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

Annotations only tell the agent this is a mutating, open-world, idempotent, non-destructive write. The description adds substantial context beyond that: the follow may trigger notifications via email or push depending on author settings, and it returns success=false when already following or on failure. This is exactly the kind of side-effect and error semantics annotations cannot express.

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 sentences, each earning its place: the action, the notification side effect, the error/return semantics, plus the sibling redirect. Front-loaded with the action and 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?

For a single-parameter mutation with an output schema present, the description covers the side effects (notifications), the key return/error condition (success=false when already following), and the sibling boundary. Nothing material is missing 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?

Schema coverage is 100% and the single userId parameter is already documented with format and provenance ('Synoptic user ID... such as id from get_user'). The description adds nothing about the parameter, so the baseline 3 is appropriate.

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 ('Follow an author') plus the acting identity ('as the signed-in user'), and explicitly names the sibling it differs from ('Use follow_stream to follow a stream'). An agent can distinguish it from follow_stream and unfollow_user 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?

Clearly signals the alternative follow_stream for a different resource, so the agent knows when not to use this tool. It doesn't cover preconditions like checking is_following_user first, but the routing guidance is explicit for the main ambiguity.

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