CSS SOTA

What's new in CSS

whats_new

Read-only

List CSS features that crossed a Baseline threshold within a date range, newest first. Use this to answer "what CSS can I start using now?", to write release notes, or to catch up after time away. Defaults to the last 180 days. The date is when a feature became interoperable across browsers, which is usually well after it first shipped in one.

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "whats_new"
  }
}

Input Schema

Name	Required	Description	Default
`limit`	No	Maximum number of features to return.
`since`	No	Start of the window (YYYY-MM-DD), inclusive. Defaults to 180 days before today.
`until`	No	End of the window (YYYY-MM-DD), inclusive. Defaults to today.
`transition`	No	Which threshold was crossed in the window: "newly" for features that became interoperable across all major engines (the usual meaning of new), "widely" for those that reached the 30-month Widely available mark, or "any" for both.	newly

Output Schema

Name	Required	Description	Default
`count`	Yes
`since`	Yes
`until`	Yes
`features`	Yes
`truncated`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedAug 2, 2026

TDQS

A4.3/5.0

Behavior4/5

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

The annotations indicate a read-only operation, and the description adds valuable behavioral context beyond that: it mentions the default window, ordering (newest first), and the semantic that the date is the interop date rather than first-ship date. This helps set expectations.

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?

The description is three sentences, with the first stating the action, the second giving usage, and the third providing default behavior and a key semantic caveat. Every sentence earns its place and there is no redundancy.

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?

Given the simple nature of the tool, the comprehensive schema, annotations, and the presence of an output schema, the description fully covers purpose, usage, and an important nuance. It does not need to explain return values.

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%, with all parameters described in detail. The description reinforces the date-range and threshold concepts but does not need to restate parameter specifics. It meets the baseline for adequate semantics.

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?

The description clearly states the action: 'List CSS features that crossed a Baseline threshold within a date range, newest first.' It specifies a distinct verb and resource plus a date-range scope, which differentiates it from siblings like check_support and search_css_features.

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

Usage Guidelines4/5

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

It explicitly names use cases: 'to answer "what CSS can I start using now?", to write release notes, or to catch up after time away.' This provides clear context, though it does not explicitly name alternative tools for other scenarios.

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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