@Cyanheads/Mailchimp Server

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Overview

Mailchimp campaign management over the Mailchimp Marketing API v3. Draft, test, and send email campaigns, manage audiences and subscribers, and review post-send analytics from any MCP client. Runs as a stdio process or a local Streamable HTTP server.

Tools

Tool	Description
mailchimp_account	Account profile, plan, data center, and the Chimp Chatter activity feed.
mailchimp_audiences	Manage audiences (lists) — read, create/update, per-audience analytics, signup-form config. No delete.
mailchimp_audience_overview	One-call audience health digest: info, stats, growth history, top email clients, merge-field schema.
mailchimp_subscribers	Subscriber CRUD + tags/notes/activity. archive is the strongest delete available.
mailchimp_upsert_subscriber	Add or update a subscriber idempotently with status, merge fields, tags, and an optional note.
mailchimp_find_subscriber	Locate a subscriber by email in one audience or across the account.
mailchimp_import_subscribers	Batch add/update subscribers (capped at 500/call). Status defaults to pending.
mailchimp_segments	CRUD for audience segments (saved, static, fuzzy) plus member listing and batch add/remove.
mailchimp_merge_fields	Read + create/update custom subscriber attributes. No delete.
mailchimp_campaigns	Campaign record management: list/get/create/update, replicate, content, checklist, RSS/resend controls.
mailchimp_send_campaign	Compose and send (or schedule/test) a campaign in one call.
mailchimp_replicate_campaign	Duplicate a campaign with optional overrides, then draft/test/send/schedule.
mailchimp_reports	Campaign reports — generic slicer across ten dimensions.
mailchimp_campaign_report	Post-send analytics digest — headline metrics plus top-N slices in one response.
mailchimp_templates	Mailchimp-hosted templates: reads work on free; writes require a paid plan.
mailchimp_files	File Manager (Content Studio) — upload, list, fetch, rename, delete files on Mailchimp's CDN.
mailchimp_search	Global search across members or campaigns.
mailchimp_assets (conditional — set MAILCHIMP_ASSETS_DIR)	Local-assets surface — inspect and pre-warm uploads for @assets/<path> references in campaign HTML.
mailchimp_local_templates (conditional — set MAILCHIMP_TEMPLATES_DIR)	Author and render local .eta templates — the write path for templates on free-tier Mailchimp.
mailchimp_playbook	Structured procedural playbook merged with live account state. Advice-only.

Resources

Resource	Description
mailchimp://account	Account info snapshot — profile, plan, data center, total subscribers.
mailchimp://audiences/{audienceId}	Audience snapshot — name, contact, stats, double opt-in status.
mailchimp://campaigns/{campaignId}	Campaign snapshot — status, settings, recipients summary.
mailchimp://campaigns/{campaignId}/report	Post-send campaign report headline metrics.

All resource data is also reachable via tools. Large collections (audiences, campaigns) are not exposed as resources — use the list operation on the corresponding tool instead.

Prompts

Prompt	Description
newsletter_from_source	Compose a monthly editorial newsletter from a URL or brief, chaining into mailchimp_playbook and the draft → test → send flow.

Design reference: docs/email-design-playbook.md.

Capability reference

mailchimp_account _tool

• operation: info returns profile, plan, data center, and total subscribers; operation: activity-feed returns the Chimp Chatter event stream
• activity-feed pages via count (max 100, default 20) and offset; each item carries type, timestamp, and a human-readable description
• An empty activity feed returns a plain-language note explaining likely causes instead of a bare empty array

---

mailchimp_audiences _tool

• list/get/create/update manage audience records; create requires name, contact (company/address1/city/state/zip/country), permissionReminder, and campaignDefaults (fromName/fromEmail/language)
• Analytics reads: list-activity, list-growth, list-clients, list-abuse-reports, list-locations
• get-signup-forms/customize-signup-forms manage hosted/embedded signup-form header, content sections, and CSS
• No delete — on the free plan's 1-audience cap this is destructive; delete via the Mailchimp UI
• count caps at 1000 per page

---

mailchimp_audience_overview _tool

• One call merges audience info, growthMonths of growth history (1–36, default 12), top email clients, and the full merge-field schema
• notes[] distinguishes "no data yet" (new audience, no sends) from a genuine empty-engagement signal

