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

Annotations already declare destructiveHint=true and readOnlyHint=false, so the mutating nature is covered; the description usefully adds scope (DRAFT only, channel-level default). The gap is significant, though: the schema says "Companion rules apply — see the tool description," yet the description contains no companion rules at all — a dangling pointer to guidance that does not exist — and it does not warn that AUTO wipes an existing manual selection beyond what the schema enum already says.

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 that names the target object first and then the two operating modes. No filler, no restatement of the title.

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 destructive, no-output-schema mutation with 4 parameters, the description covers what is changed and on what object, which is most of the job. It falls short by omitting the "companion rules" it is explicitly referenced as the source of, and by not indicating failure modes (e.g., non-draft campaign, mismatched channel/placement codes).

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 both enums are fully documented with their own semantics, so the baseline applies. The description's parenthetical "(Feed, Stories, Reels, ...)" lightly illustrates the placement concepts but adds no format or constraint detail beyond the schema's enum list.

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 ("Set") and a precise resource ("channel-level Meta placement default of an existing DRAFT wizard campaign's Facebook or Instagram channel"), including scope limits (DRAFT, channel-level). It is clearly distinguishable from the LinkedIn and Reddit channel-settings siblings by naming the platform and the exact object being changed.

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 description gives real preconditions: the campaign must be an existing DRAFT wizard campaign, and only Facebook/Instagram channels qualify. However, it never names an alternative tool (e.g., update_linkedin_channel_settings / update_reddit_channel_settings) or explains when an agent should reach for those instead, leaving routing to inference.

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