fuast

Set an app's server secrets

set_env

DestructiveIdempotent

Sets or removes environment variables of an app (read in handlers as ctx.env.NAME). They take effect at once, without a deploy. Values are write-only: no tool returns them, only the names.

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": {
      "app": "<app>"
    },
    "name": "set_env"
  }
}

Input Schema

Name	Required	Description
`app`	Yes	The app's slug: the first label of its address (my-app for https://my-app.fuast.com). list_apps shows the slugs you can use.
`set`	No	Variables to set, as {"NAME": "value"}.
`unset`	No	Names to remove.

Output Schema

Name	Required	Description	Default
`set`	No
`keys`	Yes
`slug`	Yes
`unset`	No

Schema Changelog

Changes observed during successful MCP inspections.

First observedNov 12, 2026

TDQS

A4.1/5.0

Behavior4/5

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

Annotations already declare destructive=true, idempotent=true and openWorld=false, and the description is consistent with all of them. It adds real behavior beyond the annotations: changes apply immediately without a deploy, and values are write-only so no tool can read them back, only names — a meaningful surprise-avoiding detail for a destructive write.

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?

Three short sentences, each front-loaded and non-redundant: what it does, when it applies, and the write-only constraint. Nothing repeats the annotations or the schema.

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?

Complete for a three-parameter mutation tool: it discloses immediacy, irreversibility of visibility (write-only values), and the read path via ctx.env.NAME. An output schema exists, so return values need not be described, and the schema covers limits and formats.

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 coverage is 100% and every parameter carries its own description (slug format, {NAME: value} map, names to remove), so the schema does the heavy lifting. The description adds only indirect semantics, mapping 'sets' and 'removes' onto the set/unset parameters and noting the NAME convention used by handlers.

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: sets or removes environment variables of an app, and adds how handlers consume them (ctx.env.NAME). No sibling tool operates on environment variables, so the agent can distinguish this operation by resource alone.

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 context is implied rather than stated: 'takes effect at once, without a deploy' tells the agent this is a live-change operation rather than a source edit or deploy. There is no explicit when-to-use vs when-not guidance, and no named alternative (e.g. update_app_files for source changes).

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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