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

With only readOnlyHint=false available, the description carries the behavioral load and does so well: it discloses re-validation of the whole markup, rejection with the validator's errors, that the mockup is left unchanged on failure, that the response is a lean confirmation rather than the markup, and that the change is visible to all viewers.

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?

Four tight sentences, no filler, with the replacement mechanism front-loaded and the workflow/visibility notes sequenced after. Every sentence adds a distinct operational fact.

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?

Despite no output schema, the description explains the return shape (lean confirmation, not the full markup) and the failure behavior, so an agent knows what to expect after calling. Nothing needed to invoke or interpret the call correctly is missing.

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 schema already defines mockup, old_string, and new_string. The description only reinforces the exact-once match constraint already stated in the schema and the origin of the mockup target, adding little parameter-level meaning.

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 and resource plus the mechanism: 'Edit a mockup by exact-text replacement.' An agent can immediately distinguish this from edit-document (markup editing) and from read-side siblings like get-mockup.

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?

Explicitly routes to get-mockup for reading the source markup and to show-mockup 'once after a batch of edits when the user should see the result,' which also encodes the batch-then-display workflow. The alternative tools and the conditions selecting them are named.

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