diff --git a/docs/superpowers/plans/2026-04-13-mermaid-file-viewer.md b/docs/superpowers/plans/2026-04-13-mermaid-file-viewer.md
new file mode 100644
index 00000000..1d1f6fe9
--- /dev/null
+++ b/docs/superpowers/plans/2026-04-13-mermaid-file-viewer.md
@@ -0,0 +1,360 @@
+# Mermaid File Viewer Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Open `.mmd` / `.mermaid` files and render them as live, themed diagrams with a source/diagram toggle in the editor header.
+
+**Architecture:** Orca already has a `MermaidBlock` component that renders mermaid syntax to SVG, and a `MarkdownViewToggle` that switches between source and rich modes. This plan registers `.mmd`/`.mermaid` as a new language (`'mermaid'`), adds a `MermaidViewer` component that wraps `MermaidBlock` for full-file rendering with scroll caching and centering, and wires it into the existing `EditorPanel` / `EditorContent` routing alongside the markdown path. The view mode store (`markdownViewMode`) is reused since it is already keyed by file ID.
+
+**Tech Stack:** React, Zustand (existing store), mermaid (already installed v11.14), DOMPurify (already installed), Monaco (source mode), existing `MermaidBlock` component.
+
+---
+
+## File Map
+
+| Action | Path | Responsibility |
+|--------|------|---------------|
+| Modify | `src/renderer/src/lib/language-detect.ts` | Register `.mmd` and `.mermaid` extensions |
+| Create | `src/renderer/src/components/editor/MermaidViewer.tsx` | Full-file mermaid diagram viewer with scroll caching |
+| Modify | `src/renderer/src/components/editor/EditorPanel.tsx` | Add `isMermaid` flag, show view toggle for mermaid files |
+| Modify | `src/renderer/src/components/editor/EditorContent.tsx` | Add `isMermaid` prop, route to `MermaidViewer` or Monaco |
+| Modify | `src/renderer/src/assets/markdown-preview.css` | Add `.mermaid-viewer` styles |
+
+---
+
+### Task 1: Register `.mmd` and `.mermaid` file extensions
+
+**Files:**
+- Modify: `src/renderer/src/lib/language-detect.ts:10-76` (add two entries to `EXT_TO_LANGUAGE`)
+
+- [ ] **Step 1: Add mermaid extensions to the language map**
+
+In `src/renderer/src/lib/language-detect.ts`, add these two entries to the `EXT_TO_LANGUAGE` object (between `.mdx` and `.css`):
+
+```typescript
+'.mmd': 'mermaid',
+'.mermaid': 'mermaid',
+```
+
+- [ ] **Step 2: Verify the change compiles**
+
+Run: `cd src/renderer && npx tsc --noEmit --pretty 2>&1 | head -20`
+Expected: No errors related to `language-detect.ts`.
+
+- [ ] **Step 3: Commit**
+
+```bash
+git add src/renderer/src/lib/language-detect.ts
+git commit -m "feat: register .mmd and .mermaid file extensions"
+```
+
+---
+
+### Task 2: Create `MermaidViewer` component
+
+**Files:**
+- Create: `src/renderer/src/components/editor/MermaidViewer.tsx`
+
+This component renders the entire file content as a mermaid diagram. It reuses the existing `MermaidBlock` for rendering and follows the same dark-mode detection and scroll-caching patterns as `MarkdownPreview`.
+
+- [ ] **Step 1: Create the `MermaidViewer` component**
+
+Create `src/renderer/src/components/editor/MermaidViewer.tsx`:
+
+```tsx
+import React, { useLayoutEffect, useRef } from 'react'
+import { useAppStore } from '@/store'
+import { scrollTopCache, setWithLRU } from '@/lib/scroll-cache'
+import MermaidBlock from './MermaidBlock'
+
+type MermaidViewerProps = {
+  content: string
+  filePath: string
+}
+
+// Why: MermaidViewer is the full-file counterpart to MermaidBlock (which
+// renders fenced mermaid blocks inside markdown). When a user opens a .mmd
+// or .mermaid file in diagram mode, the entire file content is the diagram
+// source — no markdown wrapper, no frontmatter, just mermaid syntax.
