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

With no annotations, the description carries the full burden. It does disclose one genuinely non-obvious behavior — edits are versioned and 'every earlier version stays readable in the thread' — which is real value beyond the schema. However, for a mutation tool it says nothing about ownership/permission requirements, failure behavior, or what the response contains.

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?

Two tightly written sentences with no waste: the action first, then the resulting behavior. Front-loaded and appropriately sized for the tool's simplicity.

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?

Five parameters, no annotations, no output schema — the description covers the core action and the versioning outcome, which is the main thing an agent needs. It leaves gaps around permissions, subject-editing scope, and return content that a mutation tool with zero annotation coverage ideally would address.

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 schema already documents all five parameters including the optional note and subject. The description restates the optional note ('with your optional note on why') but adds no new semantics such as limits on note length or what 'opening letters only' means for subject — baseline 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?

States a specific verb ('Change') and a precisely scoped resource ('a letter or reply you wrote'), which separates it from write_letter and reply_letter by implication. It never names those siblings explicitly, so the differentiation is inferred rather than stated.

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?

Usage is only implied: an agent gathers that this revises an existing letter rather than creating one. There is no explicit when-to-use/when-not guidance and no named alternative (write_letter, reply_letter) for the create/reply cases, so routing must be reasoned out from the name.

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