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fal.ai MCP Server

fal.ai MCP Server

by thenavidm
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TypeScript
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Sign file URL

sign_storage_file_url
Destructive

Create a time-limited signed URL for a fal CDN file to share access-restricted assets, with expiration up to 7 days.

Instructions

Creates a signed URL that grants temporary access to a fal CDN file, regardless of its ACL. Useful for sharing access-restricted files.

The signature is valid for expiration_seconds (up to 7 days).

Authentication: Required. The API key must have the assets:read permission.

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>",
      "output_file": "<output_file>"
    },
    "name": "sign_storage_file_url"
  }
}

Input Schema

NameRequiredDescriptionDefault
urlYesFull URL of the fal CDN file, as returned by the upload APIs (https://v3.fal.media/files/b/<id>/<filename>). Must not contain query parameters.
accountNoExact private account key profile label, not an authenticated provider owner ID.
confirmNoSet true only when the user asked for exactly this action.
payloadNoComplete native JSON body, mutually exclusive with flat body flags and payload_file. Use schema for union bodies.
output_fileYesRequired absolute new owner-private file; signed credential URL is never echoed.
payload_fileNoAbsolute regular non-symlink private JSON file, at most 1 MiB. Cannot mix with other body inputs.
expiration_secondsNoHow long the signed URL stays valid, in seconds (max 7 days)

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changedv3.0.1
    • changedInput schema / properties / confirm / description
      Previous value: -"Must be true for the requested mutation, paid work or private output file."New value: +"Set true only when the user asked for exactly this action."
  2. First observedv2.0.0

TDQS

A4.1/5.0
Behavior4/5

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

Annotations already declare destructiveHint=true, readOnlyHint=false and idempotentHint=false, so the safety profile is covered. The description adds genuinely new context beyond that: the assets:read permission requirement, the authentication requirement, and the 7-day validity ceiling on the generated signature. It never explains why a signing operation is flagged destructive (the required output_file write), which is the one notable gap.

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

Conciseness4/5

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

Three short blocks, front-loaded with the core action and scope before the validity constraint and auth note. Every sentence carries information; only the parenthetical restatement of the 7-day limit is mildly redundant with the schema.

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 7-parameter tool with a nested payload object and no output schema, the description covers the essentials: what it produces, its ACL-bypassing behavior, validity window, and auth/permission requirements. It leaves the output_file contract and the destructive write side-effect implicit, relying on the schema to convey them.

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 url, account, confirm, payload, payload_file, output_file and expiration_seconds are all documented in the schema itself. The description only echoes the expiration_seconds 7-day cap, adding nothing about the payload vs. flat-flag mutual exclusion or the confirm flag's meaning. Baseline 3 applies when the schema does the heavy lifting.

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 ('Creates a signed URL') and resource ('fal CDN file') plus the key behavioral distinction 'regardless of its ACL'. This clearly separates it from siblings like get_storage_file_acl and set_storage_file_acl, which inspect or mutate ACLs rather than mint access credentials.

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?

'Useful for sharing access-restricted files' gives a concrete when-to-use context, which is the main reason an agent would pick this over the ACL siblings. It does not name an explicit alternative or state exclusions (e.g., that ACL changes should use set_storage_file_acl instead), so it stops short of full routing 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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