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

Annotations are minimal (readOnlyHint=false, openWorldHint=true, idempotentHint=false, destructiveHint=false), so the description carries the full burden — and it delivers. It discloses that agents never spend money directly (human pays on Stripe), that nothing is charged until payment, that downgrades are executed by a person, the admin-only gating, and the member rejection message. No contradiction with annotations.

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 description is dense but front-loaded with the core purpose and admin gate, then branches logically through each path. Every sentence carries a distinct piece of information — listing, checkout URL, downgrade handoff, member message. It's long, but the multi-path behavior genuinely needs the space; nothing is redundant.

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 output schema, the description must explain return values, and it does: the plan-listing format (id · monthly price · monthly credits), the ready-to-pay Stripe Checkout URL, the exact-do-this message for downgrades, and the 'ask an admin' message for members. For a branching tool with four distinct user states, this is highly complete.

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 both parameters are already documented in the schema, placing the baseline at 3. The description adds genuine value beyond the schema by explaining the interaction between the two modes (omitting `plan` lists plans, setting it triggers a change) and enriching `period` with the '2 months free' yearly incentive that the schema also mentions. This pushes it above baseline.

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 + resource ('Change this account's SUBSCRIPTION plan (admin only)'), and immediately disambiguates from siblings like buy_credits and billing_status by scoping the action to plan changes, not credit purchases or status reads. The dual-mode behavior (list vs. change) makes the tool's function unmistakable.

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 spells out a sequencing protocol ('Call with no argument to list the plans... call again with `plan` set to a plan id') and writes conditional paths for new subscribers, existing paid plans, downgrades, and members. It doesn't explicitly name alternative sibling tools, but it clearly explains when the tool hands off to a human instead of acting, which is strong guidance.

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