+export default function MermaidViewer({
+  content,
+  filePath
+}: MermaidViewerProps): React.JSX.Element {
+  const rootRef = useRef<HTMLDivElement>(null)
+  const settings = useAppStore((s) => s.settings)
+  const isDark =
+    settings?.theme === 'dark' ||
+    (settings?.theme === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches)
+
+  // Why: Each viewing mode (source vs diagram) produces different DOM heights.
+  // Mode-scoped keys prevent restoring a source-mode scroll position in diagram
+  // mode (same reasoning as MarkdownPreview's scrollCacheKey).
+  const scrollCacheKey = `${filePath}:mermaid-diagram`
+
+  useLayoutEffect(() => {
+    const container = rootRef.current
+    if (!container) {
+      return
+    }
+
+    let throttleTimer: ReturnType<typeof setTimeout> | null = null
+
+    const onScroll = (): void => {
+      if (throttleTimer !== null) {
+        clearTimeout(throttleTimer)
+      }
+      throttleTimer = setTimeout(() => {
+        setWithLRU(scrollTopCache, scrollCacheKey, container.scrollTop)
+        throttleTimer = null
+      }, 150)
+    }
+
+    container.addEventListener('scroll', onScroll, { passive: true })
+    return () => {
+      setWithLRU(scrollTopCache, scrollCacheKey, container.scrollTop)
+      if (throttleTimer !== null) {
+        clearTimeout(throttleTimer)
+      }
+      container.removeEventListener('scroll', onScroll)
+    }
+  }, [scrollCacheKey])
+
+  useLayoutEffect(() => {
+    const container = rootRef.current
+    const targetScrollTop = scrollTopCache.get(scrollCacheKey)
+    if (!container || targetScrollTop === undefined) {
+      return
+    }
+
+    let frameId = 0
+    let attempts = 0
+
+    // Why: mermaid.render() is async, so the SVG may not exist on the first
+    // frame. Retry up to 30 frames (~500ms) to match MarkdownPreview's pattern.
+    const tryRestore = (): void => {
+      const maxScrollTop = Math.max(0, container.scrollHeight - container.clientHeight)
+      const nextScrollTop = Math.min(targetScrollTop, maxScrollTop)
+      container.scrollTop = nextScrollTop
+
+      if (Math.abs(container.scrollTop - targetScrollTop) <= 1 || maxScrollTop >= targetScrollTop) {
+        return
+      }
+
+      attempts += 1
+      if (attempts < 30) {
+        frameId = window.requestAnimationFrame(tryRestore)
+      }
+    }
+
+    tryRestore()
+    return () => window.cancelAnimationFrame(frameId)
+  }, [scrollCacheKey, content])
+
+  return (
+    <div
+      ref={rootRef}
+      className="mermaid-viewer h-full min-h-0 overflow-auto scrollbar-editor"
+    >
+      <div className="mermaid-viewer-canvas">
+        <MermaidBlock content={content.trim()} isDark={isDark} />
+      </div>
+    </div>
+  )
+}
+```
+
+- [ ] **Step 2: Verify it compiles**
+
+Run: `cd src/renderer && npx tsc --noEmit --pretty 2>&1 | head -20`
+Expected: No errors.
