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

Annotations already cover the safety profile (readOnlyHint, idempotentHint, destructiveHint=false), and the description reinforces 'nothing is sent' while adding real behavioral detail: the would_send/blocked_code/blocked_details contract and the quota_exceeded case with reset date. It does not mention rate limits or cost, so it stops short of a 5.

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 most important reassurance ('Read-only: nothing is sent') and structured around would-send, exclusions, and quota. The final sentence is more instructional than definitional, but it still earns its place by shaping correct usage.

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?

There is no output schema, so the description must carry return-value meaning, and it does so for the key fields (would_send, blocked_code, blocked_details). It does not enumerate the per-address exclusion reason vocabulary exhaustively, but gives representative examples.

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 coverage is 100%, so baseline is 3, but the description adds meaning beyond the schema: with `to` it reports address-by-address who is kept or excluded and why, whereas omitting it evaluates the whole audience. That is a semantic payoff not present in the schema text.

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 names a specific verb+resource (preview a campaign send) and immediately scopes it: 'Returns exactly what send_campaign would do right now.' It is unambiguously distinct from the sibling send_campaign and from read tools like get_campaign_stats.

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?

Explicit routing: 'Call it BEFORE send_campaign, and give that number to the person when you ask them to confirm.' It states both the sequencing relative to the alternative tool and the intended workflow, leaving nothing to inference.

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