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FastSaver API

by io.fastsaver
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YouTube download link

youtube_download
Read-only

Get a direct download link for a YouTube video, as audio only or as video at a chosen height. Returns download_url, filename, duration and thumbnails. The link is temporary: a *.fastsaver.io/dl link works for 5 minutes and for at most 3 downloads; a /v1/tunnel link (every 1440p and 2160p link, and any fallback link) works for 10 minutes. Cost: 15 credits, 25 for 1440p and 2160p; a failed call costs 0.1.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "url": "<url>",
      "format": "<format>"
    },
    "name": "youtube_download"
  }
}

Input Schema

NameRequiredDescriptionDefault
urlYesThe link as copied from the app or the browser.
formatYesaudio, or the video height.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A4.3/5.0
Behavior4/5

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

Annotations already declare readOnlyHint and openWorldHint, yet the description adds substantial context: the exact return fields, the 5-minute/3-download limit on *.fastsaver.io links versus 10 minutes on /v1/tunnel links, and the credit cost including the 0.1 charge on failure. The only gap is what happens when an expired link is reused or how failures surface.

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?

Purpose is front-loaded in one sentence, followed by returns, link lifetime rules, and pricing in a tight sequence. Every clause carries operational information, with no filler.

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

Completeness5/5

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

With no output schema, the description still enumerates the returned fields, and it documents expiry, download caps, and per-call cost for a paid, stateful-link tool. Nothing an agent needs to invoke it correctly or interpret the result is missing.

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 earns more by explaining that 'format' selects audio-only versus a specific video height and by tying 1440p/2160p choices to higher cost and different link types. This adds meaning the enum alone does not convey.

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?

States a specific verb and resource ('Get a direct download link for a YouTube video') and immediately scopes the variants (audio only or video at a chosen height). An agent can clearly distinguish this from youtube_info or fetch_media without opening the schema.

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?

Usage is implied by the format enum and the cost notes, but there is no explicit when-to-use/when-not guidance or reference to alternatives such as checking youtube_info first or falling back when a link expires. The agent must infer the workflow.

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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