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

Annotations already declare readOnlyHint=false and destructiveHint=true, so the write nature is covered. The description adds real context beyond that: it is an upsert that overwrites the project's default template for that channel/message type, and it discloses a specific failure mode (template_hardcoded_name) with its trigger. It does not say whether the change is reversible or how it interacts with a previously set custom template.

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?

The operational sentence is front-loaded and the auth prerequisite follows, so the agent reads the important scoping first. The ACCESS block is slightly verbose, but every clause (authenticate first, start_login does not apply) carries actionable information.

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 template-upsert tool with no output schema, the description covers scope, auth prerequisite, and a key validation rule. It leaves minor gaps around return values and whether the new template is effective immediately or needs preview/render to verify, but nothing critical for correct invocation 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 baseline is 3. The description adds meaning beyond the schema by tying channel and message_type to the template's targeting scope and by specifying that the brand placeholder must be {{business_name}} inside the body. It does not clarify subject/button field interactions beyond the schema's own 'email channel only' notes.

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 states a specific verb set (create or update) plus resource (custom template) and narrows the scope to a single sub-account, channel, and message type. The phrase 'replacing the default for that project only' distinguishes it from account-wide default templates (get_default_templates) and from read-only siblings like get_template/list_templates. An agent can identify the upsert semantics even though the tool name says only 'update'.

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 gives an explicit prerequisite (needs a Proof account; authenticate first) and warns that start_login does not unlock this tool, which prevents a common wrong path. It also states a hard usage constraint (brand name must be written as {{business_name}}). It stops short of naming alternatives such as preview_template or get_default_templates for checking output before committing.

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