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

Beyond the annotations (which signal a non-read-only mutation but nothing else), the description discloses real behavioral details: automatic leg placement, two specific 400 scenarios, the no-retargeting semantics of ruleset, and the required permission. This is exactly the kind of context that prevents an agent from misusing the tool. It is consistent with annotations, so there is no contradiction.

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?

Three sentences deliver the core action, the critical placement behavior, the error conditions, the optional parameter semantics, and the permission requirement. Information is front-loaded and every clause earns its place. There is no filler or repetition of schema/annotations.

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 mutating add-operation with an output schema and clear annotations, the description covers the prerequisites (leg exists), the caller permission, the exact failure modes, and the optional parameter's effect. Nothing needed to invoke the tool correctly appears to be missing. The success return is already covered by the output schema, so its omission is appropriate.

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?

Although the schema already covers 100% of parameters, the description adds meaning beyond the raw field docs: it explains that ruleset is only a check and 'never retargets', and that ruleId refers to a vault rule whose own ruleset governs placement. This turns a generic 'optional confirmation' schema field into an actionable behavioral contract. It reinforces campaignId's role as the target allowlist.

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 opens with a specific verb and resource: 'Add a vault rule to a campaign's allowlist,' and immediately clarifies the unique placement rule ('lands on the leg matching the RULE's own ruleset'). This clearly distinguishes it from related campaign/vault tools like play_add_campaign_item or vault_create_rule. It also names the permission and failure mode, pinning down the exact operation.

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 description gives strong contextual guidance: the campaign must have a leg for the rule's ruleset, the optional ruleset param is only a confirmation, and the caller needs 'contribute' permission. It does not explicitly name sibling alternatives or state 'use this when ... instead of ...', but its conditions make the intended usage unmistakable. No misleading exclusions are present.

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