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

Adds real behavioral context beyond the annotations: the no-op-when-app-closed semantics and the resulting player state. That is valuable for an agent that needs to know the call will not fail. It never explains why destructiveHint=true — skipping a track discards playback position — which is the one annotation an agent would most want justified.

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 short sentences, zero filler, with the core action stated first and preconditions and outcome trailing. Nothing to cut.

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 the return-value sentence is bonus rather than necessity, and the no-op condition covers the main failure mode. The only gap is edge behavior at the start of a queue or playlist, which the description leaves unstated.

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% and the single parameter ('confirm' — 'Ignored; kept for older clients') is fully documented in the schema itself. The description adds nothing about it, which is acceptable at this coverage level, 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?

States a specific verb and resource ('Goes back to the previous track in the Music app') with the direction of travel front and center, which is exactly what separates it from music_next_track, music_play and music_pause. An agent can disambiguate 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 precondition/negative case: 'Does nothing when the Music app is not open,' which tells the agent the call is a safe no-op rather than an error. It stops short of naming an alternative (e.g. music_next_track or music_play) or saying what to do when the app is closed, so it is clear context without explicit routing.

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