How to Build an Email Tool for an AI Agent

Do not give an AI agent a raw sendEmail(to, subject, html) tool and call it done.

That is how you get duplicate sends, wrong audiences, broken personalization, missing unsubscribe links, and production emails that nobody reviewed. Email tools for agents need to be designed around safety, context, and reversibility.

This guide shows a practical design for an agent email tool. It works whether you expose it through MCP, function calling, an internal API, or a CLI.

Start With the Workflow, Not the Endpoint

Most bad agent tools are too low level.

Bad tool:

{
  "name": "send_email",
  "description": "Send an email",
  "input": {
    "to": "string",
    "subject": "string",
    "html": "string"
  }
}

This gives the agent too much power and too little guidance. It can send anything to anyone with no preview, no audience validation, and no audit trail.

Better tool design separates the workflow:

create_email_draft
send_test_email
validate_audience
request_send_approval
schedule_approved_campaign
send_transactional_email
get_delivery_status

The agent can still complete the job, but each step is safer.

Agent Email Tool Design Table

Tool	Should it send email?	Safety purpose
`create_email_draft`	No	Creates a reviewable artifact before any delivery
`validate_audience`	No	Checks recipient count, consent, suppression, and restrictions
`send_test_email`	Test only	Verifies rendering, links, variables, and footer requirements
`request_send_approval`	No	Captures explicit human approval and scope
`schedule_approved_campaign`	Yes, later	Gives the system a final validation and cancel window
`send_transactional_email`	Yes, controlled	Sends only approved templates with idempotency

Tool 1: Create a Draft

The draft tool should not send anything.

It should create a reviewable object with subject, preview text, body, audience intent, links, and metadata.

Example input:

{
  "campaignType": "product_update",
  "audienceDescription": "active Pro plan users",
  "goal": "announce the new API endpoint",
  "tone": "concise, developer-friendly",
  "requiresApproval": true
}

Return a draft ID. Every later action should reference that ID.

Why this matters: the draft becomes an audit object. You can inspect what the agent generated before anything leaves the system.

Tool 2: Validate the Audience

Audience mistakes are more dangerous than copy mistakes.

Your tool should let the agent ask:

How many recipients match?
Are unsubscribed users excluded?
Are bounced users excluded?
Are suppressed domains excluded?
Does this include users in restricted regions?
Is this transactional or marketing?

Example output:

{
  "audienceId": "seg_123",
  "recipientCount": 1842,
  "excludedUnsubscribed": 211,
  "excludedBounced": 17,
  "requiresApproval": true,
  "reason": "Audience exceeds autonomous send limit of 500"
}

Agents should never infer that a segment is safe only because it exists.

Audience Validation Table

Check	Why it matters	Block or warn?
Recipient count	Prevents accidental blasts	Block above autonomous cap
Unsubscribed excluded	Enforces consent	Block
Bounced or suppressed excluded	Protects deliverability	Block
Restricted regions	Avoids compliance problems	Block or require approval
Marketing vs transactional purpose	Determines footer and consent rules	Block if mismatched
Duplicate recent send	Prevents repeated campaigns	Warn or block depending on recency

Tool 3: Send a Test Email

Every campaign-like message should have a test path.

Inputs:

draftId
testRecipients
renderMode
includeDiagnostics

Diagnostics should include:

Rendered subject
Rendered preview text
Rendered personalization variables
Link list
Missing variables
Unsubscribe presence
Plain-text fallback

If the test fails, the agent can fix the draft before requesting approval.

Tool 4: Request Approval

Approval should be explicit.

Do not let the agent bury approval inside a natural language message like "looks good, sending now." Store approval as an object:

{
  "draftId": "draft_123",
  "audienceId": "seg_123",
  "approvedBy": "user_456",
  "approvedAt": "2026-05-12T12:30:00Z",
  "approvalScope": "send_once",
  "maxRecipients": 2000
}

Approval should expire. A campaign approved last month should not be sendable today without re-checking the audience.

Tool 5: Schedule, Do Not Immediately Blast

For marketing campaigns, prefer scheduled sends over immediate sends.

Scheduling gives humans a last-minute cancel window and gives the system time to run final checks:

DNS verified
Links valid
Required footer present
Audience count below cap
No recent complaint spike
No duplicate send
Send time inside allowed window

Transactional email can send immediately, but marketing email should usually schedule.

Tool 6: Send Transactional Email

Transactional sends are different. They are product events, not campaigns.

The tool should require a template ID and event context:

{
  "templateId": "payment_failed",
  "recipientId": "sub_123",
  "idempotencyKey": "invoice_in_456_payment_failed_v1",
  "variables": {
    "firstName": "Ada",
    "invoiceUrl": "https://example.com/invoices/in_456"
  }
}

Do not let agents send arbitrary transactional HTML in production. Use templates. Use idempotency keys. Store message IDs.

Tool 7: Read Delivery Status

Agents need feedback.

Give them tools to read:

Accepted
Delivered
Opened
Clicked
Bounced
Complained
Deferred
Suppressed
Replied

This lets the agent answer, "Did the launch email work?" without manual dashboard checks.

