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

Annotations already declare the safety profile (readOnly=false, destructive=false, idempotent=true, openWorld=false), so the bar is lower. The description nonetheless adds real behavioral context: duplicate handling returns already_exists, creation is immediate ('created at once'), and the returned slug/exercise_id are immediately usable in save_routine, save_plan and edit_routine. It does not cover error cases or what happens with optional fields like reasoning, keeping it short of a 5.

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 prerequisite search instruction is front-loaded before the mechanics, and the closing sentence routes to update_custom_exercise. It is dense but nearly every sentence carries a distinct instruction; the 'tell the user in one sentence what you assumed' clause is slightly tangential to tool invocation.

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 return values need not be documented, yet the description usefully notes slug/exercise_id interoperability with three sibling tools. Combined with the search-first prerequisite and the update route, an agent has enough to call this correctly; only the undocumented optional parameters remain a gap.

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 0% across 7 parameters, so the description carries the full burden and only partially delivers. It gives actionable guidance for two of them ('Fill in muscle_groups and equipment from what the user described'), but leaves name, type, notes, reasoning and unilateral entirely unexplained in both schema and description.

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 states a specific verb and resource ('added to their own Reps exercises') and immediately frames the exact case it covers: an exercise the library lacks, such as a gym-specific machine or variation. It also names the sibling it is not (get_exercise_library for search, update_custom_exercise for changes), so an agent can route without opening any 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?

It gives an explicit prerequisite ('Search get_exercise_library FIRST'), explains why (a match returns already_exists instead of a duplicate), and names the alternative for later modification ('use update_custom_exercise'). This is a rare case of when-to-use, ordering, and when-not-to-use all being stated.

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