nex_browser_fill_credentials

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

With no annotations provided, the description carries the full burden and does well: it discloses that values cross the model/MCP boundary, that they must never be stored or repeated, and explicitly states the tool never submits the form. It could add more about return values or errors, but the security-relevant behavior is well covered.

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 tight sentences, each earning its place: purpose, alternative/security rationale, and non-behavior. No fluff, information-dense, and front-loaded with the core action.

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 an 8-parameter tool with no output schema, the description covers the critical aspects: what it does, when to use an alternative, and key security guarantees. It could mention what happens on success/failure or how it interacts with the visible page state, but overall it's quite complete for its purpose.

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 63% (5 of 8 params described); the description doesn't add parameter-specific detail beyond emphasizing the sensitivity of the literal values and their placement 'into visible fields.' The username/password/totpCode params lack descriptions in the schema, and the description doesn't elaborate on them either, so it doesn't fully compensate for the coverage gap.

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?

The description uses a specific verb+resource construction: 'Fill literal login credentials... into visible fields.' It clearly distinguishes itself from the sibling nex_browser_fill_account by contrasting literal vs. stored credentials, and it adds important scope with 'never submits the form.'

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 explicitly names the alternative tool (nex_browser_fill_account) and gives the exact condition for preferring it ('when a stored vault credential is available'). This directly fulfills the when-to-use vs. alternatives guideline.

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