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

Annotations already declare destructiveHint=true and readOnlyHint=false, so the mutation profile is covered. The description adds genuinely new behavioral context: the prompt is applied additively alongside event-specific prompts, and the call requires recordings:write scope. It does not state that an existing series prompt is overwritten or point to hub_clear_series_custom_prompt for removal, which is a gap given the destructive hint.

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 tight sentences, front-loaded with the action and scope, followed by the layering rule and the auth requirement. No filler.

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?

With no output schema and a fully documented parameter schema, the description covers the essential semantics: what changes, at what granularity, and how it interacts with the sibling tool. Missing only the overwrite/clear behavior for an existing series 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 all five parameters including the auth/workspace context are documented in the schema. The description adds no parameter-level detail beyond the series-wide semantics, so the baseline 3 applies.

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?

Specific verb+resource ('Set the custom prompt for an entire recurring series') with explicit scope ('entire recurring series', 'every occurrence'). It clearly differentiates from the sibling hub_set_event_custom_prompt by naming it and describing the layering relationship.

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 semantic context in which this tool applies (series-wide, applied to every occurrence, cumulative with event-level prompts) and names the alternative sibling. It stops short of stating when an agent should prefer series-level over occurrence-level prompting, but the layering explanation makes the distinction inferable.

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