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

With no annotations, the description carries the full burden and does the most important part: it discloses that mode=replace overwrites the day's entire text, which is the one piece of destructive behavior an agent must know before calling. It also gives the append-vs-replace default. It doesn't say whether a missing day is created implicitly (relevant to the 'default today' claim) or what the call returns, leaving a modest gap.

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?

Three short sentences, front-loaded with scope and gating, then the overwrite semantics. Every clause carries a fact an agent would otherwise have to guess; nothing is filler.

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 three-parameter write tool with no annotations and no output schema, the description supplies the decision gate, the default values, and the destructive-replace warning. It is nearly complete; only the implicit day creation and the date format remain unspecified.

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 0%, so the description must compensate, and it does: date is explained as defaulting to today, mode is given as append-by-default versus replace-overwrites, and text is Markdown matching create_note. The only shortfall is that the date string format is never specified.

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 — writing to the user's journal for a given day — and immediately delimits it against the sibling journal_read and against create_note, which is used only as a format reference. An agent can distinguish this from create_note or append_to_note without opening any schema.

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?

"Only when the user asked you to write in their journal" is an explicit gating condition that excludes proactive writes, which is exactly the kind of guidance an agent needs. It stops short of naming the near-miss alternatives (create_note, append_to_note, journal_read) as the tools to prefer when it isn't a journal write.

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