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

Goes beyond the annotations by disclosing that the change is deferred ('takes effect the next time that meeting is processed') and that recordings:write scope is required — neither is derivable from destructiveHint/openWorldHint. It does not say whether an existing per-event prompt is overwritten or how to revert, which matters given destructiveHint=true.

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?

Three short sentences, scope constraint front-loaded in the first, with no filler. Every clause (composition, timing, auth scope) conveys callable information.

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?

Covers the essentials for a mutation tool: what it targets, how it composes with existing prompts, when it takes effect, and the auth scope. The only gap is overwrite/clear behavior, which is partly covered by the sibling hub_clear_event_custom_prompt.

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 account, eventId, workspace and customPrompt are already documented in the schema. The description adds only the implicit 'one occurrence' framing for eventId; baseline 3 applies when the schema carries the parameter burden.

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 ('Set the custom prompt for ONE calendar occurrence') and immediately scopes it against the series-level and workspace-level prompts, so an agent can distinguish it from hub_set_series_custom_prompt and hub_set_custom_prompt without opening schemas.

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?

Explains the composition semantics for a series occurrence versus a one-off meeting, which tells the agent when this tool is the correct or only target. It stops short of explicitly naming the sibling alternatives (series/workspace/clear-event tools) or stating how to remove a prompt, so it is clear context without full routing.

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