives or state when not to use it, but the no-body distinction and listed use cases provide clear contextual guidance.

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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