+
+- [ ] **Step 3: Commit**
+
+```bash
+git add src/renderer/src/components/editor/MermaidViewer.tsx
+git commit -m "feat: add MermaidViewer component for .mmd file rendering"
+```
+
+---
+
+### Task 3: Add `.mermaid-viewer` styles
+
+**Files:**
+- Modify: `src/renderer/src/assets/markdown-preview.css` (append after existing `.mermaid-error` block, around line 242)
+
+- [ ] **Step 1: Add viewer styles**
+
+Append after the `.mermaid-error` rule block (line ~242) in `markdown-preview.css`:
+
+```css
+/* Full-file mermaid diagram viewer — used when opening .mmd/.mermaid files
+   in diagram mode. Centers the rendered SVG and adds padding so diagrams
+   don't press against viewport edges. */
+.mermaid-viewer {
+  background: var(--color-background, #fff);
+}
+.mermaid-viewer-canvas {
+  display: flex;
+  align-items: flex-start;
+  justify-content: center;
+  min-height: 100%;
+  padding: 32px 24px;
+}
+.mermaid-viewer-canvas .mermaid-block {
+  max-width: 100%;
+}
+.mermaid-viewer-canvas .mermaid-block svg {
+  max-width: 100%;
+  height: auto;
+}
+```
+
+- [ ] **Step 2: Commit**
+
+```bash
+git add src/renderer/src/assets/markdown-preview.css
+git commit -m "feat: add mermaid-viewer CSS for full-file diagram display"
+```
+
+---
+
+### Task 4: Wire mermaid files into `EditorPanel` and `EditorContent`
+
+**Files:**
+- Modify: `src/renderer/src/components/editor/EditorPanel.tsx:427-519`
+- Modify: `src/renderer/src/components/editor/EditorContent.tsx:1-67,262-270`
+
+This task threads the `isMermaid` flag through the same path as `isMarkdown`, reusing the existing `markdownViewMode` store (keyed by file ID, so no collision) and `MarkdownViewToggle` (icon-only, works for both).
+
+- [ ] **Step 1: Update `EditorPanel.tsx`**
+
+**Change 1** — add `isMermaid` flag and widen the view-mode condition (around line 427):
+
+Replace:
+```typescript
+  const isMarkdown = resolvedLanguage === 'markdown'
+  const mdViewMode: MarkdownViewMode =
+    isMarkdown && activeFile.mode === 'edit'
+      ? (markdownViewMode[activeFile.id] ?? 'rich')
+      : 'source'
+```
+
+With:
+```typescript
+  const isMarkdown = resolvedLanguage === 'markdown'
+  const isMermaid = resolvedLanguage === 'mermaid'
+  // Why: mermaid files reuse the same per-file view mode store as markdown.
+  // Both default to 'rich' (rendered view) and fall back to 'source' (Monaco).
+  const hasViewModeToggle = (isMarkdown || isMermaid) && activeFile.mode === 'edit'
+  const mdViewMode: MarkdownViewMode = hasViewModeToggle
+    ? (markdownViewMode[activeFile.id] ?? 'rich')
+    : 'source'
+```
+
+**Change 2** — update the toggle condition in the header (around line 515):
+
+Replace:
+```tsx
+          {isMarkdown && activeFile.mode === 'edit' && (
+            <MarkdownViewToggle
+```
+
+With:
+```tsx
+          {hasViewModeToggle && (
+            <MarkdownViewToggle
+```
+
+**Change 3** — pass `isMermaid` to `EditorContent` (around line 531):
+
+Add `isMermaid={isMermaid}` after the `isMarkdown` prop:
+
+```tsx
+          isMarkdown={isMarkdown}
+          isMermaid={isMermaid}
+```
+
+- [ ] **Step 2: Update `EditorContent.tsx`**
+
+**Change 1** — add lazy import for `MermaidViewer` (after the `ImageDiffViewer` lazy import, around line 18):
+
+```typescript
+const MermaidViewer = lazy(() => import('./MermaidViewer'))
+```
+
+**Change 2** — add `isMermaid` to the component props type and destructuring (around line 34-67):
+
+Add `isMermaid: boolean` to the props type (after `isMarkdown: boolean`), and add `isMermaid` to the destructuring.
+
+**Change 3** — add mermaid routing in the edit-mode branch (around line 266):
+
+Replace:
+```tsx
+          {isMarkdown ? renderMarkdownContent(fc) : renderMonacoEditor(fc)}
+```
+
+With:
+```tsx
+          {isMarkdown
+            ? renderMarkdownContent(fc)
+            : isMermaid && mdViewMode === 'rich'
+              ? <MermaidViewer
+                  key={activeFile.id}
+                  content={editBuffers[activeFile.id] ?? fc.content}
+                  filePath={activeFile.filePath}
+                />
+              : renderMonacoEditor(fc)}
+```
+
+- [ ] **Step 3: Verify everything compiles**
+
+Run: `cd src/renderer && npx tsc --noEmit --pretty 2>&1 | head -20`
+Expected: No errors.
