resend · danilowoz · Feb 20, 2026
diff --git a/INVESTIGATION_TEMPLATE_HTML_TRANSFORMATION.md b/INVESTIGATION_TEMPLATE_HTML_TRANSFORMATION.md
@@ -0,0 +1,289 @@
+# Investigation: Template HTML Transformation and Content Duplication
+
+**Related Issues:**
+- [resend-python#186](https://github.com/resend/resend-python/issues/186) - Templates.create() transforms raw HTML
+- [resend-python#187](https://github.com/resend/resend-python/issues/187) - Templates.update() appends HTML instead of replacing it
+- Linear: PRODUCT-1427 - Opening template rewrites HTML and duplicates content
+
+**Investigation Date:** February 20, 2026
+
+## Summary
+
+The Resend dashboard template editor processes raw HTML through a React Email rendering pipeline and persists the transformed result back to storage **every time a template is opened**, even when no edits are made. This causes:
+
+1. **HTML Transformation:** Original HTML gets bloated with React Email artifacts (data-id attributes, HTML comments, inline styles)
+2. **Content Duplication:** Subsequent updates via API cause content to be duplicated in the editor
+3. **Unexpected Behavior:** HTML sent via API differs from what's retrieved after dashboard viewing
+
+## Root Cause Analysis
+
+### The Problem Flow
+
+1. **Create Template via API** (3,267 chars of HTML)
+   ```python
+   resend.Templates.create({
+       "name": "test-template",
+       "html": "<html>...</html>"  # Raw HTML
+   })
+   ```
+
+2. **GET /templates/{id}** → Returns identical HTML (3,267 chars) ✅
+
+3. **Open Template in Dashboard** (no edits, just view it)
+   - Dashboard renders HTML through React Email pipeline
+   - React Email components add artifacts:
+     - `data-id="__react-email-column"` attributes
+     - `<!--$-->` and `<!--/$-->` comment markers  
+     - Injected inline styles
+     - Transformed HTML structure
+   - **Dashboard saves transformed HTML back to storage** ❌
+
+4. **GET /templates/{id}** → HTML now ~21,000+ chars ❌
+
+5. **Update via API with original HTML**
+   ```python
+   resend.Templates.update({
+       "id": template_id,
+       "html": "<html>...</html>"  # Same original HTML
+   })
+   ```
+
+6. **Open Dashboard Again** → Content appears duplicated ❌
+
+### Source of Artifacts
+
+Investigation of the [react-email](https://github.com/resend/react-email) repository reveals React Email components hardcode `data-id` attributes:
+
+**packages/column/src/column.tsx:**
+```typescript
+export const Column = React.forwardRef<HTMLTableCellElement, ColumnProps>(
+  ({ children, style, ...props }, ref) => {
+    return (
+      <td {...props} data-id="__react-email-column" ref={ref} style={style}>
+        {children}
+      </td>
+    );
+  },
+);
+```
+
+**packages/markdown/src/markdown.tsx:**
+```typescript
+<div data-id="react-email-markdown">
+  {/* content */}
+</div>
+```
+
+When raw HTML is rendered through React Email (even just for viewing), these components inject their attributes and transform the HTML structure.
+
+## Evidence
+
+### Reproduction Script
+
+A [Node.js reproduction script](https://gist.github.com/drish/5352959c0dca9abd96141b301ea317a1) demonstrates:
+
+- API behavior is correct (PATCH replaces HTML exactly as sent)
+- Transformation only occurs after opening template in dashboard
+- Issue affects both Python SDK and Node.js SDK identically
+- **Conclusion: Platform/dashboard issue, not SDK-specific**
+
+### Example Transformation
+
+**Original HTML:**
+```html
+<table>
+  <tr>
+    <td>Cell 1</td>
+    <td>Cell 2</td>
+  </tr>
+</table>
+```
+
+**After Dashboard View:**
+```html
+<!--$--><table>
+  <tr>
+    <td data-id="__react-email-column" style="...">Cell 1</td>
+    <td data-id="__react-email-column" style="...">Cell 2</td>
+  </tr>
+</table><!--/$-->
+```
+
+## Impact
+
+### User-Reported Problems
+
+1. **HTML Bloat:** Templates grow 6-7x in size after dashboard viewing
+2. **Content Duplication:** Updates cause content to appear multiple times
+3. **Unexpected Behavior:** HTML sent via API ≠ HTML retrieved after dashboard interaction
+4. **Loss of Control:** Users lose ability to maintain exact HTML structure
+
+### Affected Users
+
+- Users creating templates via API with custom HTML
+- Users expecting API to preserve exact HTML formatting
+- Users with CSS that relies on specific HTML structure
+- Users with dark mode or complex styling (as reported in issues)
+
+## Proposed Solutions
+
+### Solution 1: Prevent Auto-Save on View (Recommended)
+
+**Location:** Dashboard template editor code (private repository)
+
+**Fix:** Only persist HTML changes when user explicitly saves, not on template view/load.
