create_post_tagCreates a post tag on your Substack publication and derives its slug from the tag name. Use it to group posts, then assign posts with tag_post; empty tags remain hidden.
New tag for grouping your posts.
Creates a post tag on your publication; Substack makes the slug from the name. A tag with no posts shows nowhere. Put posts under it with tag_post.
| Name | Required | Description | Default |
|---|---|---|---|
| name | Yes | Tag name, e.g. Habits. |
Changes observed during successful MCP inspections.
v0.1.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare the write, non-idempotent, non-destructive profile, and the description adds real value on top: the slug is derived server-side from the name, and an empty tag is invisible in the product. It stops short of warning that repeating the call with the same name will likely create a duplicate despite the non-idempotent hint.
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?
Four short sentences, front-loaded with the action and followed by consequences and next steps; every sentence carries information. The opening fragment 'New tag for grouping your posts' slightly restates the title and could be merged, keeping it just under 5.
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 single-parameter creation tool with no output schema, the description covers the action, the slug side effect, the visibility consequence, and the follow-up tool. It leaves open what the call returns and how duplicate names behave, which are the only remaining gaps.
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 baseline is 3, but the description adds meaning beyond the schema by explaining that the supplied name is transformed into a slug rather than used verbatim. That materially affects what the agent should pass.
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 ('Creates a post tag on your publication') and scopes it to posts, which distinguishes it from create_subscriber_tag and tag_subscribers in the sibling list. An agent can tell it apart without touching the 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?
The sentence 'Put posts under it with tag_post' gives the natural follow-up call, which is genuinely useful routing. However it never says when to create a new tag versus reusing one (list_post_tags), nor what happens if the name already exists, so selection guidance is only implied.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.