---

mailchimp_subscribers _tool

• CRUD except permanent delete: list/get/update, plus archive (removes from the active audience, preserves the record so the email can resubscribe)
• set-tags is declarative — the provided set becomes the full active tag list; anything not included is removed unless named in preserveTags
• Mailchimp stores static-segment membership as a tag, so an unguarded set-tags can silently drop segment membership
• list-notes/add-note/update-note/delete-note manage CRM-style notes; list-activity/list-events/list-goals are engagement reads
• No GDPR permanent-delete — do that in the Mailchimp UI

---

mailchimp_upsert_subscriber _tool

• Idempotent add-or-update by email — creates via PUT, updates via PATCH, skipping merge-field revalidation on updates that don't touch merge fields
• Declarative tag sync computes the add/remove delta from a desired set; preserveTags protects named tags (including static-segment names) from removal
• status: 'pending' triggers Mailchimp's double opt-in email; 'subscribed' requires documented consent
• updateExistingStatus: false applies status only to newly-created subscribers, leaving existing ones untouched

---

mailchimp_find_subscriber _tool

• Searches one audience (audienceId) or every audience on the account by email, returning separate exactMatches/fuzzyMatches arrays
• Each match is enriched with merge fields and engagement stats; includeTags: true (default) adds the full active tag list at one extra call per match

---

mailchimp_import_subscribers _tool

• Batch add/update, capped at 500 rows per call — chunk larger imports client-side
• status defaults to pending (double opt-in) to prevent accidental mass-sends; a per-row status overrides the top-level default
• updateExisting: false (default) skips rows that already exist instead of overwriting them
• Returns per-row succeeded/failed with Mailchimp error codes and messages

---

mailchimp_segments _tool

• CRUD for saved, static, and fuzzy segments; free tier supports static and basic saved segments — advanced dynamic conditions require Premium
• list-members pages current segment membership
• batch-update-members adds/removes many subscribers from a static segment in one call — not reversible in one shot
• delete is exposed here (unlike audiences/merge-fields), since removing a segment doesn't destroy subscriber data

---

mailchimp_merge_fields _tool

• list/get/create/update for custom subscriber attributes (merge tags such as FNAME, limited to 10 characters)
• No delete — removing a merge field drops that data column across every subscriber; do it in the Mailchimp UI
• options carries type-specific config: choices for dropdown/radio, date_format for date/birthday, phone_format for phone

---

mailchimp_campaigns _tool

• list/get/create/update/replicate for campaign records; get-content/set-content manage the HTML/plaintext payload
• get-checklist runs Mailchimp's send-readiness checklist without sending; cancel-send aborts an in-flight send
• create-resend builds a resend-to-non-openers draft; pause-rss/resume-rss control RSS-driven campaigns
• No send/send-test/schedule/delete — use mailchimp_send_campaign or mailchimp_replicate_campaign for dispatch (checklist-validated, gated on confirmSend: true); delete would destroy report history on a sent campaign
• set-content accepts html, plainText, templateId + templateSections, an archive payload, a fetch url, or localTemplate (mutually exclusive with html/templateId)

---

mailchimp_send_campaign _tool

• Chains create → set-content → checklist → optional test → send/schedule in one call
• mode defaults to draft; send/schedule require confirmSend: true plus a re-entrant confirmation round before any campaign mutation
• Blocking checklist errors throw pre_send_checklist_failed before dispatch; cleanupOnError: true (default) deletes the orphaned draft on any mid-flow failure
• scheduleTime must be at least 15 minutes in the future; testEmails caps at 50 recipients

---

mailchimp_replicate_campaign _tool

• Duplicates an existing campaign, then optionally overrides subject/previewText/fromName/replyTo/title/audience/segment/content before draft/test/send/schedule
• Same confirmSend: true plus re-entrant confirmation and cleanupOnError semantics as mailchimp_send_campaign
• overridesApplied[] in the output lists which overrides actually took effect

---

mailchimp_reports _tool

• list/get are report-index reads; slice pulls one dimension via dimension (abuse-reports, advice, click-details, open-details, domain-performance, eepurl, email-activity, locations, sent-to, unsubscribed)
• click-details drills into one URL with linkId; open-details drills into one member with subscriberHash
• get/slice throw campaign_not_sent when the campaign has no send yet
• For a full digest in one call, prefer mailchimp_campaign_report

