Locus

Describe API

describe_api

Read-onlyIdempotent

Get one endpoint's full contract by slug: input schema, example, output shape, price. Use before the first execute or after an args rejection; get the slug from search_apis. Free.

Input Schema

Name	Required	Description
`slug`	Yes	provider/endpoint slug.
`model`	No	Exact fal model ID.
`model_cursor`	No	Fal page cursor.
`operation_query`	No	Search operations or fal/generate models.
`operation_offset`	No	MPP operation index or continuation offset.

Output Schema

Name	Required	Description	Default
`slug`	No
`tool`	No
`title`	No
`inputSchema`	No
`max_credits`	No
`likely_credits`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedOct 20, 2026
- addedOutput schema / properties / likely_credits
```
Added value: +{
+  "type": "string"
+}
```
- addedOutput schema / properties / max_credits
```
Added value: +{
+  "type": "string"
+}
```
Changed5 schema fields changedOct 7, 2026
- removedOutput schema / additionalProperties
```
Removed value: -true
```
- removedOutput schema / properties / inputSchema / description
```
Removed value: -"JSON Schema for execute args."
```
- removedOutput schema / properties / slug / description
```
Removed value: -"provider/endpoint slug."
```
- removedOutput schema / properties / title / description
```
Removed value: -"Endpoint title."
```
- removedOutput schema / properties / tool / description
```
Removed value: -"Typed tool name for direct calls."
```

Changed3 schema fields changedSep 28, 2026

addedInput schema / properties / model

Added value: +{
+  "description": "Exact fal model ID.",
+  "maxLength": 240,
+  "type": "string"
+}

addedInput schema / properties / model_cursor

Added value: +{
+  "description": "Fal page cursor.",
+  "maxLength": 2048,
+  "type": "string"
+}

changedInput schema / properties / operation_query / description

Previous value: -"Search intent for directory operations."New value: +"Search operations or fal/generate models."

Changed2 schema fields changedSep 17, 2026

addedInput schema / properties / operation_query

Added value: +{
+  "description": "Search intent for directory operations.",
+  "maxLength": 256,
+  "type": "string"
+}

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "additionalProperties": true,
+  "properties": {
+    "inputSchema": {
+      "description": "JSON Schema for execute args.",
+      "type": "object"
+    },
+    "slug": {
+      "description": "provider/endpoint slug.",
+      "type": "string"
+    },
+    "title": {
+      "description": "Endpoint title.",
+      "type": "string"
+    },
+    "tool": {
+      "description": "Typed tool name for direct calls.",
+      "type": "string"
+    }
+  },
+  "type": "object"
+}

First observedSep 15, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Annotations already declare readOnly, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description adds a cost trait not present in annotations ('Free') and frames the retrieve-before-execute workflow. It stops short of noting auth requirements or rate limits, but with annotations doing that work a 4 is warranted.

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 short sentences, zero filler, with the primary action ('get one endpoint's full contract by slug') front-loaded and the workflow guidance following. Every clause carries information.

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?

An output schema exists, so return values need no explanation, and the description covers when to call it and how to get the slug. The only residual gap is that the five-parameter schema hints at search/pagination behavior that the description doesn't reconcile with the 'one endpoint' framing.

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 slug, model, model_cursor, operation_query, and operation_offset are all documented in the schema; the baseline is 3. The description reinforces that the lookup is keyed by slug but does not explain why model/operation_query appear on a single-endpoint describe call.

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 one endpoint's full contract by slug') and enumerates what the contract contains (input schema, example, output shape, price). This is clearly distinguishable from siblings like search_apis (discovery) and execute (invocation).

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 explicit timing conditions ('before the first execute or after an args rejection') and names where to obtain the required slug ('get the slug from search_apis'). The workflow relationship to execute and search_apis is unambiguous.

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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