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

Annotations already declare readOnly=false, openWorld=true, idempotent=false, destructive=false. The description adds genuinely new behavior: an invalid scheme returns `code: invalid_field` and does not call the portal, and the destination key is never returned in the response. It stops short of covering duplicate-target behavior, which matters given idempotentHint=false.

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?

Front-loaded with the action, then validation rules, then output handling — a sensible ordering, and every sentence carries information. Minor redundancy: 'the destination key is not in the response' at the top and 'Do not echo the destination key back' at the bottom say nearly the same thing.

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?

An output schema exists, so return values need no explanation, yet the description usefully confirms which fields to surface (id, host) and which to suppress (key). For a 3-parameter mutation with full schema coverage and annotations, this is close to complete; only duplicate-target and stream-state preconditions are absent.

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%, so `stream_id`, `simulcast_url`, and `simulcast_stream_key` are already fully documented, including the rtmp/rtmps constraint and the 'never returned' note. The description largely restates these facts; its only real addition is the error-code consequence of a bad scheme, which is behavioral rather than 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?

States a specific verb+resource: 'Add a simulcast target.' An agent can distinguish it from the sibling remove_simulcast_target by the verb alone, though the description never explicitly names a sibling or the parent live stream context.

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?

Usage is implied rather than stated: it never says when to add a simulcast target vs. other setup steps, nor what preconditions (e.g. an existing live stream in a particular state) are required. It does give clear post-call usage instructions ('Tell the user the returned target id and host'), which is a partial substitute.

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