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

Annotations only declare the mutation/idempotency/safety profile, and the description adds real behavioral context on top: loads never carry over between exercises, the new exercise brings its own rules, an undo is returned, and untilWeek determines whether the swap is one-off or persists through a program week. None of that is derivable from the annotations or schema.

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 trigger condition is front-loaded and the remaining sentences each carry a distinct load-bearing fact (replacement source, load-carry-over, untilWeek scope, undo, exclusion). A few phrasings are terse to the point of ambiguity, notably 'brings its own rules,' but there is no filler.

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 four-required-parameter mutation with an output schema, the description covers sourcing the exercise, durability scope, side effects, reversibility, and sibling exclusion. The dual meaning of 'slot' and the exact effect of 'its own rules' are the only gaps an agent might stumble on.

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?

With only 20% schema description coverage, the description carries must of the burden and does so for exerciseKey and especially untilWeek, spelling out the null-versus-week durability semantics that the bare schema does not convey. The 'slot' parameter remains unexplained in both places, which keeps this short of a 5.

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 opens with a specific verb and resource plus the triggering condition: swapping an exercise in a session. It clearly distinguishes itself from list_alternatives (used to pick the replacement) and move_session (for relocating sessions), so an agent can route correctly 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?

It states when to use the tool, the two paths for choosing the replacement (user-chosen exerciseKey or a same-movement one from list_alternatives), and an explicit exclusion with the correct alternative: 'Do not use to move a session; use move_session.' That is about as complete as routing guidance gets.

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