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

Annotations already cover the safety profile (readOnly, non-destructive), so the description's job is to add behavior — and it does: accepted formats (JPEG/PNG/WebP), the returned structure (each detected food with weight and nutrition), and the operational gotcha that clients do not attach images automatically. It does not discuss failure modes for non-food or low-quality images, so not a 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?

Four short sentences, front-loaded with purpose, then format, then return shape, then usage condition. No filler and nothing repeated from annotations.

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?

With no output schema and only two fully-documented parameters, the description fills the remaining gaps: accepted input formats, the return contents, and the explicit-passing requirement. An agent has everything needed to call it 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 coverage is 100%, so the baseline is 3, but the description adds meaning the schema does not: it reinforces that the image must be passed explicitly as image_base64 and enumerates the supported formats beyond the raw enum-less string. That extra framing justifies a 4.

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 ('Estimate calories and macros from a photo of food') and immediately distinguishes the tool from text-based siblings by noting the photo input. An agent can tell this apart from get_food_nutrition or search_foods 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 gives the selecting condition: 'Use when the user supplies a photo rather than naming a food,' which routes the agent correctly away from the name-based lookup siblings. It does not name a specific alternative tool or state exclusions (e.g., non-food images, multi-image), so it falls short of a 5.

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