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

Annotations declare non-destructive and idempotent, and the description adds genuine context beyond that: a closed timeline is not deleted, all tools work on it as before, and get_timeline reflects the `open` state as last saved. This resolves the main behavioral ambiguities for a UI-state mutation.

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 purpose is front-loaded in the first clause, the parenthetical clarifies tab semantics without waste, and the comparison to set_active_timeline follows immediately. Every sentence 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?

For a four-parameter UI-state tool with full schema coverage and annotations, the description covers the remaining gaps: what opening a tab means, what closing means, how activate changes behavior, and how the state is reported. Nothing an agent needs is missing.

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 coverage is 100%, so the baseline is 3, but the description adds real meaning for `activate` ('By default it does not change what the user is looking at; activate:true also shows it') beyond the schema's terse 'Also show it (default false).' The other parameters are already fully documented in the schema.

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 and resource ('Opens a timeline's TAB in the editor') and immediately distinguishes the action from set_active_timeline by noting the default does not change what the user is looking at. An agent can tell this apart from siblings like set_active_timeline and close_timeline without opening the 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?

Explicitly describes the default behavior and the condition that selects the alternative: 'activate:true also shows it, like set_active_timeline.' It also clarifies that closing tabs is cosmetic and every tool still works, which is exactly the context needed to decide whether to call this tool at all.

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