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

Annotations already declare readOnlyHint=false, destructiveHint=false and openWorldHint=true, so the safety profile is covered. The description adds meaningful semantics beyond that: partial-update behavior ('leaves the others as they are'), the drafts-only constraint, that nothing reaches subscribers, and that it runs without confirmation plus the API-key access requirement.

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?

Front-loaded with purpose, then constraints and preconditions in a logical order. Four short sentences with only mild redundancy ('Changes fields of a draft and leaves the others as they are' slightly restates the opening).

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 two-parameter mutation tool with no output schema, the description covers the key behaviors an agent needs: partial update, drafts-only scope, no subscriber impact, and auth requirement. Only the absence of an explicit sibling alternative keeps it from a 5.

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 100%, so the schema already documents post_id and fields. The description restates the editable fields (title, subtitle, body, audience) but adds no syntax or format detail, and its 'body' differs slightly from the schema's 'body_markdown'. Baseline 3 is appropriate when the schema does the heavy lifting.

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) plus the exact resource (a draft) and enumerates the fields it touches, so the agent immediately knows this is a partial update rather than a create/publish. It is clearly distinguished from siblings like substack_create_draft and substack_publish_post.

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?

Explicitly says it 'works on drafts only; a published post is not changed', which is a real when/when-not rule. It stops short of naming the alternative tool (e.g., publish_post) to use for live posts, so it is strong but not fully routing.

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