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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds real behavioral context the annotations do not: default ordering (newest first) and, importantly, that a workspace lacking the paid add-on raises an error rather than returning an empty list — a non-obvious failure mode an agent would otherwise misinterpret as 'no tests'.

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 short sentences, front-loaded with what the tool returns and its ordering, then the domain definitions, then the failure caveat. Nothing is redundant with the title and every sentence carries a distinct 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?

With no parameters and no output schema, the description carries the burden of explaining what comes back, and it does so by defining the fields a test contains (content, sending mailboxes, recurrence). It omits whether results are paginated or truncated despite the 'lists every' claim, which is the one remaining ambiguity for a caller.

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?

The tool takes zero parameters, so there is nothing beyond the schema to document and the baseline of 4 applies. The description adds no misleading parameter hints.

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 ('Lists every inbox placement test defined in the workspace') and adds a clear test-vs-run distinction that separates it from list_inbox_placement_runs and get_inbox_placement_run without needing the sibling schemas opened.

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?

The test/run distinction implicitly tells the agent when to pick this tool versus the run-listing siblings, and the paid-add-on note warns of a failure mode. It stops short of explicitly naming an alternative tool ('to see executions, use list_inbox_placement_runs'), so routing is inferable rather than stated.

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