List per-site permissions

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

readOnlyHint=true already declares the safety profile, so the description's main added value is the 'read through Services.perms' provenance and the enumeration of permission kinds — useful but not behavioral in the sense of permissions required, result shape, or ordering. It does not describe what the returned entries contain, which matters since no output schema exists.

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, front-loaded with what is listed and followed by filtering and alternative guidance. Minor cost: the trailing '…' enumeration is slightly loose, but nothing is genuinely wasted.

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 2-param read-only listing with no output schema, the description covers what is returned (permission grants by type), how to narrow it, and the counterpoint tool. A brief note on the result item shape (e.g., origin/type/state triplets) would make it fully 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% and both parameter descriptions already say 'Only this permission type' and 'Only this origin'. The description restates those semantics rather than adding format, case-sensitivity, or wildcard guidance, so the 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?

States a specific verb and resource ('List the per-site permission grants Firefox has stored'), enumerates concrete permission types, and contrasts implicitly with the mutation sibling browser_set_permission. An agent can distinguish this from other browser_* list tools at a glance.

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?

Explicitly gives the two filtering modes ('Filter by origin to look at one site, or by type') and names the alternative tool for the write path ('Use browser_set_permission to change one'). The when-to-use boundary is fully stated.

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