Run Mobile Friendly Test

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

With no annotations, the description carries the full behavioral burden and does so well: it discloses the API shutdown date, that calling it returns an error, and that it exists only for API-surface completeness. It stops short of describing the concrete error shape or whether the call still consumes quota, but the essential behavior is unambiguous.

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?

The critical warning is front-loaded in the first line, followed by the reason and the alternative. Every sentence earns its place and nothing is wasted.

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 no explanation. Combined with the deprecation status, error behavior, and the explicit redirect to inspect_url, an agent has everything needed to decide not to call this tool.

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% and both parameters (url, request_screenshot) are documented in the schema, so the baseline is 3. The description adds nothing about parameter semantics, but the schema already does the heavy lifting for a two-param tool.

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 the specific verb+resource ('Run the Mobile-Friendly Test on a URL') and immediately qualifies it with 'DEPRECATED AND NON-FUNCTIONAL', so an agent understands both the nominal purpose and its unusability. It also names a sibling (inspect_url) so it can be distinguished without opening schemas.

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?

Explicitly says 'Do not call it expecting a result' and redirects to the alternative ('use inspect_url instead, whose response carries a mobileUsabilityResult'), naming the exact field to look for. This is textbook when-not-to-use plus the replacement path.

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