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

The description discloses that the tool mutates the library, affects personalization (strongest signal in For You feed and email digest), and is idempotent. This adds context beyond the annotations (readOnlyHint=false, destructiveHint=false). No contradiction.

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, each adding value: purpose, mutation effect, idempotency, alternative tool. No unnecessary words, well front-loaded.

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?

Given the simple single-parameter tool with an output schema, the description provides complete context: purpose, behavior, idempotency, auth requirement, and sibling alternative.

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 only parameter (arxiv_id) is fully described in the input schema (100% coverage). The description does not add extra details about the parameter beyond what the schema provides, so baseline 3 is appropriate.

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 clearly states the tool saves a paper to the user's library (bookmark). It distinguishes from the sibling add_to_collection, which also saves but additionally files into a collection.

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?

The description explicitly tells when to use this tool vs. add_to_collection, mentions the requirement for SF_API_KEY, and notes idempotency, providing clear usage guidance.

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