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

Annotations already declare readOnlyHint, idempotentHint and non-destructive, so the safety profile needs no restating. The description adds genuinely new behavior: the photo comes from the DataLikers database rather than a live fetch, and a text message is returned when no photo is stored. Failure-mode disclosure is the valuable part; it omits nothing critical.

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, front-loaded with the action before the usage triggers and return behavior. The bilingual trigger list is slightly bulky but directly serves tool-selection, so it 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?

Despite there being no output schema, the description covers the return contract (image content vs. text message on no stored photo) and the source of the data. Together with read-only annotations, an agent has everything needed to call and interpret this tool.

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, and the description's 'pass `username` or `user_id`' merely echoes the schema's 'Use either username or user_id.' It adds no precedence rule, format detail, or behavior for when neither is supplied.

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 ('Show an Instagram user's profile photo as an image'), which unambiguously separates it from the profile-data siblings like get_user_by_username or get_user_by_id. An agent can tell exactly what this returns without inspecting any 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?

Gives explicit trigger phrases in two languages ('show photo', 'show picture', 'show avatar', 'покажи фото') and names the inputs to pass, so routing conditions are clear. It stops short of naming alternatives or stating when NOT to use it (e.g. when a live fetch or a non-stored avatar is wanted), which keeps it out of the top band.

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