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

Annotations already declare readOnly, idempotent, non-destructive. Beyond that, the description adds a genuinely useful behavioral trait: signed public picture URLs expire, so the agent knows not to cache results and why 'current' matters. It omits what the return payload actually is (a URL), which keeps it short of 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?

One front-loaded sentence with no filler; the expiry caveat follows the core action and every clause 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 read with annotations covering safety, the description plus schema give enough to call correctly, and 'signed public picture URLs' implies the return is a URL. No output schema exists, so a brief statement of the returned value would have closed the last gap.

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 50% and the user_id branch already documents the numeric-ID requirement and exclusions exhaustively; account_id is also documented. The description only reiterates 'resolved profile' without adding format or sourcing guidance, so baseline 3 is appropriate.

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 ('Download the current picture of a ... profile'), which is unambiguous against most siblings. It does not explicitly distinguish itself from facebook_get_profile or facebook_get_chat_picture, so the differentiation rests on the tool name rather than the description.

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 phrase 'resolved Facebook profile' implies the profile must already be resolved (via facebook_resolve_person/facebook_get_profile) before calling, which is a useful prerequisite. However, it never names an alternative or states when not to use it, so usage is only implied.

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