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

Annotations already declare readOnlyHint=false, destructiveHint=false and openWorldHint=true, so the bar is lower, yet the description still adds substantive behavior: upgrade is charged immediately (prorated), downgrade defers to period end, and dryRun returns a quote without mutating. It omits error/failure behavior and permission requirements, so not 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?

The opening two clauses say the same thing twice ('Move a subscription to another plan' / 'Moves one subscription to another plan with the same currency and interval'), which is redundant. The dryRun sentence does earn its place, but the duplication wastes space.

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 mutation tool with no output schema, the description covers the essential outcome semantics (immediate prorated charge vs. deferred change) and the no-op quote mode. It leaves out failure conditions and any return-value expectations, so it is solid but not exhaustive.

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 largely restates what the schema already says for 'to' (same currency and interval) and dryRun ('price quote without changing anything'), adding only marginal interpretation rather than new semantics.

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 ('Move a subscription to another plan'), which is clearly distinct from siblings like subscriptions_cancel and subscriptions_credit. It does not, however, explicitly name or differentiate itself from any sibling tool, which keeps it below a 5.

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?

Gives real usage context: the same-currency-and-interval precondition, when an upgrade is charged vs. when a downgrade applies, and the dryRun escape hatch for a quote. It stops short of naming alternative tools (e.g. cancel) or stating when-not to use it.

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