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

Annotations only declare the mutation profile; the description adds real behavioral context on top: the return is a browser-opening link, the session supports immediate composition edits, frame rendering/playback/media require the tab to be open, and the scene expires 24 hours after the last change. The state lifetime and capability boundary are exactly what an agent cannot infer from the annotations.

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?

Four sentences, front-loaded with the selection condition, and each carries distinct information (trigger, return value, limitation, retention). The clause 'Compositions can be created and edited at once' is the only slightly loose phrasing, but nothing is padded.

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 zero parameters and no output schema, the description carries the full burden and meets it: it explains the trigger, what is returned, what the session can and cannot do without the tab, and how long state persists. Nothing an agent needs to call this correctly is missing.

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 tool takes zero parameters, so there are no semantics to explain and the baseline of 4 applies. The description spends its words on behavior rather than restating an empty schema, which is the right allocation.

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 ('Starts a Motion session') plus the exact condition that scopes it ('when the user has no pairing code'), which implicitly separates it from connect. It also names the returned artifact (a link) and references wait_for_tab, so an agent can identify the tool without opening the schema.

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?

Gives a clear trigger condition ('no pairing code') and an explicit exclusion with a pointer to the alternative for media work ('see wait_for_tab'). It never names the sibling that handles the with-pairing-code case, so the selection rule is strong but not fully closed.

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