check_usernameCheck if ONE username is free on 12 resale platforms. Example: username='vintage_finds' returns available, taken, or unverified per platform.
| Name | Required | Description | Default |
|---|---|---|---|
| username | Yes | Username, no @ or spaces. | |
| platforms | No | Platform ids. Omit for all. |
| Name | Required | Description | Default |
|---|---|---|---|
| results | Yes | ||
| summary | Yes | ||
| username | Yes | ||
| disclaimer | Yes |
Changes observed during successful MCP inspections.
Input schema / properties / platforms / descriptionPrevious value: -"Platform ids to check. Omit to check all of them: ebay, ebaystore, poshmark, grailed, vinted, whatnot, bonanza, chairish, depop, etsy, mercari, rubylane."New value: +"Platform ids. Omit for all."Input schema / properties / username / descriptionPrevious value: -"The username or store name to check, without an @ and without spaces."New value: +"Username, no @ or spaces."Output schema / properties / disclaimer / descriptionRemoved value: -"How to read the results."Output schema / properties / results / descriptionRemoved value: -"One entry per platform checked."Output schema / properties / results / items / properties / check_url / descriptionRemoved value: -"Profile page address to open, present when the result is taken, unverified, or error."Output schema / properties / results / items / properties / name / descriptionRemoved value: -"Platform display name."Output schema / properties / results / items / properties / note / descriptionRemoved value: -"Why the result is unverified or an error."Output schema / properties / results / items / properties / platform / descriptionRemoved value: -"Platform id."Output schema / properties / results / items / properties / status / descriptionRemoved value: -"Result for this platform."Output schema / properties / summary / descriptionRemoved value: -"Platform names grouped by status."Output schema / properties / summary / properties / available / descriptionRemoved value: -"Names of platforms where the username looks free."Output schema / properties / summary / properties / error / descriptionRemoved value: -"Names of platforms whose check failed."Output schema / properties / summary / properties / taken / descriptionRemoved value: -"Names of platforms where a profile exists."Output schema / properties / summary / properties / unverified / descriptionRemoved value: -"Names of platforms that need a manual check."Output schema / properties / username / descriptionRemoved value: -"The username that was looked up."Output schema / (root)Previous value: -nullNew value: +{
+ "$schema": "http://json-schema.org/draft-07/schema#",
+ "additionalProperties": false,
+ "properties": {
+ "disclaimer": {
+ "description": "How to read the results.",
+ "type": "string"
+ },
+ "results": {
+ "description": "One entry per platform checked.",
+ "items": {
+ "additionalProperties": false,
+ "properties": {
+ "check_url": {
+ "description": "Profile page address to open, present when the result is taken, unverified, or error.",
+ "type": "string"
+ },
+ "name": {
+ "description": "Platform display name.",
+ "type": "string"
+ },
+ "note": {
+ "description": "Why the result is unverified or an error.",
+ "type": "string"
+ },
+ "platform": {
+ "description": "Platform id.",
+ "type": "string"
+ },
+ "status": {
+ "description": "Result for this platform.",
+ "enum": [
+ "available",
+ "taken",
+ "unverified",
+ "error"
+ ],
+ "type": "string"
+ }
+ },
+ "required": [
+ "platform",
+ "name",
+ "status"
+ ],
+ "type": "object"
+ },
+ "type": "array"
+ },
+ "summary": {
+ "additionalProperties": false,
+ "description": "Platform names grouped by status.",
+ "properties": {
+ "available": {
+ "description": "Names of platforms where the username looks free.",
+ "items": {
+ "type": "string"
+ },
+ "type": "array"
+ },
+ "error": {
+ "description": "Names of platforms whose check failed.",
+ "items": {
+ "type": "string"
+ },
+ "type": "array"
+ },
+ "taken": {
+ "description": "Names of platforms where a profile exists.",
+ "items": {
+ "type": "string"
+ },
+ "type": "array"
+ },
+ "unverified": {
+ "description": "Names of platforms that need a manual check.",
+ "items": {
+ "type": "string"
+ },
+ "type": "array"
+ }
+ },
+ "required": [
+ "available",
+ "taken",
+ "unverified",
+ "error"
+ ],
+ "type": "object"
+ },
+ "username": {
+ "description": "The username that was looked up.",
+ "type": "string"
+ }
+ },
+ "required": [
+ "username",
+ "results",
+ "summary",
+ "disclaimer"
+ ],
+ "type": "object"
+}Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds useful behavioral context beyond that: results are three-state per platform (available, taken, unverified), which signals that 'unverified' is a distinct non-definitive outcome an agent must handle. It does not mention auth or rate limits, but for a read-only lookup that is minor.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
Two short sentences, zero filler, with the core operation front-loaded before the illustrative example. Every clause carries information.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
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 not be spelled out, and the description still gestures at the per-platform result set. With annotations covering safety and the schema covering parameters, nothing an agent needs to call this tool correctly is missing.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
Schema description coverage is 100%, so both parameters are already documented in the schema, including the 'Omit for all' default on platforms. The description's example (username='vintage_finds') illustrates the input but adds no format, validation, or default semantics beyond what the schema provides.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description states a specific verb and resource ('Check if ONE username is free') and quantifies scope with '12 resale platforms'. The capitalized 'ONE' makes the single-username scope unambiguous, and it is clearly distinct from the unrelated sibling compare_fees.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
Usage is implied by the availability-check framing, but there is no explicit when-to-use/when-not guidance, no mention of batching multiple usernames, and no reference to how this relates to alternatives. The only sibling, compare_fees, is a different domain, so no disambiguation is offered or needed.
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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