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

Annotations already indicate readOnlyHint=false and destructiveHint=false. The description adds context that only the account and debit/credit amounts are changed, and that the line is identified by id. It doesn't mention side effects or error behavior, but for a simple update tool with non-destructive annotations, this is adequate transparency.

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 two sentences, concise and front-loaded with the core action. It includes the key identification method ('Identify the line by its id') and sibling relationships without unnecessary detail.

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 low complexity (5 optional parameters, no output schema) and the presence of sibling guidance, the description fully covers what an agent needs to know to correctly invoke the tool: what it does, which fields can change, how to identify the line, and when to use alternatives.

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 descriptions cover all 5 parameters with clear definitions (e.g., 'Line id (for updating an existing line)' and 'Debit amount for this line (0 if this is a credit line)'). The main description adds high-level context but does not enhance parameter-specific semantics beyond the schema. Since coverage is 100%, the baseline score of 3 applies.

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 tool's action: 'Update a single debit/credit line on an existing journal entry — change its account, debit or credit amount.' It specifies the resource (journal line) and distinguishes it from siblings: 'Pairs with add_journal_line; to edit the whole entry use save_journal_entry.'

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 explicitly tells when to use this tool versus alternatives: 'Pairs with add_journal_line; to edit the whole entry use save_journal_entry.' This gives clear guidance on when to choose this tool over adding a line or editing the whole entry.

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