credential_fill

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

With no annotations, the description carries the full burden and does contribute one important behavioural fact: the agent never receives the credential, which tells the caller not to expect secret material back. However, it omits whether a profile must be started/leased first, what happens on auth failure, and whether the action is reversible or has side effects on the profile.

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?

Two short sentences with zero filler, and the core action is front-loaded before the security caveat. Nothing is wasted or padded.

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 zero-parameter action tool the description covers the essential action and the credential-handling property, but it leaves out the prerequisite state (which profile, whether it must be leased or started) that an agent needs to invoke it successfully in the right sequence.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate; the baseline for a parameterless tool is 4. The description correctly implies the target is implicitly the active profile rather than an argument.

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 concrete action (sign in) performed against a specific resource (this profile's stored login), which is more informative than a tautology. It is distinguishable from sibling tools like credential_store or credential_new in kind, though the description never names those siblings to make the boundary explicit.

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?

There is no when-to-use guidance, no stated prerequisites, and no mention of alternatives such as credential_store (to save a login) or lease_profile/profile.start (to obtain a profile context). The agent must infer that this is the tool for authenticating with previously stored credentials.

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