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Customer.io MCP by usefulapi

by io.usefulapi
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Send transactional email

customerio_send_transactional_email
Destructive

⚠️ SENDS A REAL EMAIL. Send a transactional email using a transactional message template. Delivers immediately to the to recipient. App API: POST /v1/send/email.

Input Schema

NameRequiredDescriptionDefault
toYesRecipient email address.
bodyNoOverride the email body.
fromNoOverride the sender address.
subjectNoOverride the subject line.
identifiersYesPerson identifiers, e.g. {"id":"..."} or {"email":"..."}.
message_dataNoTemplate variables (liquid) to interpolate into the message.
send_to_unsubscribedNoIf true, send even to unsubscribed recipients.
transactional_message_idYesThe transactional message template id (or trigger name).
disable_message_retentionNoIf true, don't retain the message body.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 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 / transactional_message_id / anyOf
      Removed value: -[
-  {
-    "type": "string"
-  },
-  {
-    "type": "number"
-  }
-]
    • addedInput schema / properties / transactional_message_id / type
      Added value: +[
+  "string",
+  "number"
+]
  2. First observed

TDQS

A3.7/5.0
Behavior4/5

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

Annotations already declare destructiveHint=true, so the safety bar is lower; the description reinforces this with the real-email warning and immediate-delivery note, and adds the API endpoint (POST /v1/send/email). It stops short of mentioning rate limits, idempotency, or that unsubscribed recipients are skipped unless overridden.

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?

Three short sentences, with the critical warning front-loaded before purpose and endpoint. No filler or restatement of the title.

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

Completeness3/5

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

For a 9-parameter mutation tool with nested objects and no output schema, the description covers the destructive nature and delivery target but omits preconditions (template setup), default unsubscribe-skipping behavior, and override interactions. Adequate but with clear gaps.

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 the schema fully documents all 9 parameters; baseline 3 applies. The description adds only that the `to` recipient is the delivery target and that a template is referenced, without clarifying identifiers vs message_data usage.

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 and resource ('Send a transactional email using a transactional message template') plus delivery semantics ('delivers immediately to the `to` recipient'). It does not explicitly differentiate from the sibling customerio_trigger_broadcast, which an agent could plausibly confuse with this tool.

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?

'⚠️ SENDS A REAL EMAIL' implies caution but gives no explicit when-to-use/when-not-to guidance or routing versus trigger_broadcast. The precondition that a transactional_message_id must already exist is left implicit.

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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