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

Annotations already declare readOnlyHint=false and destructiveHint=true, so the agent knows this is a mutating and potentially destructive operation. The description adds that it targets an 'existing' txn and requires Id/SyncToken, but does not detail the sparse/full update behavior or sensitive-field restrictions (those live in the schema description, not the tool description).

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 a single sentence that immediately states the action, target, and a critical prerequisite. No redundant wording, and it avoids repeating information already present in the schema or 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?

Given the rich schema description and the annotations (readOnlyHint=false, destructiveHint=true), the brief description suffices as a high-level summary. It tells the agent what the tool does and the key requirement, while the schema covers all invocation details; no output schema exists, so return-value expectations are not required.

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%, with the entity parameter thoroughly documented (required Id/SyncToken, sparse update semantics, prohibited fields). The tool description itself adds no parameter detail beyond the name and the Id/SyncToken mention, so the schema carries the semantic load.

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 clearly states the action ('Update'), the specific resource ('existing credit card payment txn'), and a key prerequisite ('requires Id and SyncToken'), which distinguishes it from create_credit_card_payment_txn. It does not explicitly name sibling alternatives, but the verb and resource are unambiguous.

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 implies usage for modifying an already-existing txn by requiring Id and SyncToken, but it does not explicitly state when to choose this over create/delete/search variants. The schema adds prerequisites (fetch first), but the description itself offers no when-not-to-use guidance.

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