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

Annotations declare readOnlyHint=false and idempotentHint=false, so the agent already knows this is a non-idempotent mutation surface. The description adds nothing about merge irreversibility, the effect of delete_branch_after_merge/force_merge, or auth/rate-limit constraints, so it goes only marginally beyond the structured fields.

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 sentences, zero filler. The purpose statement is front-loaded and the action-routing hints follow, so a skimming agent gets the tool's identity first and the disambiguation second.

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 21-parameter, eight-action dispatcher with no output schema, the description covers identity and partial action routing but omits how actions map to required inputs (index for everything but list/create), pagination behavior on list, and the difference between this tool and reviews/comments. The complete schema prevents this from being worse, but the description alone is thin for the tool's complexity.

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% across all 21 parameters, including per-action annotations ('Required for create', 'For merge', 'For list'), so the schema carries the parameter burden. The description adds no syntax, format, or default-value detail beyond it, making 3 the correct baseline.

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 resource (pull requests) and enumerates the operation set (create, update, merge, inspect), and the second sentence disambiguates three of the eight actions. It does not distinguish this tool from siblings like reviews, comments, or commits, which an agent could reasonably confuse with PR-related operations.

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 routes the agent among internal actions ('diff' for unified diff, 'list_files' for changed paths, 'list_commits' for commits), which is genuine guidance, but it never says when to pick pull_requests over the sibling reviews/comments tools, nor does it surface prerequisites beyond what the schema already encodes.

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