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

Annotations already declare idempotent, non-destructive mutation; the description adds the non-obvious side effect that a closed timeline tab will be opened, and that the change is essentially cosmetic. No contradiction with readOnlyHint=false since it persistently changes project state.

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?

Two tight sentences with the core behavior front-loaded and the caveat about necessity placed after it. Nothing is redundant.

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 simple, fully-documented three-parameter tool with annotations covering safety, the description supplies purpose, side effect, and applicability. Only the absence of any return-value or failure hint (e.g. unknown timelineId) keeps it off a 5.

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?

All three parameters are fully documented in the schema, so the baseline applies. The description only implicitly identifies timelineId as the timeline to activate; it adds no format or identifier details beyond what get_projects/get_timeline already supply.

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?

Specific verb and resource: it sets the project's active/default timeline, and additionally clarifies the editor-tab side effect ("opening its tab if it was closed"). The parenthetical about what the user sees distinguishes it from a plain open operation.

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 clearly frames when this matters (project default and editor view) and warns that it is "purely a convenience — every tool here takes an explicit timelineId regardless," which effectively tells the agent it usually can be skipped. It stops short of naming the sibling alternative (open_timeline) for agents who only want to open a tab.

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