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

Annotations already establish that this is a non-read-only, non-destructive write, so the description need not restate safety. What it omits is the key behavioral question for a 'set' tool: whether calling it replaces previously declared families or adds to them, and what happens with an invalid or unavailable family name.

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 sentence, front-loaded with the action and resource, with no filler. The trailing clause about `family` syntax is slightly awkward but still serves a purpose.

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 two-parameter mutation with no output schema, the definition is workable but thin. It leaves out replacement semantics, validation behavior, and whether the font declaration applies retroactively to existing ad pieces, which an agent needs before calling.

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%, and the schema already supplies the format example 'Inter:wght@400;700'. The description's pointer to Google Fonts `family` parameter syntax adds a small amount of meaning (the format is externally defined), but it largely duplicates the schema example.

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?

The description gives a specific verb and resource: 'Declare the Google Fonts families the set uses.' That distinguishes it from nearby siblings like ads_set_title and ads_edit_piece, which operate on different resources. It stops short of stating the effect on already-set fonts.

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?

Usage is only implied: an agent can infer this is the call for registering font families on a document. There is no explicit when-to-use, no statement about whether it should follow start_ads, and no mention of alternatives such as styling entered elsewhere.

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