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

Annotations already establish readOnlyHint=true and openWorldHint=false, and the description adds real behavior: what gets returned (first/last decode dates, decode_count, dated change entries) and the meaning of an empty result ("measured, stable so far"). That empty-case semantics is genuinely useful beyond the annotations, though it omits any volume/pagination limits for long histories.

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?

Front-loaded with the core identity and the return shape up front; the parenthetical examples (radius, hover color, motion duration) are dense but they earn their place by showing the diff granularity. Slightly longer than needed — the "live-decoded domain / Decode Ledger" framing and the trailing aside about screenshot libraries could be trimmed.

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?

With no output schema and no annotations about returns, the description fully specifies what comes back (first/last decode dates, decode_count, dated entries) and the empty-history case. Nothing an agent needs in order to call this correctly appears to be 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 coverage is 0%, so the description carries the burden for the single `site` parameter, and it does: "a domain ('stripe.com') or product name" with an example. This resolves the main ambiguity (domain vs product key) but leaves unresolved what happens on an unknown site.

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?

States a specific verb+resource (measured design change history / Decode Ledger) and explains it concretely as token-level diffs between deep decodes over time, with inline examples of the diff format. It does not, however, distinguish itself from close siblings like list_design_changes or get_design_drift, so an agent must infer the boundary.

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?

"Use it to see how a product's design system is EVOLVING (no screenshot library can backfill this)" gives a clear when-to-use rationale and even a contrast against an alternative source of truth. It stops short of naming sibling tools or stating exclusions, so no explicit routing guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.