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

Annotations already declare non-read-only, idempotent, non-destructive; the description adds real context on top: the call returns an updated preview, an empty fallback_body is a deletion, and links from unverified numbers may be blocked. Billing-by-segment and merge-field behavior round out behavior beyond the annotation set.

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?

Purpose and the replacement-mode rule are correctly front-loaded, but the trailing SMS guidance paragraph mixes tool-relevant facts (link-free fallback, merge-field reference) with generic copywriting advice ('identify yourself, keep it conversational') that does not help an agent select or invoke the tool.

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 7-parameter mutation with full schema coverage and annotations, the description covers invocation modes, the destructive edge case, and the response behavior, so an agent can call it correctly. Only minor gaps remain, such as whether replacements fail atomically when a find string is absent.

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?

With 100% schema coverage the baseline is 3, but the description adds meaning beyond the schema by framing text/fallback_body as a mutually exclusive alternative to replacements and by defining the one notable sentinel value (""). It does not explain name, is_default, or description, though those are self-documenting in the schema.

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+resource (edit an SMS template) and adds a visible outcome (show the updated preview), which distinguishes it from the sibling spur_update_email_template and spur_update_letter_template. There is no ambiguity about what gets mutated.

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 lays out the two invocation modes — full replacement of text/fallback_body versus targeted 'replacements' — and calls out the special case of passing fallback_body="" to delete the fallback. It stops short of naming when to prefer this over sibling tools, but the mode-selection guidance is genuinely actionable.

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