Type credentials into a login form

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

Annotations only carry readOnlyHint=false, so the description adds real value: it discloses that input is real keystroke synthesis rather than value assignment (matters for JS-validated forms) and explains why the Firefox password manager is not used, with observed evidence. It does not cover failure modes (field not found, selector mismatch) or whether credentials appear in logs.

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-loads the action and the keystroke distinction in one sentence, with the submit behavior appended. The final sentence about MEASURED Firefox autofill is longer than needed for invocation decisions, but it does preempt a plausible wrong approach.

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 6-parameter mutation tool with full schema coverage and no output schema, the description covers what the tool does, how it types, and what it deliberately does not do. Only error/verification behavior (what happens when selectors don't match, how success is observable) is left unaddressed.

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 defaults for submit, context and the CSS selector fallbacks are already documented. The description only echoes the optional-submit behavior ('then optionally submit') and adds nothing about selector resolution beyond the schema. Baseline 3 applies.

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+resource ('Type a username and password into a form') with the scope narrowed further by 'real keystroke input (not a scripted value assignment), then optionally submit'. It also positions itself against browser_type and the password-manager siblings (browser_list_logins / autofill) via the 'you give me credentials, I fill them' framing.

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?

Clear usage context: it is for the case where the agent already holds credentials and should type them, and it explicitly rules out the native-autofill path. It does not name generic siblings like browser_type or browser_find_login_form as alternatives, so the routing is implied rather than spelled out.

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