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Customer.io MCP by usefulapi

by io.usefulapi
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List messages

customerio_list_messages
Read-only

List sent messages (deliveries) across the workspace, filterable by type/campaign/newsletter. App API: GET /v1/messages.

Input Schema

NameRequiredDescriptionDefault
typeNoMessage type filter, e.g. email | push | sms | in_app | webhook | slack | twilio.
limitNoMax messages to return.
startNoPagination cursor from a previous response's `next`.
draftsNoInclude drafts.
campaign_idNoFilter to a specific campaign.
newsletter_idNoFilter to a specific newsletter.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changed
    • changedInput schema / $schema
      Previous value: -"http://json-schema.org/draft-07/schema#"New value: +"https://json-schema.org/draft/2020-12/schema"
    • removedInput schema / properties / campaign_id / anyOf
      Removed value: -[
-  {
-    "type": "string"
-  },
-  {
-    "type": "number"
-  }
-]
    • addedInput schema / properties / campaign_id / type
      Added value: +[
+  "string",
+  "number"
+]
    • removedInput schema / properties / newsletter_id / anyOf
      Removed value: -[
-  {
-    "type": "string"
-  },
-  {
-    "type": "number"
-  }
-]
    • addedInput schema / properties / newsletter_id / type
      Added value: +[
+  "string",
+  "number"
+]
  2. First observed

TDQS

A3.6/5.0
Behavior3/5

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

readOnlyHint=true already tells the agent this is a safe read. The description adds useful context beyond that: it returns only *sent* messages, is workspace-scoped, and maps to the App API endpoint GET /v1/messages. It does not address pagination behavior or the (slightly inconsistent) presence of a drafts filter alongside a 'sent messages' scope.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two short sentences with the scope and filters front-loaded and the API endpoint tacked on compactly. No wasted wording.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a read-only list tool with full schema coverage, the definition covers scope, filters, and endpoint adequately. There is no output schema, and the description says nothing about the shape of returned message objects or pagination via the `next` cursor, which is a minor remaining gap.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so all six parameters are already documented in the schema. The description only summarizes the filters at a high level (type/campaign/newsletter) and omits drafts, limit, and the start cursor, adding essentially nothing over the schema. Baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb+resource (list sent messages/deliveries) and scope (across the workspace), which implicitly separates it from the sibling customerio_get_customer_messages. It never names an alternative tool explicitly, so it stops short of full sibling differentiation.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines3/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The mention of workspace-wide listing and type/campaign/newsletter filters implies when the tool is appropriate, but there is no explicit when-to-use guidance or exclusion against the per-customer sibling customerio_get_customer_messages. Usage is left to inference.

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

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