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

Annotations already establish readOnlyHint=false, so the mutation is expected; the description adds concrete behavioral details: author-only enforcement, content length bounds (5-10000 chars), and possible error conditions (not found, not author, validation). This goes beyond what annotations convey.

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 main purpose is front-loaded in a single sentence, followed by compact Args and Returns sections. The format is slightly verbose with docstring-style labels, but every element adds useful information and there is no padding.

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 tool's simplicity (2 required parameters, no output schema, no nested objects), the description is fully self-contained: it states the permission model, parameter constraints, success return shape, and failure modes. An agent has everything needed to invoke it correctly.

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 carry the full meaning of both parameters. It does: reply_id is identified as 'the reply to update,' and content is specified as 'new raw markdown content (5-10000 chars),' providing both format and validation constraints.

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?

Description starts with a specific verb and resource: 'Update an existing forum reply (author only).' It clearly distinguishes from sibling create_forum_reply, delete_forum_reply, and update_forum_post, removing ambiguity about what action applies to which object.

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 '(author only)' parenthetical provides a clear eligibility constraint, telling the agent this tool cannot be used to modify others' replies. While it doesn't explicitly name alternatives, the sibling context and the 'existing' wording imply this is for editing an already-created reply, not creating or deleting.

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