gmail_inbox.fetch_image

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

With no annotations, the description carries the full behavioral burden. It usefully discloses the return encoding (base64) and that it acts as a proxy for CSP-blocked images, but says nothing about permission/auth requirements, URL validation, or failure behavior for an outbound network fetch — a meaningful gap for a proxy-style tool. Output schema covers the response shape, lowering the bar somewhat.

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?

A single front-loaded sentence with the purpose first and the triggering context in a parenthetical. No filler, nothing that fails to earn 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?

The output schema covers return values, and the description covers purpose plus trigger, so the core is complete. But with no annotations and two undocumented parameters (notably user_id), an agent still lacks enough to invoke it confidently in every case.

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 0%, so the description must compensate, and it does not. It implies 'url' is a remote image URL but gives no format or validation detail, and the 'user_id' parameter is never mentioned at all — an agent has no basis for knowing why fetching an image needs a user identity.

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 precise verb+resource ('fetch a remote image referenced by an email's HTML') and the return format ('base64'), which distinguishes it from attachment-oriented siblings like gmail_get_attachment and gmail_composer.get_attachment. An agent can tell what this does 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?

The parenthetical names the triggering condition clearly ('when the host CSP blocks remote img-src'), which tells the agent exactly when this tool is the right call. It stops short of naming alternative tools or exclusions, but the invocation context is unambiguous.

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