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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds that the two surfaces are measured independently, which is genuinely useful behavioral context, but says nothing about response shape, caching, or what happens when the requested surface has no measurements.

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 tight sentences, no filler: the first defines the return payload and the second front-loads the key splitting behavior. Every clause earns its place.

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 two-parameter, read-only lookup with full schema coverage and no output schema, the description covers what the tool returns and how the surfaces divide. Minor gaps remain around error cases (unknown productId) and result shape, but nothing essential to correct invocation 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%: both parameters are fully documented in the schema, including the enum meaning and the productId example set. The description's remark that surfaces are measured separately restates the context parameter's semantics rather than adding new syntax or constraints, so the baseline 3 applies.

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 and resource ('Get the measured design guide for a product') and enumerates the contents (font, palette, type scale, layout, component notes), so the agent knows exactly what comes back. It does not explicitly contrast itself with siblings like check_my_ui or build_handoff, so it falls short of a 5.

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?

The note that 'marketing site and product UI are measured separately' implies when the context parameter matters, and the schema points to listProducts for the id. However there is no explicit when-to-use/when-not guidance or routing against alternatives such as pick_reference or get_screen, leaving the agent to infer the boundary.

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