designing-email-templates — Claude Code skill for email templates

---
name: designing-email-templates
description: 'Author, save, and edit email templates in the PostHog workflows library — compose email design JSON with Liquid personalization and create and round-trip-edit templates over MCP. Use when asked to design, build, update, or fix an email template for workflows, broadcasts, or campaigns.'
---

# Designing email templates

Use this skill when creating or editing email templates for PostHog workflows — broadcast campaigns and `function_email` workflow actions send the rendered template.

## How authoring works

You author the **design JSON** (`content.email.design`) and save it with `workflows-create-email-template`. The server renders the sent email from your design with the same renderer PostHog's visual editor uses, so the template opens as editable blocks for humans and sends exactly what the design describes. Schema, the native blocks and spacing rules every email follows, and a working example in [references/unlayer-design-json.md](references/unlayer-design-json.md).

When talking to the user, call it the template's **design** — the design document format is an internal implementation detail. Always share the template's `_posthogUrl` edit link in your reply after creating or updating, so the user can open it in PostHog directly.

Read [references/design-guidelines.md](references/design-guidelines.md) before composing — it covers committing to a design direction, typography, color, and the patterns that make an email look designed rather than generated. For one fragment the block editor can't express, use an `html`-type content block inside the design.

## Personalization with Liquid

Email content uses Liquid templating. Liquid tags pass through the renderer as plain text, so use them anywhere — block text, subject, links:

```liquid
Hi {{ person.properties.first_name | default: 'there' }},
```

Marketing emails must include an unsubscribe link — render it with the built-in variables:

```html
<a href="{{ unsubscribe_url }}">Unsubscribe</a>
```

(`{{ unsubscribe_url_one_click }}` is also available for one-click list-unsubscribe flows.)

Output is HTML-escaped by default, so a value renders as text even when it contains markup:

- Don't add `| escape`. It escapes a second time, and `&` reaches the inbox as `&amp;amp;`.
- For HTML your own backend builds, such as a summary list, add `| raw`: `{{ variables.summary_html | raw }}`. Never use `raw` on text a person typed.
- Format money with `currency`: `{{ variables.amount | currency: "USD" }}` renders `$1,234.50`. A value that isn't a number renders unchanged.

## Click tracking and opt-out

Every link is automatically rewritten through a click-tracking redirect. This breaks mobile universal links / app deeplinks, which only resolve when the href stays on their own domain. To keep a link untracked, mark its anchor (use an `html` block) with `clicktracking="off"` or `data-ph-no-track`:

```html
<a href="https://app.example.com/deeplink" data-ph-no-track>Open in app</a>
```

The marker must be on the `<a>` tag itself, not a child element. Opted-out links get no click metrics.

## Images

Call `media-images-list` with `purpose="email"` first — reuse an existing image (a logo, a header banner) instead of uploading a duplicate.

To add a new image, upload it with the presigned flow rather than embedding bytes in the design:

1. `media-image-upload-start` with the file's name and `purpose="email"` — returns `id`, `upload_url`, and `form_fields`.
2. From a shell, POST the local file to `upload_url`: `curl -X POST <upload_url> -F key=value... -F file=@/path/to/image.png` (the `form_fields` from the response, `file` last). Never base64-encode image bytes into a tool call — a flipped token corrupts the image.
3. `media-image-upload-complete` with the `id` — returns the permanent `url`.

Put that `url` in the image block's `values.src.url` (see [references/unlayer-design-json.md](references/unlayer-design-json.md)). The block also takes `values.src.width`/`height`; the media tools don't return dimensions, so if you have shell access to the local file, measure it yourself rather than guessing.

Images must be under 4MB and decode as PNG, JPEG, GIF, WebP, AVIF or BMP.

