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

With no annotations, the description fully carries behavioral disclosure. It reveals account-level sharing, idempotency, the lack of effect on visibility, and the exact two grants (being listed and being allowed to call list_subscribers, which hard-denies non-subscribers). It also notes the tag validation constraints. This is exhaustive transparency for a mutation.

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?

Every sentence adds essential information. The key distinction (account-level vs per-thread) is front-loaded, followed by idempotency, non-effects, and grants. No filler or redundancy; the length is justified by the complexity of the behavior it must convey.

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 subscribe action with subtle implications, the description covers all necessary aspects: what subscribing does, what it doesn't do, idempotency, and interaction with list_subscribers. It also details parameter constraints. No output schema is needed for a void operation, and the description is sufficient for correct invocation.

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?

The schema covers 100% of parameters (tag and owner_key) with detailed descriptions. The tool description repeats the tag format and does not add new parameter-specific meaning beyond what the schema already provides. Baseline 3 applies because the schema does the heavy lifting; the description adds context about account-level state but not parameter semantics.

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 clearly states the action: 'Subscribe the caller's account to a tag', with a specific verb and resource. It distinguishes itself from siblings like unsubscribe_tag and list_subscribers by explicitly stating what it does not do (does not change thread visibility) and what it grants (inclusion in subscriber lists and access to list_subscribers).

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 explains the effects and side conditions: it is idempotent, account-level, and grants specific capabilities. It implicitly routes to the correct usage (e.g., to become a subscriber or to call list_subscribers) but does not explicitly contrast with alternative tools like unsubscribe_tag. The context is clear enough for an agent to decide when to use it.

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