+
+- [ ] **Step 4: Commit**
+
+```bash
+git add src/renderer/src/components/editor/EditorPanel.tsx src/renderer/src/components/editor/EditorContent.tsx
+git commit -m "feat: wire mermaid file viewer into editor panel routing"
+```
+
+---
+
+### Task 5: Manual verification
+
+- [ ] **Step 1: Create a test `.mmd` file in any worktree**
+
+Place a file like `test-diagram.mmd` in a worktree directory with content:
+
+```
+graph TD
+    A[Open .mmd file] --> B{Rendered?}
+    B -->|Yes| C[Diagram view]
+    B -->|No| D[Source view]
+    C --> E[Toggle to source]
+    D --> F[Toggle to diagram]
+```
+
+- [ ] **Step 2: Open the file in Orca and verify**
+
+Check:
+1. File opens in diagram mode by default (rendered SVG, centered)
+2. Source/diagram toggle appears in the editor header
+3. Clicking the Code icon switches to Monaco with mermaid syntax
+4. Clicking the Eye icon switches back to the rendered diagram
+5. Dark mode: diagram respects the current theme
+6. Large diagrams scroll properly
+7. Scroll position is preserved when switching tabs and back
+
+- [ ] **Step 3: Verify no regressions**
+
+Check:
+1. Opening a `.md` file still works with source/rich toggle
+2. Mermaid fenced blocks inside `.md` files still render
+3. Non-markdown, non-mermaid files open normally in Monaco
+
+- [ ] **Step 4: Clean up the test file and commit everything**
+
+```bash
+rm test-diagram.mmd
+```
diff --git a/src/renderer/src/assets/markdown-preview.css b/src/renderer/src/assets/markdown-preview.css
index abc8fec7..f2b729c5 100644
--- a/src/renderer/src/assets/markdown-preview.css
+++ b/src/renderer/src/assets/markdown-preview.css
@@ -240,6 +240,26 @@
   font-size: 0.85em;
   margin-bottom: 0.5em;
 }
+/* Full-file mermaid diagram viewer — used when opening .mmd/.mermaid files
+   in diagram mode. Centers the rendered SVG and adds padding so diagrams
+   don't press against viewport edges. */
+.mermaid-viewer {
+  background: var(--color-background, transparent);
+}
+.mermaid-viewer-canvas {
+  display: flex;
+  align-items: flex-start;
+  justify-content: center;
+  min-height: 100%;
+  padding: 32px 24px;
+}
+.mermaid-viewer-canvas .mermaid-block {
+  width: 100%;
+}
+.mermaid-viewer-canvas .mermaid-block svg {
+  width: 100%;
+  height: auto;
+}
 
 .markdown-body ul,
 .markdown-body ol {
diff --git a/src/renderer/src/components/editor/EditorContent.tsx b/src/renderer/src/components/editor/EditorContent.tsx
index f094de96..2ef84d4b 100644
--- a/src/renderer/src/components/editor/EditorContent.tsx
+++ b/src/renderer/src/components/editor/EditorContent.tsx
@@ -16,6 +16,7 @@ const RichMarkdownEditor = lazy(() => import('./RichMarkdownEditor'))
 const MarkdownPreview = lazy(() => import('./MarkdownPreview'))
 const ImageViewer = lazy(() => import('./ImageViewer'))
 const ImageDiffViewer = lazy(() => import('./ImageDiffViewer'))
+const MermaidViewer = lazy(() => import('./MermaidViewer'))
 
 const richMarkdownSizeEncoder = new TextEncoder()
 // Why: encodeInto() with a pre-allocated buffer avoids creating a new
@@ -40,6 +41,7 @@ export function EditorContent({
   worktreeEntries,
   resolvedLanguage,
   isMarkdown,
+  isMermaid,
   mdViewMode,
   sideBySide,
   pendingEditorReveal,
@@ -55,6 +57,7 @@ export function EditorContent({
   worktreeEntries: GitStatusEntry[]
   resolvedLanguage: string
   isMarkdown: boolean
+  isMermaid: boolean
   mdViewMode: MarkdownViewMode
   sideBySide: boolean
   pendingEditorReveal: {
@@ -282,7 +285,17 @@ export function EditorContent({
       <div className="flex flex-1 min-h-0 flex-col">
         {activeFile.conflict && <ConflictBanner file={activeFile} entry={activeConflictEntry} />}
         <div className="min-h-0 flex-1 relative">
-          {isMarkdown ? renderMarkdownContent(fc) : renderMonacoEditor(fc)}
+          {isMarkdown ? (
+            renderMarkdownContent(fc)
+          ) : isMermaid && mdViewMode === 'rich' ? (
+            <MermaidViewer
+              key={activeFile.id}
+              content={editBuffers[activeFile.id] ?? fc.content}
+              filePath={activeFile.filePath}
+            />
+          ) : (
+            renderMonacoEditor(fc)
+          )}
         </div>
       </div>
     )
diff --git a/src/renderer/src/components/editor/EditorPanel.tsx b/src/renderer/src/components/editor/EditorPanel.tsx
index 1a8a5ce0..1dcb4ce5 100644
--- a/src/renderer/src/components/editor/EditorPanel.tsx
+++ b/src/renderer/src/components/editor/EditorPanel.tsx
@@ -151,6 +151,10 @@ function EditorPanelInner({
             // last active.
