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

Annotations only flag non-destructive and non-idempotent; the description explains WHY: every call creates a NEW entry even for the same URL because the current filter is stored alongside, so repeated calls with identical arguments produce duplicates. It also discloses the account scoping and that object data is untouched — context 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?

The action is front-loaded, followed by scope, then the duplicate-creation caveat, then the alternatives — a sensible ordering with no filler. The trailing bilingual keyword tag ('| EN: favorite, bookmark, quick access') is minor search metadata rather than content, so not a full 5.

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 two-parameter mutation with no output schema, the description supplies everything needed to call it safely: scope, required input role, the non-idempotency trap, and the sibling to check first or use to undo.

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 both fields (url required, title optional with default) are already documented in the schema. The description only implies that url is the identity key ('auch bei gleicher URL') without adding format or validation detail, 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+resource ('Ein Objekt zu den eigenen Favoriten hinzufuegen') and constrains the effect to the caller's own account. It explicitly names the retrieval sibling (onto_favorite_index) and the inverse operation (onto_favorite_delete), so an agent can place it among the favorite_* family without opening a 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 an explicit use case ('fuer haeufig gebrauchte Objekte'), a pre-check workflow ('vorher mit onto_favorite_index pruefen') and a removal alternative (onto_favorite_delete). It does not mention onto_favorite_toggle, the closest sibling, so the agent is left to infer when to use additive add versus toggle.

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