Set the active Compass session

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

Annotations already say readOnlyHint=false and idempotentHint=true, but the description adds the critical behavioral fact that this state change is cosmetic: the registry is a label only and does not alter which Compass account requests use, since all calls route through the extension-signed browser tab. That is exactly the kind of context an agent cannot infer from annotations and prevents a serious misuse.

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 sentences, no filler. The action is front-loaded, the scope caveat follows immediately, and the routing explanation closes the loop. Every sentence carries distinct information.

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 one-parameter, non-destructive state tool with no output schema, the description supplies everything needed: the action, the prerequisite, the fact that the effect is bookkeeping-only, and how account switching actually works. Nothing material 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% and describes session_id as 'Session id to make active', so the baseline is 3. The description goes further by establishing that the id must reference a 'previously registered' session, which constrains valid values in a way the schema does not.

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 ('Mark a previously registered session_id as the current account label') and immediately disambiguates the scope of that action from what an agent might wrongly assume (it does not change account routing). The reader can distinguish it from compass_register_session and compass_get_session_context 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 names the use case ('your own bookkeeping across a conversation') and a prerequisite ('previously registered'), and gives a when-not: to actually read another account, sign that browser tab in instead. It stops short of explicitly naming register_session as the required precursor tool by name, so not quite a 5.

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