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

Annotations declare non-read-only, non-idempotent, open-world, non-destructive, so the agent already knows this mutates externally-visible state. The description adds the concrete mechanism — session cookies persisted for later calls — which explains the mutation and its downstream effect, though it omits failure/expiry 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?

A single front-loaded clause naming the action, credentials, and side effect, with zero 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?

An output schema exists and both parameters are documented, so return values and inputs need no explanation here. What remains thin is session lifecycle detail — expiry, reuse, and behavior on invalid credentials — which matters for an auth entry point but is not strictly required to invoke it.

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% and both parameters carry their own descriptions, so the description adds nothing beyond what the schema states. Baseline 3 applies since the schema does the heavy lifting.

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 ('Sign in to AFFiNE') plus the credential mechanism, which no sibling tool does. It does not explicitly contrast with the closest sibling (current_user), but the auth scope is unambiguous.

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?

'Sets session cookies for subsequent calls' implies this is the prerequisite step before other authenticated tools, which is actionable context. There is no explicit statement about when not to use it (e.g., when a session already exists) or how to re-authenticate.

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