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

readOnlyHint=true already establishes the safety profile, so the description is free to add value — and it does: it discloses the validation categories, the error output shape (each error names its JSON Pointer path and the fix), and the accepted input encodings. It stops short of describing success behavior or any size/rate limits, but the added context is well beyond what the annotation provides.

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?

Front-loaded with the core action and scope; the long enumerations of checked conditions and the error format are dense but each item carries distinct information. Slightly list-heavy, but no filler sentences and nothing is restated verbatim from the schema.

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 shoulders the burden of explaining returns and does so via the JSON-Pointer-path fix format, plus it covers all three accepted input encodings. The only gap is what a clean pass looks like (empty result vs. success payload), which an agent would have to discover at call time.

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?

One parameter with 100% schema description coverage sets a baseline of 3, since the schema already documents `messages` and its anyOf forms. The description adds a genuine semantic constraint the schema cannot express: pass ALL messages of the surface, not a subset, since cross-message checks (missing root, child loops) depend on completeness.

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 (checks/validates) and resource (A2UI messages against the Mlola Render catalog), and enumerates the exact message types it accepts (createSurface, updateComponents, updateDataModel, deleteSurface). The 'before they render' framing cleanly separates it from render_ui and from check_markup, so an agent can route without opening a schema.

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?

Gives clear timing context ('before they render') and an explicit input instruction: 'Pass every message of the surface: one object, an array, or JSON Lines.' However, it never names or contrasts with sibling tools such as check_markup or render_ui, so the agent gets context but no exclusions or explicit alternative-selection rule.

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