---

mailchimp_campaign_report _tool

• One call parallelizes headline delivery/engagement metrics with top-N clicked links, top locations, and recent unsubscribes (includeTopN, 1–100, default 10)
• Throws campaign_not_sent if the campaign hasn't been dispatched yet
• Includes industry benchmarks when Mailchimp reports them for the account's declared vertical

---

mailchimp_templates _tool

• list/get/get-default-content are reads that work on free for base/user template types; create/update/delete are paid-tier writes regardless of type
• gallery (drag-and-drop) is read-gated to paid plans too
• Per-section edits aren't supported here — Mailchimp's PATCH only accepts name/html/folderId; per-section overrides at send time go through mailchimp_campaigns (set-content) or mailchimp_send_campaign's templateSections
• Prefer mailchimp_local_templates for authoring — it works on every plan tier

---

mailchimp_files _tool

• upload/list/get/update/delete manage Mailchimp File Manager (Content Studio); works on every plan tier including free
• Size caps: 1 MB per image, 10 MB per other file; fileData is base64 with no data: prefix
• Upload response's fullSizeUrl is the public CDN URL to embed in campaign HTML
• update with folderId: 0 moves a file to root; folder CRUD isn't exposed — use the Mailchimp UI or upload to root
• WebP/AVIF aren't in Mailchimp's allowed-extension list — convert to PNG/JPG first

---

mailchimp_search _tool

• scope: members matches across all audiences (or one via audienceId); scope: campaigns matches subject/title/preview/archive text
• Member results split into exact and fuzzy arrays with separate upstream totals
• Capped at includeTopN results (1–100, default 10); use mailchimp_find_subscriber for full subscriber detail and tags

---

mailchimp_assets _tool

• Conditional — registered only when MAILCHIMP_ASSETS_DIR is set
• list/info/sync/clear-cache inspect and pre-warm the local-assets pipeline; most workflows never call this directly
• @assets/<path> references in campaign HTML auto-upload via mailchimp_send_campaign, mailchimp_replicate_campaign, or mailchimp_campaigns set-content — hash → upload cache misses → cache sha256 → fileId/URL at <assetsDir>/.mailchimp-cache.json → rewrite to the CDN URL
• Path traversal (.., absolute paths) is rejected; deleting the cache file forces re-upload on next reference
• Node-only

---

mailchimp_local_templates _tool

• Conditional — registered only when MAILCHIMP_TEMPLATES_DIR is set
• list/get/render-preview/seed-from-mailchimp manage .eta template files (Eta v4 — partials, conditionals, loops) with optional YAML frontmatter (subject, previewText, vars) or a legacy <name>.meta.yaml sidecar
• Canonical write path for templates on free-tier Mailchimp accounts, where the upstream /templates API is read-only
• When a template declares vars, every declared name must be present in render-preview's vars input or the render fails — undeclared lookups fall back to an empty string
• seed-from-mailchimp bootstraps a local template from an existing Mailchimp base/user template by ID
• Reference from any campaign tool via content.localTemplate + content.localTemplateVars; mutually exclusive with html/templateId

---

mailchimp_playbook _tool

• topic selects a procedural playbook: send, post-send-review, deliverability, list-hygiene, onboarding, subscriber-triage, design-campaign
• Returns markdown instructions tailored to live account/audience state, a liveState snapshot, and nextToolSuggestions with pre-filled arguments
• Advice-only — makes no writes; the agent executes the suggested follow-up tool calls
• design-campaign includes the editorial-design reference (palette, typography, layout, graphics via CDN) tailored by audience size and engagement

---

mailchimp://account _resource

• Account snapshot as application/json — profile, plan, data center, total subscribers, fetchedAt timestamp
• No parameters; equivalent to mailchimp_account operation: info

---

mailchimp://audiences/{audienceId} _resource

• Audience snapshot — name, contact, stats, campaign defaults, double opt-in status
• audienceId comes from mailchimp_audiences operation: list

---

mailchimp://campaigns/{campaignId} _resource

• Campaign snapshot — status, settings, recipients summary, tracking, report summary
• campaignId comes from mailchimp_campaigns operation: list