Required Guardrails

Minimum production controls:

Separate API keys for read, draft, test, and send.
Daily send cap.
Per-campaign recipient cap.
Test-send requirement.
Approval requirement above a threshold.
Unsubscribe enforcement.
Suppression list enforcement.
Bounce and complaint auto-pause.
Audit log for every tool call.
Idempotency keys for transactional sends.
No root-level complex JSON Schema keywords if publishing MCP tools; validate conditional rules in handlers.

The last point matters because some MCP clients reject schemas with unsupported root-level constructs. Keep tool schemas simple and enforce complex rules in code.

Approval Gate Table

Send type	Approval requirement	Reason
Draft creation	No approval needed	No external effect
Test send to internal address	Usually no approval	Low risk and reversible
Marketing campaign under small cap	Optional policy approval	Depends on team maturity
Marketing campaign above cap	Explicit approval	Audience mistakes are expensive
Transactional template send	Template pre-approval plus idempotency	Must be fast but controlled
Sensitive segment send	Explicit approval regardless of size	Compliance, billing, or account-risk context

MCP Tool Schema Pattern

For MCP, prefer plain object schemas:

{
  "type": "object",
  "properties": {
    "draftId": {
      "type": "string",
      "description": "Draft to send. Required unless templateId is provided."
    },
    "templateId": {
      "type": "string",
      "description": "Transactional template to send. Required unless draftId is provided."
    },
    "recipientId": {
      "type": "string",
      "description": "Recipient subscriber ID."
    }
  }
}

Then validate "draftId or templateId, but not both" inside the handler. That keeps the tool loadable across more clients.

Build vs Buy

Build your own email tool if:

Email is a core product capability.
You already own the notification system.
You have deliverability expertise.
You need custom approval logic.

Use a platform like Sequenzy if:

You want agents to manage campaigns, subscribers, sequences, and analytics.
You need both transactional and marketing email.
You want MCP tools without building the entire wrapper yourself.
You want audit logs, segments, and sequence generation as product features.

Use a pure send API like Resend or Postmark if:

Your agent only needs to add transactional email to an app.
Marketing workflows live somewhere else.
You prefer templates in code.

Final Checklist

Before giving an agent production email access, confirm:

It can draft without sending.
It can send tests.
It cannot bypass suppression lists.
It cannot send to large audiences without approval.
It uses templates for transactional email.
It has idempotency keys for retries.
It logs every action.
It can read delivery results.
You can revoke its key quickly.

Email agents are useful when they reduce repetitive work. They are dangerous when they skip review. Design the tool so good behavior is the easiest path.

Do not give an AI agent a raw sendEmail(to, subject, html) tool and call it done.

This guide shows a practical design for an agent email tool. It works whether you expose it through MCP, function calling, an internal API, or a CLI.

Start With the Workflow, Not the Endpoint

Most bad agent tools are too low level.

Bad tool:

{
  "name": "send_email",
  "description": "Send an email",
  "input": {
    "to": "string",
    "subject": "string",
    "html": "string"
  }
}

This gives the agent too much power and too little guidance. It can send anything to anyone with no preview, no audience validation, and no audit trail.

Better tool design separates the workflow:

create_email_draft
send_test_email
validate_audience
request_send_approval
schedule_approved_campaign
send_transactional_email
get_delivery_status

The agent can still complete the job, but each step is safer.

Agent Email Tool Design Table

Tool	Should it send email?	Safety purpose
`create_email_draft`	No	Creates a reviewable artifact before any delivery
`validate_audience`	No	Checks recipient count, consent, suppression, and restrictions
`send_test_email`	Test only	Verifies rendering, links, variables, and footer requirements
`request_send_approval`	No	Captures explicit human approval and scope
`schedule_approved_campaign`	Yes, later	Gives the system a final validation and cancel window
`send_transactional_email`	Yes, controlled	Sends only approved templates with idempotency

Tool 1: Create a Draft

The draft tool should not send anything.

It should create a reviewable object with subject, preview text, body, audience intent, links, and metadata.

Example input:

{
  "campaignType": "product_update",
  "audienceDescription": "active Pro plan users",
  "goal": "announce the new API endpoint",
  "tone": "concise, developer-friendly",
  "requiresApproval": true
}

Return a draft ID. Every later action should reference that ID.

Why this matters: the draft becomes an audit object. You can inspect what the agent generated before anything leaves the system.

Tool 2: Validate the Audience

Audience mistakes are more dangerous than copy mistakes.

Your tool should let the agent ask:

How many recipients match?
Are unsubscribed users excluded?
Are bounced users excluded?
Are suppressed domains excluded?
Does this include users in restricted regions?
Is this transactional or marketing?

Example output:

{
  "audienceId": "seg_123",
  "recipientCount": 1842,
  "excludedUnsubscribed": 211,
  "excludedBounced": 17,
  "requiresApproval": true,
  "reason": "Audience exceeds autonomous send limit of 500"
}

Agents should never infer that a segment is safe only because it exists.

