diff_state

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

With no annotations, the description carries the full burden. It discloses that it compares against the last cached snapshot, returns added/removed/text-changed element lists (stable-id diff), and both screen hashes. It also clarifies its relationship to automatic evidence. It does not explicitly state it is read-only or what happens if no snapshot exists, but 'diff' strongly implies non-mutating behavior, and the output description adds context beyond the tool name.

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, tightly packed with meaningful information: what it does, what it returns, and its relationship to other tools. No filler or redundancy.

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?

The description covers the core behavior and return values, and an output schema exists to detail the exact structure. It doesn't mention edge cases (e.g., no snapshot), but given the tool's simplicity and available schema, this is sufficient. Could be a 5, but a note about snapshot prerequisites 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 description coverage is 100%, so the parameter (device) is fully documented in the schema. The description does not mention parameters, which is fine because the schema already handles it. Baseline 3 applies; no extra value needed from description.

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 clearly states the tool's function: 'Diff the current screen against the last cached snapshot.' It uses a specific verb ('diff') and resource ('screen' vs 'cached snapshot'), and distinguishes itself by mentioning it is the 'explicit counterpart of the automatic evidence returned by every action tool.' This separates it from sibling tools like get_screen or get_screen_hash.

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 provides clear context by calling itself the 'explicit counterpart of the automatic evidence returned by every action tool,' implying use when explicit diff evidence is needed. However, it does not explicitly state when not to use it or name alternative tools. This is clear context without exclusions, meriting a 4.

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