---

mailchimp://campaigns/{campaignId}/report _resource

• Post-send headline metrics — bounces, opens, clicks, industry stats
• Throws a validation error if the campaign hasn't been sent yet

---

newsletter_from_source _prompt

• Arguments: source (URL or free-form brief, required), audienceId (optional — feeds live engagement state into design-campaign), seasonalContext (optional)
• Chains into mailchimp_playbook (topic: design-campaign) for audience-aware design guidance, then walks draft → test → send via mailchimp_send_campaign
• Returns a single user-role message carrying the full workflow instructions

Features

Built on @cyanheads/mcp-ts-core: stdio and Streamable HTTP transports, pluggable auth (none / jwt / oauth), swappable storage (in-memory, filesystem, Supabase, Cloudflare KV/R2/D1), structured logging with optional OpenTelemetry tracing.

Mailchimp-specific:

• Auto-derives the API base URL and data center from the -dc suffix on the API key
• Safe-by-default send workflows — re-entrant confirmation before dispatch, pending-status imports, no permanent deletes from the agent surface
• Upstream requests share a configurable concurrency limit and 429 cooldown (MAILCHIMP_CONCURRENCY_LIMIT)
• Local-first authoring for assets and templates — bypasses the free tier's read-only Templates API and File Manager UI friction
• Declarative tag sync computes add/remove deltas instead of requiring callers to track state

Agent-friendly output:

• Provenance and freshness — plain-language notes/note fields explain empty results (e.g. "no growth history yet" vs. zero engagement)
• Graceful partial failure — mailchimp_import_subscribers and mailchimp_segments batch operations return per-row succeeded/failed with Mailchimp error codes instead of failing the whole request
• Discriminated output contracts — operation/dimension fields on multi-operation tools tell the caller which optional fields are populated
• Typed error contracts with required recovery hints — every declared error reason carries a recovery string describing the caller's next move

Getting started

Add the following to your MCP client configuration file. See docs/api-key.md for how to generate a Mailchimp API key.

{
  "mcpServers": {
    "mailchimp-mcp-server": {
      "type": "stdio",
      "command": "bunx",
      "args": ["@cyanheads/mailchimp-mcp-server@latest"],
      "env": {
        "MCP_TRANSPORT_TYPE": "stdio",
        "MCP_LOG_LEVEL": "info",
        "MAILCHIMP_API_KEY": "your-key-with-dc-suffix-e.g.-us22"
      }
    }
  }
}

Or with npx (no Bun required):

{
  "mcpServers": {
    "mailchimp-mcp-server": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "@cyanheads/mailchimp-mcp-server@latest"],
      "env": {
        "MCP_TRANSPORT_TYPE": "stdio",
        "MCP_LOG_LEVEL": "info",
        "MAILCHIMP_API_KEY": "your-key-with-dc-suffix-e.g.-us22"
      }
    }
  }
}

Or with Docker:

{
  "mcpServers": {
    "mailchimp-mcp-server": {
      "type": "stdio",
      "command": "docker",
      "args": [
        "run", "-i", "--rm",
        "-e", "MCP_TRANSPORT_TYPE=stdio",
        "-e", "MAILCHIMP_API_KEY=your-key-with-dc-suffix-e.g.-us22",
        "ghcr.io/cyanheads/mailchimp-mcp-server:latest"
      ]
    }
  }
}

For Streamable HTTP, set the transport and start the server:

MCP_TRANSPORT_TYPE=http MCP_HTTP_PORT=3010 MAILCHIMP_API_KEY=... bun run start:http
# Server listens at http://localhost:3010/mcp

Prerequisites

• Bun v1.4.0 or higher (or Node.js v24+).
• A Mailchimp Marketing API key — the key's -dc suffix (e.g. -us22) identifies your data center and is parsed at startup.

