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

Annotations already mark this as read-only, and the description adds useful behavioral details: output is base64 and 'refused' signals that no retrievable media exists for the id. This goes beyond the structured annotations without contradicting them.

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 sentences with no filler. The core capability is stated first, followed by the key input source and the most important return sentinel. Every sentence 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 simple two-parameter fetch operation with a readOnlyHint and an output schema, the description covers the essential behavior: what is fetched, how it is encoded, where the id comes from, and what 'refused' means. Nothing critical 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%, so both parameters are already documented. The description's mention of using message_id from wa_list_messages reinforces the schema but adds no significant new semantic beyond what the schema already provides.

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?

Description clearly states a specific verb ('Fetch'), the resource ('attachment...a received WhatsApp message'), and even enumerates media types. It distinguishes itself from the many sibling tools by focusing on retrieving media rather than sending or managing chats.

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 instruction 'Use the message_id from wa_list_messages' gives a concrete prerequisite and source for valid input. It does not explicitly name alternative tools or when not to use this tool, but the received-message scope makes the usage context reasonably clear.

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