Save a login

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

Annotations already flag this as non-read-only and destructive, so the description adds genuine value by disclosing the failure mode ('Can't add a login without a httpRealm or formActionOrigin'), the defaulting behavior, and that the password is written to the browser and never echoed back.

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 compact sentences, front-loaded with the purpose, then the constraint. The 'MEASURED:' framing is a slightly internal tic but each sentence carries real 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 four-parameter mutation with annotations and no output schema, the description covers the operation, the likely failure condition, and credential handling. It stops short of stating the return behavior (e.g., whether a login ID comes back) or duplicate-handling, which would make it complete.

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%, so baseline is 3; the description goes beyond it by explaining why formActionOrigin matters (the nsILoginInfo.init requirement and the httpRealm alternative) and by reinforcing that password is write-only, which the schema only states tersely.

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?

Specific verb plus resource: 'Save a username/password pair in the Firefox password manager.' An agent can tell this persists credentials rather than filling a page (browser_fill_login) or listing existing entries (browser_list_logins), though no sibling is named explicitly.

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?

The description never says when to choose this over browser_fill_login, browser_import_logins, or browser_import_logins_from_file, nor what prerequisites exist (e.g., profile unlocked, existing duplicate logins). Only an internal init constraint is mentioned, which is not usage guidance.

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