Installation

1. Clone the repository:

git clone https://github.com/cyanheads/mailchimp-mcp-server.git

2. Navigate into the directory:

cd mailchimp-mcp-server

3. Install dependencies:

bun install

4. Configure environment:

cp .env.example .env
# edit .env and set MAILCHIMP_API_KEY

Configuration

Variable	Description	Default
MAILCHIMP_API_KEY	Required. Mailchimp Marketing API key including -dc suffix (e.g. abc…-us22).	—
MAILCHIMP_BASE_URL	Override API base URL (for mock servers or tests).	https://{dc}.api.mailchimp.com/3.0
MAILCHIMP_TIMEOUT_MS	Per-request timeout in milliseconds.	60000
MAILCHIMP_MAX_RETRIES	Max retry attempts for transient upstream failures (0-10).	3
MAILCHIMP_CONCURRENCY_LIMIT	Max in-flight upstream requests across all tools in this process (1-10).	4
MAILCHIMP_ASSETS_DIR	Absolute path to a local assets directory. Enables mailchimp_assets and auto-upload of @assets/<path> references in campaign HTML. Node-only.	unset
MAILCHIMP_TEMPLATES_DIR	Absolute path to a local templates directory. Enables mailchimp_local_templates and content.localTemplate on campaign tools. Node-only.	unset
MCP_TRANSPORT_TYPE	Transport: stdio or http.	stdio
MCP_SESSION_MODE	auto, stateful, or stateless; auto resolves to stateful. HTTP requires stateful for campaign confirmation and rejects a stateless override. Stdio is unaffected.	stateful
MCP_HTTP_HOST	HTTP server hostname.	127.0.0.1
MCP_HTTP_PORT	HTTP server port.	3010
MCP_HTTP_ENDPOINT_PATH	MCP endpoint path.	/mcp
MCP_AUTH_MODE	Auth mode: none, jwt, or oauth.	none
MCP_LOG_LEVEL	Log level (RFC 5424).	info
LOGS_DIR	Directory for log files (Node.js only).	<project-root>/logs
OTEL_ENABLED	Enable OpenTelemetry.	false

See .env.example for the full list of optional overrides.

Local assets (optional)

Set MAILCHIMP_ASSETS_DIR to enable a local-image workflow on top of Mailchimp's File Manager. Drop image files into the directory, reference them in HTML as @assets/<relative-path>, and the server uploads + rewrites at send time.

export MAILCHIMP_ASSETS_DIR=/Users/me/Pictures/email-assets

Then in a campaign:

<img src="@assets/hero.png" alt="Hero">
<a href="@assets/whitepaper.pdf">Download</a>

When mailchimp_send_campaign (or mailchimp_campaigns set-content / mailchimp_replicate_campaign contentOverride) sees these references, it:

1. Hashes each referenced file (SHA-256).
2. Uploads cache misses to Mailchimp File Manager via the mailchimp_files tool surface.
3. Caches sha256 → file_id + URL at <assetsDir>/.mailchimp-cache.json (atomic writes; safe to delete to force re-upload).
4. Rewrites every @assets/<path> to the public CDN URL before passing content upstream.

The mailchimp_assets tool exposes list, info, sync (pre-warm), and clear-cache for direct inspection — most workflows don't need it.

Caveats:

• Mailchimp caps images at 1 MB and other files at 10 MB. Oversize files fail before upload with an actionable error.
• Allowed extensions: see the mailchimp_files tool description. WebP and AVIF are NOT in the allowlist — convert to PNG/JPG.
• Path traversal is rejected (../ and absolute paths throw Forbidden).
• The mailchimp_assets tool is Node-only.

Local templates (optional)

Set MAILCHIMP_TEMPLATES_DIR to enable a local-template authoring workflow on top of Eta (v4 — fast, ESM-native, supports partials/conditionals/loops). This is the canonical write path for templates on free-tier Mailchimp accounts, where the upstream /templates API is read-only.

export MAILCHIMP_TEMPLATES_DIR=/Users/me/email-templates

email-templates/
  welcome.eta              # body + optional YAML frontmatter
  newsletter.eta
  partials/
    header.eta
    footer.eta

Template (welcome.eta) — YAML frontmatter on top, Eta body below:

---
subject: "Welcome to {{brand}}"
previewText: "Onboarding starts here"
vars:
  - firstName
  - brand
---
<%~ include('partials/header', it) %>
<h1>Hello <%= it.firstName %></h1>
<p>Welcome to <%= it.brand %>.</p>
<img src="@assets/hero.png" alt="Hero">

Frontmatter and its fields are optional. When vars: is present, every listed variable must be supplied; undeclared variable lookups render as an empty string.