## Creating a template

Call `workflows-create-email-template` with:

```json
{
  "name": "Welcome email",
  "description": "Sent to new signups on day 0",
  "type": "email",
  "content": {
    "templating": "liquid",
    "email": {
      "subject": "Welcome to {{ person.properties.company | default: 'our product' }}",
      "design": {
        "counters": { "u_row": 1 },
        "schemaVersion": 16,
        "body": { "rows": ["…"], "values": { "preheaderText": "Three short steps to get you set up" } }
      },
      "text": "Plain-text fallback of the same message"
    }
  }
}
```

- `subject` is required for email templates.
- Put the preview text that most inboxes show after the subject in the design's `body.values.preheaderText`. The rendered html carries it, so it reaches the inbox on every send path. Leave `content.email.preheader` unset, so the email carries one preview text.
- Always author the `design` and omit `html` - the server renders html from the design. Sending hand-written html without a design produces an email the visual editor can only show as one raw block.
- Always provide `text` as a real plain-text rendering of the message - clients that block rich content show only `text`, so filler like "placeholder" reaches real inboxes, and a text part that doesn't match the html hurts deliverability.
- The tool result returns an edit link into the PostHog library.
- After creating (or updating), call `workflows-show-email-template` — it renders an inline preview so the user sees the result.

### Payload mechanics

Pass the design directly in the tool call — no scratch files, no pre-validation subprocesses, no payload preview rounds. Liquid tags (`{{ }}`, `{% %}`), apostrophes, single quotes, and emoji are ordinary characters inside JSON strings; only standard JSON escaping applies. Never rewrite content to avoid them — converting Liquid's single quotes to double quotes inside markup attributes breaks the markup. If the tool call is rejected as malformed, fix the JSON escaping and resend the same content unchanged.

## Converting an existing HTML email

To move an email from another platform, rebuild it from native blocks instead of pasting its HTML into one `html` block, so people can still edit it:

- Each part of the source becomes the native block it shows: a `heading`, a `text` block per paragraph, a `button` per call to action, an `image` per picture. Keep the source's colors, font sizes, and padding on each block.
- Re-host every image through the media library (see Images). Put the file's real dimensions in `src.width` and `src.height`, and set the displayed width with `src.maxWidth` as a percentage of the content width. Source HTML often declares a height that doesn't match the file.
- A section the source wraps in `{% if %}` becomes a block with a display condition (see [references/unlayer-design-json.md](references/unlayer-design-json.md)).
- Rename the source platform's Liquid variables to the ones this send provides. A transactional API send provides everything the caller passes in `message_data` as `{{ variables.* }}`.
- Buttons have no `box-shadow`. A 3px bottom border in a darker shade (`border.borderBottomWidth`, `borderBottomStyle`, `borderBottomColor`) gives the same raised look.
- The `social` block draws only its own icon sets. To keep custom icons, put the icon row in an `html` block with the re-hosted images.

## Editing a template (read–modify–write)

`content` is replaced as a whole on update, never merged — and humans may have edited the design in PostHog's visual editor since you last saw it:

1. `workflows-get-email-template` — always fetch fresh; the returned `design` is the current source of truth.
2. Modify the `design` (keep subject/text alongside it).
3. `workflows-update-email-template` — send the complete `content` back. The server re-renders the sent email from the edited design.
4. `workflows-show-email-template` — render the updated template so the user sees the change; its response carries the final rendered html, so read it before describing the result.

For small changes to an existing design, prefer `workflows-patch-email-template`: id-addressed operations over the Unlayer blocks, so you send only the edit instead of the whole design.

## Editing the email inside a workflow step

A `function_email` step carries its own email snapshot (`config.inputs.email.value` with subject/text/html/design), independent of any library template.
Edit it with `workflows-patch-action-email`: the same design operations as `workflows-patch-email-template`, plus an `email_patch` merge for subject, preheader, text, and recipients.

1. `workflows-get` — the step's current design (and its block ids) is in `config.inputs.email.value.design`.
2. `workflows-patch-action-email` with the workflow id, the step's `action_id`, and your operations and/or `email_patch`. When the operations change the copy, the same call's `email_patch` sets a matching `text`.
3. The HTML is re-rendered server-side from the patched design, so it never goes stale.
4. On an active workflow the edit stages a draft — test with `workflows-test-run` (`use_draft=true`) and apply it with `workflows-publish`.

## Using templates

- List what exists with `workflows-list-email-templates` (metadata only; fetch one for its content).
- When the user asks to see a template, call `workflows-show-email-template` — it renders an inline preview.
- Reference a template from a workflow's `function_email` action (its UUID in `config.template_uuid`), or start a broadcast from it in the PostHog UI. The step takes a snapshot of the template's body at save - editing the library template later does not change steps that already used it. To change a step's email, patch that step with `workflows-patch-action-email`.
- Templates are soft-deleted by setting `deleted: true` via `workflows-update-email-template`.