Register a signed-in Zillow session

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

Annotations cover the safety profile (idempotent, non-destructive, not read-only), and the description adds non-obvious behavior beyond them: the registry is a label only and does not change which account requests use, plus the browser-tab constraint governing every zillow call. This is exactly the kind of caveat an agent cannot get from structured fields.

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?

Front-loaded with the core action, followed by idempotency, return value, parameter tip, and the routing caveat. Every sentence carries distinct information with no filler.

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?

Covers the action, the update semantics, the returned session_id (important since there is no output schema), and the critical constraint that the registry does not affect request routing. Nothing needed to call it correctly 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 description coverage is 100%, so all three parameters are already documented; the description's mention of 'keyed by account_identity' and 'pass mark_active: true' largely restates the schema. auth_expires_at is not elaborated on at all, so no value is added beyond the baseline.

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 ('Record (or refresh) which Zillow account the signed-in session belongs to') and names the keying field, which distinguishes it from siblings like zillow_set_active_session and zillow_get_session_context. An agent can identify the tool's scope 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?

Gives clear context: re-registering the same identity updates the existing entry, and mark_active can promote it in the same call. However, it never explicitly names or excludes the sibling zillow_set_active_session, leaving the agent to infer when to use that instead.

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