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

Annotations already declare it is a write (readOnlyHint=false) that is non-destructive and closed-world, so the safety profile is covered. The description adds real behavioral context beyond that: rename semantics (a rename starts a history over), the confirmation requirement, the "Saves on return" persistence behavior, and the inverse "unlink separates" mode.

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?

Three dense sentences, zero filler, with the purpose front-loaded before the precondition and parameter notes. Every clause 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?

No output schema exists and the annotations cover the safety profile, so the description need not explain return values. It covers purpose, precondition, behavior, and all three params; only edge details (e.g., what happens when names overlap an existing history, or the effect beyond "saves") are left unstated.

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%, so the description must carry the burden, and it does: "`names` continue `as`" documents the direction of the merge for the two required params, and "`unlink` separates" documents the third. This meaningfully compensates for the undocumented schema, though it stops short of noting array limits (maxItems 6) or the return/error shape.

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 ("Join the lift histories of one movement logged under two names") and immediately clarifies the rename-causes-split semantics that motivate the tool. This distinguishes it from siblings like rename_activity or edit_activity, which do not merge history.

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 a hard precondition: "ONLY after the athlete confirmed it is the same exercise," which is exactly the gating an agent needs before merging histories. It does not name an explicit alternative tool or spell out a when-not case, but the confirmation gate is a strong, actionable usage rule.

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