replay_diff

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

Annotations already declare readOnlyHint=true and openWorldHint=false, covering the no-mutation behavior. The description adds useful detail about what the comparison surfaces (metric deltas and file-touch differences), but it does not disclose limitations such as how missing or invalid session IDs are handled or whether the diff is a full side-by-side report.

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?

A single, front-loaded sentence that states the core action and the concrete comparison dimensions. Every phrase earns its place, with no filler, tautology, or unnecessary restatement of the tool name.

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 simple two-parameter read-only tool, the description is largely complete: it names the inputs' purpose, the operation, and the expected output content. A small gap is the absence of any note about how the comparison is presented or whether the returned data is a summary or a detailed report, but this is not critical given the tool's low 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%, with both parameters described as session IDs. The description confirms that the two IDs represent the sessions being compared, but adds no extra meaning beyond the schema, so the baseline score of 3 is appropriate.

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 ('Compare') and resource ('two sessions side by side'), and enumerates the exact kinds of differences produced: metric deltas for tool calls, errors, duration, files, plus per-file touches. This makes it clearly distinct from siblings like replay_sessions or replay_stats, which list or aggregate sessions rather than diffing two of them.

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 implies the tool is for when a user wants to compare two sessions, but it does not explicitly state when to prefer it over alternatives or when not to use it. It also gives no context about prerequisites, such as whether sessions must be replayed or in the same workspace.

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