Audience Validation Table

Check	Why it matters	Block or warn?
Recipient count	Prevents accidental blasts	Block above autonomous cap
Unsubscribed excluded	Enforces consent	Block
Bounced or suppressed excluded	Protects deliverability	Block
Restricted regions	Avoids compliance problems	Block or require approval
Marketing vs transactional purpose	Determines footer and consent rules	Block if mismatched
Duplicate recent send	Prevents repeated campaigns	Warn or block depending on recency

Tool 3: Send a Test Email

Every campaign-like message should have a test path.

Inputs:

draftId
testRecipients
renderMode
includeDiagnostics

Diagnostics should include:

Rendered subject
Rendered preview text
Rendered personalization variables
Link list
Missing variables
Unsubscribe presence
Plain-text fallback

If the test fails, the agent can fix the draft before requesting approval.

Tool 4: Request Approval

Approval should be explicit.

Do not let the agent bury approval inside a natural language message like "looks good, sending now." Store approval as an object:

{
  "draftId": "draft_123",
  "audienceId": "seg_123",
  "approvedBy": "user_456",
  "approvedAt": "2026-05-12T12:30:00Z",
  "approvalScope": "send_once",
  "maxRecipients": 2000
}

Approval should expire. A campaign approved last month should not be sendable today without re-checking the audience.

Tool 5: Schedule, Do Not Immediately Blast

For marketing campaigns, prefer scheduled sends over immediate sends.

Scheduling gives humans a last-minute cancel window and gives the system time to run final checks:

DNS verified
Links valid
Required footer present
Audience count below cap
No recent complaint spike
No duplicate send
Send time inside allowed window

Transactional email can send immediately, but marketing email should usually schedule.

Tool 6: Send Transactional Email

Transactional sends are different. They are product events, not campaigns.

The tool should require a template ID and event context:

{
  "templateId": "payment_failed",
  "recipientId": "sub_123",
  "idempotencyKey": "invoice_in_456_payment_failed_v1",
  "variables": {
    "firstName": "Ada",
    "invoiceUrl": "https://example.com/invoices/in_456"
  }
}

Do not let agents send arbitrary transactional HTML in production. Use templates. Use idempotency keys. Store message IDs.

Tool 7: Read Delivery Status

Agents need feedback.

Give them tools to read:

Accepted
Delivered
Opened
Clicked
Bounced
Complained
Deferred
Suppressed
Replied

This lets the agent answer, "Did the launch email work?" without manual dashboard checks.

Required Guardrails

Minimum production controls:

Separate API keys for read, draft, test, and send.
Daily send cap.
Per-campaign recipient cap.
Test-send requirement.
Approval requirement above a threshold.
Unsubscribe enforcement.
Suppression list enforcement.
Bounce and complaint auto-pause.
Audit log for every tool call.
Idempotency keys for transactional sends.
No root-level complex JSON Schema keywords if publishing MCP tools; validate conditional rules in handlers.

The last point matters because some MCP clients reject schemas with unsupported root-level constructs. Keep tool schemas simple and enforce complex rules in code.

Approval Gate Table

Send type	Approval requirement	Reason
Draft creation	No approval needed	No external effect
Test send to internal address	Usually no approval	Low risk and reversible
Marketing campaign under small cap	Optional policy approval	Depends on team maturity
Marketing campaign above cap	Explicit approval	Audience mistakes are expensive
Transactional template send	Template pre-approval plus idempotency	Must be fast but controlled
Sensitive segment send	Explicit approval regardless of size	Compliance, billing, or account-risk context

MCP Tool Schema Pattern

For MCP, prefer plain object schemas:

{
  "type": "object",
  "properties": {
    "draftId": {
      "type": "string",
      "description": "Draft to send. Required unless templateId is provided."
    },
    "templateId": {
      "type": "string",
      "description": "Transactional template to send. Required unless draftId is provided."
    },
    "recipientId": {
      "type": "string",
      "description": "Recipient subscriber ID."
    }
  }
}

Then validate "draftId or templateId, but not both" inside the handler. That keeps the tool loadable across more clients.

Build vs Buy

Build your own email tool if:

Email is a core product capability.
You already own the notification system.
You have deliverability expertise.
You need custom approval logic.

Use a platform like Sequenzy if:

You want agents to manage campaigns, subscribers, sequences, and analytics.
You need both transactional and marketing email.
You want MCP tools without building the entire wrapper yourself.
You want audit logs, segments, and sequence generation as product features.

Use a pure send API like Resend or Postmark if:

Your agent only needs to add transactional email to an app.
Marketing workflows live somewhere else.
You prefer templates in code.

Final Checklist

Before giving an agent production email access, confirm:

It can draft without sending.
It can send tests.
It cannot bypass suppression lists.
It cannot send to large audiences without approval.
It uses templates for transactional email.
It has idempotency keys for retries.
It logs every action.
It can read delivery results.
You can revoke its key quickly.

Email agents are useful when they reduce repetitive work. They are dangerous when they skip review. Design the tool so good behavior is the easiest path.