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

Goes well past the annotations: it explains that read receipts are a real, externally visible side effect ('sender sees the messages as read (blue ticks)'), which justifies readOnlyHint=false, while also stating no message is sent and no message.* event fires. It adds plan gating and the exact failure mode ('payment_required: feature_not_available') with the warning that retrying is futile.

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 action and the recipient constraint, and every paragraph carries information. Minor deductions for a parenthetical aside ('the number also appears online first') that is hard to parse and for line breaks splitting sentences mid-clause.

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 four-parameter mutation with an output schema and annotations already present, the description supplies the remaining essentials: side effects visible to third parties, plan prerequisite, error code, and non-retryability. An agent has everything needed to decide and call correctly.

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 each parameter already documents its format, the exactly-one-of rule, and the cross-inbox caveat for ids. The description's 'Set exactly one recipient: number, group_id, or contact_id' restates the schema rather than adding meaning, so the baseline of 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 ('Mark a conversation's unread messages as read') plus the key negative scope ('without sending anything'). An agent can immediately separate this from send_text, reply_to_message, or react_to_message, none of which exist to clear unread state.

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 this is independent of the automatic mark-as-read triggered by send/reply/react/edit/delete/typing, and gives the one condition that selects it: clearing a badge on its own, e.g. handing a conversation to a human who has already seen it. That is a real when-to-use and implicit when-not-to-use rule.

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