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

Annotations already cover idempotency (idempotentHint=true), non-destructiveness, and non-read-only status. The description adds genuinely new context: listing and reading never change read state, so this is the only way to mutate read status, plus a 100-id cap. It doesn't address side effects like notifications, so not a full 5.

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 dense sentences, front-loaded with the operation and the id source, followed by the usage rationale. No filler.

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 mutation tool with rich annotations and no output schema, the description covers purpose, timing, defaults, and limits. Only minor gaps remain (no mention of error behavior for invalid ids), which is acceptable here.

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 100%, and both parameters are documented there (read defaults true, ids are email ids, maxItems 100). The description restates read=false and the 100-id limit but adds no syntax or format meaning beyond the schema, so 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 (mark) and resource (emails) and explicitly ties the ids to their source (list_emails). The distinction from read_email and list_emails is clear without opening any 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?

Gives explicit timing ('call this once you've dealt with an email') and explains why reads don't require it, plus points to list_emails' unread_only for filtering. This is actionable when-to-use guidance with a named alternative.

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