fill_credential

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

With no annotations, the description carries the full burden and does so richly: the password never crosses the WebSocket bridge, the domain is resolved internally for security, the vault must be unlocked, and cross-agent access is rejected with TAB_NOT_OWNED. These are exactly the security and failure-mode details an agent needs before invoking.

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 what the tool does, then security model, then multi-agent/tab behavior. Five dense sentences each carry information, though the domain backward-compatibility point is stated twice (description and schema).

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 sensitive 2-param mutation of a login form with no output schema and no annotations, the description covers prerequisites (unlocked vault), security (credential never leaves extension), multi-agent ownership, and the error code — everything needed to call it correctly.

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 schema already documents that domain is ignored/derived and how tab_id resolves. The description largely restates this, so beyond the baseline 3 it adds little new parameter meaning.

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+resource (auto-fill login form) with the exact target (saved credential on the active tab). This is clearly distinguishable from siblings like list_credentials (which only lists) and use_payment_method (different resource).

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?

Explains when to pass tab_id (only when the agent owns multiple tabs) versus omitting it (auto-resolves otherwise), and flags the prerequisite that the vault must be unlocked. It doesn't explicitly name alternatives, but the use case is unambiguous.

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