Retrieve a subscriber by numeric ID to view email, name, status, timestamps, list memberships, and custom attributes; returns an error if the ID does not exist.
Get one subscriber by numeric ID: email, name, status, timestamps, list memberships and custom attributes. Returns an error if the ID does not exist. Use get_subscriber_by_email when you only know the address.
| Name | Required | Description | Default |
|---|---|---|---|
| subscriber_id | Yes | Subscriber ID |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
readOnlyHint=true already establishes the safety profile, so the bar is lower; the description adds real context by disclosing the failure mode ('returns an error if the ID does not exist') and the shape of the payload. It omits anything about permissions or error structure, but nothing it says conflicts with the annotation.
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 the payload description, then the error behavior, then the alternative. Every sentence carries distinct information with no repetition of the name or schema.
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 one-parameter read tool with no output schema, the description compensates well by naming the returned fields and the not-found error. Only the absence of any detail on how the error surfaces or how large list memberships are returned keeps it from being fully complete.
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?
With a single parameter at 100% schema description coverage, the schema already documents subscriber_id as a positive number. The description reinforces 'numeric ID' but adds no format, range, or sourcing hints beyond that. 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+resource (get one subscriber) scoped by numeric ID, and enumerates what comes back (email, name, status, timestamps, list memberships, custom attributes). It explicitly distinguishes itself from the sibling get_subscriber_by_email, so an agent can pick between them without opening either 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?
Gives a clear routing rule: use this when you have the numeric ID, and use get_subscriber_by_email when you only have the address. That covers the primary ambiguity in this sibling cluster, though it says nothing about related operations like list_subscribers when no ID is known.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.