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

Annotations already convey that this is a non-readonly, non-destructive, non-idempotent mutation. The description adds useful context by stating the mutation target, requiring API key or OAuth with scope, warning that share tokens cannot write, and listing a $0.10 cost. It does not mention duplicate-entry behavior, but the annotation context already covers the core safety traits.

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?

The key fact (mutation target) is front-loaded, followed by a clear usage rule, example, and auth/cost context. There is minor redundancy in repeating that API key is required after already saying 'Requires API key or OAuth with scope,' but overall the description is compact and every section earns its place.

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 an output schema present and annotations covering the safety profile, the description provides enough context: what is mutated, when to use it, how to format the main parameter patterns, auth requirements, and cost. It does not discuss duplicate-logging behavior or error conditions, but those are not essential for invoking the tool correctly given the schema and annotations.

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%, so the baseline is 3. The description adds a concrete usage example showing a 'track' blob mapping ('Henry Mancini - Piano And Strings (1995 Remastered)') and clarifies that title + artist can be used instead. It also highlights optional fields (year_note, source_url) in the narrative, providing practical guidance beyond the schema.

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 names the specific verb 'MUTATES', the resource ('authenticated user favorite tracks list at /music'), and explicitly distinguishes this from storing the data as chat memory or in music_preferences. It is immediately clear which action the tool performs and how it differs from read-only siblings like get_* and from other logging tools like log_event or log_food.

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?

Includes an explicit trigger: 'Use when they favorite or save a song' and a clear exclusion: 'do not store this as a chat memory or in music_preferences.' It also states auth preconditions (API key or OAuth with scope, share tokens cannot write), so the agent knows exactly when the call is appropriate and what is required to invoke it.

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