             scrollTopCache.delete(`${prevFile.filePath}:rich`)
             scrollTopCache.delete(`${prevFile.filePath}:preview`)
+            // Why: mermaid files use a mode-scoped cache key just like markdown.
+            // Without this, a reopened .mmd file would restore a stale scroll
+            // position from the previous session even if the content changed.
+            scrollTopCache.delete(`${prevFile.filePath}:mermaid-diagram`)
             cursorPositionCache.delete(prevFile.filePath)
             deleteCacheEntriesByPrefix(cursorPositionCache, `${prevFile.filePath}::`)
             break
@@ -599,10 +603,13 @@ function EditorPanelInner({
   )
 
   const isMarkdown = resolvedLanguage === 'markdown'
-  const mdViewMode: MarkdownViewMode =
-    isMarkdown && activeFile.mode === 'edit'
-      ? (markdownViewMode[activeFile.id] ?? 'rich')
-      : 'source'
+  const isMermaid = resolvedLanguage === 'mermaid'
+  // Why: mermaid files reuse the same per-file view mode store as markdown.
+  // Both default to 'rich' (rendered view) and fall back to 'source' (Monaco).
+  const hasViewModeToggle = (isMarkdown || isMermaid) && activeFile.mode === 'edit'
+  const mdViewMode: MarkdownViewMode = hasViewModeToggle
+    ? (markdownViewMode[activeFile.id] ?? 'rich')
+    : 'source'
 
   const handleOpenDiffTargetFile = (): void => {
     if (!openFileState.canOpen) {
@@ -731,7 +738,7 @@ function EditorPanelInner({
               </Tooltip>
             </TooltipProvider>
           )}
-          {isMarkdown && activeFile.mode === 'edit' && (
+          {hasViewModeToggle && (
             <MarkdownViewToggle
               mode={mdViewMode}
               onChange={(mode) => setMarkdownViewMode(activeFile.id, mode)}
@@ -749,6 +756,7 @@ function EditorPanelInner({
           worktreeEntries={worktreeEntries}
           resolvedLanguage={resolvedLanguage}
           isMarkdown={isMarkdown}
+          isMermaid={isMermaid}
           mdViewMode={mdViewMode}
           sideBySide={sideBySide}
           pendingEditorReveal={pendingEditorReveal}
diff --git a/src/renderer/src/components/editor/MermaidViewer.tsx b/src/renderer/src/components/editor/MermaidViewer.tsx
new file mode 100644
index 00000000..722a8046
--- /dev/null
+++ b/src/renderer/src/components/editor/MermaidViewer.tsx
@@ -0,0 +1,104 @@
+import React, { useLayoutEffect, useRef } from 'react'
+import { useAppStore } from '@/store'
+import { scrollTopCache, setWithLRU } from '@/lib/scroll-cache'
+import MermaidBlock from './MermaidBlock'
+
+type MermaidViewerProps = {
+  content: string
+  filePath: string
+}
+
+// Why: MermaidViewer is the full-file counterpart to MermaidBlock (which
+// renders fenced mermaid blocks inside markdown). When a user opens a .mmd
+// or .mermaid file in diagram mode, the entire file content is the diagram
+// source — no markdown wrapper, no frontmatter, just mermaid syntax.
