Find the closest CSS named colour to a hex value, with the distance score. Pure CPU, instant.
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": {
"color": "<color>"
},
"name": "color_name_lookup"
}
}| Name | Required | Description | Default |
|---|---|---|---|
| color | Yes | Hex colour |
Changes observed during successful MCP inspections.
Input schema / properties / colorAdded value: +{
+ "description": "Hex colour",
+ "type": "string"
+}Input schema / requiredAdded value: +[
+ "color"
+]Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full load; it usefully discloses that the operation is local ('pure CPU, instant') and that a distance score is returned even though no output schema exists. It does not say what happens on malformed input, whether '#' prefixes or 3-digit shorthand are accepted, or what distance metric is used.
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, front-loaded with the core action, and the performance note at the end. Nothing is wasted or repeated.
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?
For a one-parameter, deterministic lookup with no output schema, the description covers the action, the return content, and the cost profile. The remaining gap is input format edge cases, which keeps it short of a 5.
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?
There is a single parameter with 100% schema description coverage ('Hex colour'), so the schema already does the documentation work. The description's 'to a hex value' merely restates that, adding no format rules or validation hints.
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: find the closest CSS named colour for a hex value, plus the returned distance score. That is far more specific than the sibling 'color_names' would imply, though it does not explicitly distinguish itself from 'color_names' or 'color' by name.
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 only implied: the phrase 'closest CSS named colour' suggests mapping an arbitrary hex to a human-readable name, but there is no explicit when-to-use, when-not-to-use, or pointer to 'color_names' / 'color_harmony' as an alternative.
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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