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discover_capabilities

Read-onlyIdempotent

Call this first whenever a task needs an external service (email, payments, search, storage, LLMs, data...) or when a service you use is failing. Maps a plain-language goal (e.g. 'send a transactional email') to callable APIs, MCP servers or CLIs. Returns matches[] with slug, endpoint, auth, formats, rate limit, pricing and a 0-100 reliability score; if nothing fits, coverage='none' and matches[] lists reliable starting points. Rate limits: 100 calls/day anonymous, 1,000/day with a free key, 50,000/day on Agent Pro. For a known keyword or slug, use search_registry.

Input Schema

Name	Required	Description
`need`	Yes	What the agent is trying to accomplish, in plain language. 3-300 characters, required.
`limit`	No	Maximum number of matching interfaces to return, 1-20. Default 5.
`category`	No	Restrict to one interface layer ('api', 'mcp' or 'cli' — see list_categories). Default: omitted, searches all three.
`min_reliability`	No	Drop interfaces scoring below this 0-100 reliability value. Default 0 (no filter).

Output Schema

Name	Required	Description	Default
`need`	Yes
`note`	Yes
`count`	Yes
`matches`	Yes
`coverage`	Yes
`uncovered`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedSep 28, 2026

changedInput schema / properties / category / description

Previous value: -"Restrict to one interface layer. Omit to search all three (default)."New value: +"Restrict to one interface layer ('api', 'mcp' or 'cli' — see list_categories). Default: omitted, searches all three."

Changed5 schema fields changedSep 21, 2026

changedInput schema / properties / category / description

Previous value: -"Restrict to one interface layer."New value: +"Restrict to one interface layer. Omit to search all three (default)."

addedInput schema / properties / limit / description

Added value: +"Maximum number of matching interfaces to return, 1-20. Default 5."

changedInput schema / properties / min_reliability / description

Previous value: -"Drop interfaces whose reliability score is below this value."New value: +"Drop interfaces scoring below this 0-100 reliability value. Default 0 (no filter)."

changedInput schema / properties / need / description

Previous value: -"What the agent is trying to accomplish, in plain language."New value: +"What the agent is trying to accomplish, in plain language. 3-300 characters, required."

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "additionalProperties": false,
+  "properties": {
+    "count": {
+      "type": "number"
+    },
+    "coverage": {
+      "type": "string"
+    },
+    "matches": {
+      "items": {},
+      "type": "array"
+    },
+    "need": {
+      "type": "string"
+    },
+    "note": {
+      "type": "string"
+    },
+    "uncovered": {
+      "type": "boolean"
+    }
+  },
+  "required": [
+    "need",
+    "coverage",
+    "count",
+    "uncovered",
+    "note",
+    "matches"
+  ],
+  "type": "object"
+}

First observedSep 16, 2026

TDQS

A4.7/5.0

Behavior5/5

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

Annotations already declare readOnly/idempotent/non-destructive, so the bar is met; the description goes further by disclosing concrete rate limits (100/day anonymous, 1,000/day free key, 50,000/day Agent Pro) and the degraded-mode behavior (coverage='none' still returns reliable starting points). These are operational traits an agent cannot get from the structured fields.

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?

Front-loaded with the trigger condition before the mechanics, then return shape, then rate limits, then the alternative-tool pointer. Every sentence carries information an agent needs; no filler or restatement of the title.

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?

Despite an output schema being present, the description summarizes the payload (matches[] with slug, endpoint, auth, formats, rate limit, pricing, reliability) and the empty-result case, and covers the usage trigger and rate ceilings. For a 4-param discovery tool, nothing needed to call it correctly is missing.

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%, with constraints and defaults documented for need, limit, category and min_reliability, so the schema carries the parameter burden. The description only loosely implies the 'need' input ('a plain-language goal') and adds no format or syntax guidance beyond it. 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+resource ('Maps a plain-language goal to callable APIs, MCP servers or CLIs') and makes the scope unmistakable. It explicitly distinguishes itself from the closest sibling: 'For a known keyword or slug, use search_registry.' An agent can route between discover_capabilities and search_registry without opening either schema.

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

Usage Guidelines5/5

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

Gives an explicit trigger ('Call this first whenever a task needs an external service... or when a service you use is failing') plus a named alternative for the keyword/slug case. Both the when-to-use and the when-to-use-something-else conditions are spelled out, leaving nothing to inference.

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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