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fetch-mcp

by YawLabs
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TypeScript
Remote

http_post

Destructive

Send data to a URL using an HTTP POST request. Accepts raw string or JSON body, custom headers, authentication, and retry logic. Understand that each call submits twice—not idempotent.

Instructions

Perform an HTTP POST. Body can be given as a raw string (body) or as a JSON value (body_json — auto-sets content-type to application/json). NOT idempotent: calling twice submits twice.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
urlYesTarget URL (http:// or https://)
bodyNoRaw request body as a string
headersNoExtra request headers as a key/value object
retriesNoRetries on 408/425/429/500/502/503/504 with exponential backoff (default 0)
body_jsonNoRequest body as a JSON value — sets content-type to application/json if none given
max_bytesNoMax response body size in bytes before the body is truncated (default 5MiB, ceiling 100MiB)
basic_authNoHTTP Basic auth credentials
timeout_msNoRequest timeout in ms (default 10000, max 120000)
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>
content_typeNoContent-Type header to send with the body (e.g. application/json, text/plain, application/x-www-form-urlencoded)
max_redirectsNoMax redirect hops to follow (default 5)
allow_private_hostsNoAllow requests to loopback / private / link-local addresses. SSRF protection is on by default — only flip this when intentionally talking to localhost.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.6.1

TDQS

A4.1/5.0
Behavior4/5

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

Beyond the annotations (which already indicate non-idempotent, destructive, non-read-only), the description clarifies the practical consequence: 'calling twice submits twice.' It also discloses that body_json auto-sets the content-type, which is useful behavioral detail not present in the annotations.

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 tight sentences with no filler. The core operation is front-loaded, followed by the most decision-relevant parameter distinction and the critical non-idempotence warning.

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

Completeness3/5

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

The description is adequate given rich schema annotations, but there is no output schema and the description never states what the tool returns (status, headers, body, base64 behavior, etc.). For a 14-parameter HTTP client, a brief note on the response shape would make it more complete.

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

Parameters4/5

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

Schema coverage is 100%, so the baseline is 3. The description adds meaning beyond the schema by contrasting raw string body versus JSON value body_json and explaining the automatic content-type behavior, which helps agents choose the right parameter.

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 opens with a specific verb and resource ('Perform an HTTP POST') and immediately distinguishes itself from the sibling HTTP verbs through the non-idempotence warning. It also names the two body modes, so there is no ambiguity about what the tool does.

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

Usage Guidelines3/5

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

The description conveys that this tool is for HTTP POST requests, which is clear context, but it never explicitly says when to choose POST over a sibling like http_put or http_get. Usage is implied by the verb rather than stated as 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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