Set the active Zillow session

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

Annotations already declare non-readOnly, idempotent, non-destructive, but the description adds the critical behavioral fact that this is a label only and routing is controlled by the browser tab the extension is signed into. This is exactly the context that stops an agent from assuming the call changes account behavior.

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 tight sentences with the purpose front-loaded and the crucial caveat following immediately. No filler, though the routing sentence is slightly extended and could be trimmed marginally.

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 single-parameter, no-output tool, everything an agent needs is present: the action, the prerequisite that the session must already be registered, and the limitation that it does not affect request routing.

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 the single parameter is fully documented in-schema, so the baseline is 3. The description adds the 'previously registered' prerequisite, but no format or syntax detail beyond 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 object ('Mark a previously registered session_id as the current account label') and immediately scopes it as bookkeeping-only. An agent can tell this apart from zillow_register_session and zillow_get_session_context without opening schemas.

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 clear context ('for your own bookkeeping across a conversation') and an important when-not: it does not switch the account requests use. It stops short of naming a specific alternative tool, but the exclusion is explicit and prevents the most likely misuse.

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