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

Annotations already declare readOnlyHint=false, idempotentHint=true, and destructiveHint=false, so the mutation profile is covered. The description adds real behavioral context the annotations don't: what marking actually changes (links pass check_email's local-links check instead of failing) and the consequence of not marking. It stops short of saying whether a mark can be removed or what happens on repeat calls beyond the idempotent hint.

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?

Purpose is front-loaded in the first clause, with rationale following. The 'as in Project settings:' aside and stacked parentheticals add some clutter, but every sentence still contributes information about behavior or usage.

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, non-destructive, idempotent mutation with no output schema, the description covers purpose, effect on downstream checks, and when to use it. The main omission is the inverse operation (how to unmark a host), which is minor given the tool's narrow scope.

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 the single 'host' parameter is fully documented in the schema, including the 'localhost:3000' example. The description adds no format or syntax detail beyond what the schema already provides, 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 ('Marks a host as a dev environment of this project') and immediately ties the effect to a named sibling, check_email's local-links check. An agent can distinguish this from sibling tools like check_email or update_address without opening the schema.

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 names the trigger ('Use it when testing an app that runs on your machine') and the counterfactual ('links to an unmarked localhost still fail'), which tells the agent both when to call it and when calling it would be wrong. The rationale about catching staging/live mistakes reinforces the boundary.

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