+
+```typescript
+// Current (problematic):
+function onTemplateLoad(template) {
+  const transformed = renderThroughReactEmail(template.html);
+  saveToStorage(transformed);  // ❌ Don't do this
+  displayInEditor(transformed);
+}
+
+// Proposed:
+function onTemplateLoad(template) {
+  const transformed = renderThroughReactEmail(template.html);
+  displayInEditor(transformed);  // ✅ Only display, don't save
+}
+
+function onUserSave(editorContent) {
+  saveToStorage(editorContent);  // ✅ Only save on explicit user action
+}
+```
+
+**Benefits:**
+- Preserves original HTML until user makes intentional changes
+- Fixes both transformation and duplication issues
+- No SDK changes required
+
+### Solution 2: Add "plainText" Mode to Dashboard
+
+**Location:** Dashboard template editor code
+
+**Fix:** When loading templates created via API (not through React Email editor), display in plain HTML mode without React Email transformation.
+
+```typescript
+function onTemplateLoad(template) {
+  if (template.createdViaAPI && !template.usesReactEmailComponents) {
+    displayAsPlainHTML(template.html);  // No transformation
+  } else {
+    const transformed = renderThroughReactEmail(template.html);
+    displayInEditor(transformed);
+  }
+}
+```
+
+**Benefits:**
+- Preserves API-created templates
+- Allows React Email templates to work as expected
+- Clear distinction between template types
+
+### Solution 3: Add Render Options to React Email
+
+**Location:** react-email repository
+
+**Fix:** Add option to skip adding `data-id` attributes during rendering.
+
+```typescript
+export const Column = React.forwardRef<HTMLTableCellElement, ColumnProps>(
+  ({ children, style, includeDataId = true, ...props }, ref) => {
+    const dataIdProp = includeDataId ? { 'data-id': '__react-email-column' } : {};
+    return (
+      <td {...props} {...dataIdProp} ref={ref} style={style}>
+        {children}
+      </td>
+    );
+  },
+);
+```
+
+**Benefits:**
+- Allows clean HTML output when needed
+- Maintains editor functionality when attributes are included
+- Flexible for different use cases
+
+**Drawbacks:**
+- Requires changes across multiple React Email components
+- May affect editor features that rely on these attributes
+
+### Solution 4: Template Versioning
+
+**Location:** Dashboard backend
+
+**Fix:** Store both original and transformed versions, serve appropriate version based on request source.
+
+```typescript
+interface Template {
+  id: string;
+  name: string;
+  originalHtml: string;      // Preserved as submitted via API
+  transformedHtml?: string;   // For dashboard editor use
+  // ...
+}
+```
+
+**Benefits:**
+- Preserves original HTML for API consumers
+- Allows dashboard to work with transformed version
+- Non-breaking change
+
+**Drawbacks:**
+- Increased storage requirements
+- More complex synchronization logic
+
+## Recommended Action Plan
+
+1. **Immediate Fix (Critical):** Implement Solution 1 - Stop auto-saving transformed HTML on template view
+2. **Short-term:** Implement Solution 2 - Add plain HTML mode for API-created templates
+3. **Long-term:** Consider Solution 4 - Template versioning for robust handling of both API and UI workflows
+
+## SDK Considerations
+
+The Python SDK (and all other SDKs) are working correctly. No SDK changes are required. The issue is entirely in the dashboard/backend platform code.