Sidecar fallback (legacy): prior to v0.3.1, meta lived in a separate <name>.meta.yaml file next to the body. That form still works for backward compatibility — if a .eta has no frontmatter, the loader falls back to reading the sidecar. Frontmatter takes precedence when both exist.

Reference from any campaign tool:

{
  "audienceId": "abc123",
  "subject": "Welcome to Acme",
  "fromName": "Casey",
  "replyTo": "[email protected]",
  "content": {
    "localTemplate": "welcome",
    "localTemplateVars": { "firstName": "Sam", "brand": "Acme" }
  },
  "mode": "draft"
}

The render pipeline:

1. Eta renders welcome.eta with it = { firstName: 'Sam', brand: 'Acme' }.
2. If L1 is configured, @assets/hero.png is uploaded to Mailchimp File Manager and rewritten to a CDN URL.
3. Final HTML is set on the campaign via Mailchimp's set-content.

The mailchimp_local_templates tool exposes list, get, render-preview (returns HTML without sending), and seed-from-mailchimp (reads a Mailchimp base/user template by ID and writes it to disk as a starting point — useful on free where you can read but not write upstream).

Example templates in this repo

The templates/ directory holds working examples — point MAILCHIMP_TEMPLATES_DIR at it directly to try them, or copy them into your own dir as a starting point:

Template	What it shows
welcome.eta	Minimal body — frontmatter declaring subject / previewText / vars, <%= it.firstName %> interpolation, <% if %> conditional CTA block
redden-gardens-april-2026.eta	Full inline-styled HTML newsletter. Demonstrates the recommended split: Mailchimp merge tags (*|FNAME|*) for per-recipient personalization on real list sends, Eta vars (volume / issue / monthYear / URLs) for list-wide constants substituted at template-render time

Caveats:

• localTemplate is mutually exclusive with html and templateId on the same content block.
• Missing declared variables fail before rendering; extra variables are allowed.
• Path traversal is rejected.

Running the server

Local development

• Watch mode (transport via MCP_TRANSPORT_TYPE):

  bun run dev                                     # stdio (default)
  MCP_TRANSPORT_TYPE=http bun run dev             # http
  

• Build and run:

  bun run rebuild
  bun run start:stdio
  # or
  bun run start:http
  

• Run checks and tests:

  bun run devcheck   # Lint, format, typecheck, security
  bun run test       # Vitest test suite
  bun run lint:mcp   # Validate MCP definitions against spec
  

Docker

docker build -t mailchimp-mcp-server .
docker run --rm -e MAILCHIMP_API_KEY=your-key-us22 -p 3010:3010 mailchimp-mcp-server

The Dockerfile defaults to HTTP transport, stateful session mode, and logs to /var/log/mailchimp-mcp-server. Stateful HTTP preserves campaign confirmation for 2025-era clients. OpenTelemetry peer dependencies are installed by default — build with --build-arg OTEL_ENABLED=false to omit them.

Project structure

Directory	Purpose
src/index.ts	createApp() entry point — registers tools/resources/prompts and inits services.
src/config	Server-specific environment variable parsing and validation with Zod.
src/mcp-server/tools	Tool definitions (*.tool.ts). Eighteen always-on tools plus two conditional local-workspace tools.
src/mcp-server/resources	Resource definitions (*.resource.ts). Four snapshot resources.
src/mcp-server/prompts	Prompt definitions (*.prompt.ts). Newsletter starter prompt.
src/services/mailchimp	Mailchimp client wrapper — HTTP plumbing, retries, normalization, typed surface.
templates/	Example .eta templates (frontmatter + Eta syntax) — point MAILCHIMP_TEMPLATES_DIR here to try them.
tests/	Vitest coverage for configuration, services, tool workflows, output formatting, framework contracts, and regressions.

Development guide

See CLAUDE.md for development guidelines and architectural rules. The short version:

• Handlers throw, framework catches — no try/catch in tool logic
• Use ctx.log for request-scoped logging
• Register new tools and resources via the barrels in src/mcp-server/*/definitions/index.ts
• Wrap external API calls: validate raw → normalize to domain type → return output schema; never fabricate missing fields

Contributing

Issues are welcome. Run checks and tests before submitting:

bun run devcheck
bun run test

License

Apache-2.0 — see LICENSE for details.