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

readOnlyHint=true already establishes this as a safe read, so the bar is lower. The description adds the mutual-exclusivity rule for the two modes, but says nothing about pagination, rate limits, or the shape/size of the returned file list.

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 tight sentences plus an endpoint reference; the core behavior and the required parameter pattern are front-loaded with no filler.

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?

With no output schema, the description still communicates what comes back (files with changed coverage) for both modes, which is sufficient for a read-only comparison tool. Only the absence of return-format or pagination detail keeps it from a 5.

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 every parameter is already documented, including the base/head pairing hints. The description reinforces the either/or relationship between base+head and pullid, which is marginally useful, but baseline 3 applies when the schema does the heavy lifting.

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 ('List the files whose coverage changed') and scopes it to two comparison modes, which cleanly separates it from generic siblings like codecov_compare or codecov_get_commit. It does not explicitly name the sibling it is distinguished from, so it stops short of a 5.

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?

It gives explicit mode-selection logic: 'Provide EITHER both `base` and `head`, OR `pullid`.' That tells the agent exactly which invocation shape to use, though it never says when this tool is preferable to other coverage/commit tools.

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