+
+### Workaround for Users (Until Fix)
+
+Currently, users should:
+1. Create/update templates via API
+2. **Avoid opening templates in dashboard** if HTML preservation is critical
+3. Use the Resend API directly for all template operations
+
+## Testing Recommendations
+
+After implementing fixes, verify:
+
+1. ✅ Template created via API → HTML unchanged after dashboard view
+2. ✅ Template updated via API → No content duplication
+3. ✅ Template created in dashboard → React Email features work correctly
+4. ✅ Template edited in dashboard → Changes saved correctly
+5. ✅ API GET returns exact HTML that was PUT/PATCH
+
+## Related Code Repositories
+
+- **resend-python:** SDK confirmed working correctly
+- **resend-node:** SDK confirmed working correctly  
+- **react-email:** Contains components that add data-id attributes
+- **resend-openapi:** API specification (does not indicate transformation behavior)
+- **Dashboard (private):** Contains problematic auto-save logic
+
+## Verification
+
+To verify the fix works:
+
+```bash
+# Run the reproduction script from the gist
+node reproduction-script.js
+
+# Expected results after fix:
+# - Original HTML length: 3267 chars
+# - After dashboard view: 3267 chars (unchanged)
+# - After update: 3267 chars (unchanged)
+# - HTML identical to original: true (always)
+```
+
+## Contributors
+
+- **Investigation:** João (drish) - identified root cause
+- **Reproduction:** Node.js script demonstrating issue
+- **Analysis:** Cursor AI Agent - traced issue to React Email component attributes and dashboard auto-save behavior
diff --git a/TEMPLATE_ISSUE_README.md b/TEMPLATE_ISSUE_README.md
@@ -0,0 +1,71 @@
+# Template HTML Transformation Issue
+
+## Quick Links
+
+- **Full Investigation Report:** [INVESTIGATION_TEMPLATE_HTML_TRANSFORMATION.md](./INVESTIGATION_TEMPLATE_HTML_TRANSFORMATION.md)
+- **Python Reproduction Script:** [examples/investigation_template_transformation.py](./examples/investigation_template_transformation.py)
+- **Related Issues:** [#186](https://github.com/resend/resend-python/issues/186), [#187](https://github.com/resend/resend-python/issues/187)
+
+## TL;DR
+
+**Problem:** Opening a template in the Resend dashboard causes the HTML to be transformed through React Email and saved back to storage, even when no edits are made. This results in:
+- HTML bloat (6-7x size increase)
+- Content duplication on subsequent updates
+- Loss of exact HTML formatting
+
+**Root Cause:** Dashboard auto-saves React Email-transformed HTML (with `data-id` attributes and comments) instead of preserving the original HTML.
+
+**Status:** Confirmed platform issue, not an SDK bug. The Python SDK is working correctly.
+
+**Workaround:** Avoid opening templates in the dashboard if HTML preservation is critical. Use API-only operations.
+
+## Running the Investigation
+
+To reproduce the issue yourself:
+
+```bash
+export RESEND_API_KEY="re_your_api_key"
+python examples/investigation_template_transformation.py
+```
+
+The script will:
+1. Create a template with complex HTML (3,267 chars)
+2. Verify HTML is preserved after creation
+3. Prompt you to open the template in dashboard
+4. Show HTML has been transformed (~21,000+ chars)
+5. Demonstrate content duplication on updates
+
+## Proposed Solutions
+
+See the [full investigation report](./INVESTIGATION_TEMPLATE_HTML_TRANSFORMATION.md#proposed-solutions) for detailed solutions, including:
+
+1. **Stop auto-saving transformed HTML** (recommended immediate fix)
+2. **Add plain HTML mode** for API-created templates
+3. **Make React Email data-id attributes optional**
+4. **Template versioning** to store both original and transformed HTML
+
+## For Resend Team
+
+The fix needs to be implemented in the dashboard/platform code (likely a private repository). The specific area to investigate:
+
+```typescript
+// Dashboard template editor (pseudo-code)
+function onTemplateLoad(template) {
+  const transformed = renderThroughReactEmail(template.html);
+  saveToStorage(transformed);  // ❌ This line is the problem
+  displayInEditor(transformed);
+}
+```
+
+Should be:
+
+```typescript
+function onTemplateLoad(template) {
+  const transformed = renderThroughReactEmail(template.html);
+  displayInEditor(transformed);  // ✅ Display only, don't auto-save
+}
+```
+
+## SDK Impact
+
+**No SDK changes are required.** All SDKs (Python, Node.js, etc.) are functioning correctly. The issue is entirely in the platform/dashboard layer.