+export default function MermaidViewer({
+  content,
+  filePath
+}: MermaidViewerProps): React.JSX.Element {
+  const rootRef = useRef<HTMLDivElement>(null)
+  const settings = useAppStore((s) => s.settings)
+  const isDark =
+    settings?.theme === 'dark' ||
+    (settings?.theme === 'system' && window.matchMedia('(prefers-color-scheme: dark)').matches)
+
+  // Why: Each viewing mode (source vs diagram) produces different DOM heights.
+  // Mode-scoped keys prevent restoring a source-mode scroll position in diagram
+  // mode (same reasoning as MarkdownPreview's scrollCacheKey).
+  const scrollCacheKey = `${filePath}:mermaid-diagram`
+
+  useLayoutEffect(() => {
+    const container = rootRef.current
+    if (!container) {
+      return
+    }
+
+    let throttleTimer: ReturnType<typeof setTimeout> | null = null
+
+    const onScroll = (): void => {
+      if (throttleTimer !== null) {
+        clearTimeout(throttleTimer)
+      }
+      throttleTimer = setTimeout(() => {
+        setWithLRU(scrollTopCache, scrollCacheKey, container.scrollTop)
+        throttleTimer = null
+      }, 150)
+    }
+
+    container.addEventListener('scroll', onScroll, { passive: true })
+    return () => {
+      // Why: guard against writing 0 when the SVG has not rendered yet (e.g.,
+      // StrictMode double-mount or quick tab switch before mermaid.render()
+      // completes). Without this, a valid cached position gets clobbered.
+      if (container.scrollHeight > container.clientHeight || container.scrollTop > 0) {
+        setWithLRU(scrollTopCache, scrollCacheKey, container.scrollTop)
+      }
+      if (throttleTimer !== null) {
+        clearTimeout(throttleTimer)
+      }
+      container.removeEventListener('scroll', onScroll)
+    }
+  }, [scrollCacheKey])
+
+  useLayoutEffect(() => {
+    const container = rootRef.current
+    const targetScrollTop = scrollTopCache.get(scrollCacheKey)
+    if (!container || targetScrollTop === undefined) {
+      return
+    }
+
+    let frameId = 0
+    let attempts = 0
+
+    // Why: mermaid.render() is async, so the SVG may not exist on the first
+    // frame. Retry up to 30 frames (~500ms) to match MarkdownPreview's pattern.
+    const tryRestore = (): void => {
+      const maxScrollTop = Math.max(0, container.scrollHeight - container.clientHeight)
+      const nextScrollTop = Math.min(targetScrollTop, maxScrollTop)
+      container.scrollTop = nextScrollTop
+
+      if (Math.abs(container.scrollTop - targetScrollTop) <= 1 || maxScrollTop >= targetScrollTop) {
+        return
+      }
+
+      attempts += 1
+      if (attempts < 30) {
+        frameId = window.requestAnimationFrame(tryRestore)
+      }
+    }
+
+    tryRestore()
+    return () => window.cancelAnimationFrame(frameId)
+  }, [scrollCacheKey, content])
+
+  return (
+    <div ref={rootRef} className="mermaid-viewer h-full min-h-0 overflow-auto scrollbar-editor">
+      <div className="mermaid-viewer-canvas">
+        {/* Why: DOMPurify's SVG profile strips <foreignObject> elements that
+           mermaid uses for HTML labels. Force SVG-native <text> labels so
+           they survive sanitization — same fix as the markdown preview path. */}
+        <MermaidBlock content={content.trim()} isDark={isDark} htmlLabels={false} />
+      </div>
+    </div>
+  )
+}
diff --git a/src/renderer/src/lib/language-detect.ts b/src/renderer/src/lib/language-detect.ts
index 3dd1827e..543b2f4b 100644
--- a/src/renderer/src/lib/language-detect.ts
+++ b/src/renderer/src/lib/language-detect.ts
@@ -18,6 +18,8 @@ const EXT_TO_LANGUAGE: Record<string, string> = {
   '.jsonc': 'json',
   '.md': 'markdown',
   '.mdx': 'markdown',
+  '.mmd': 'mermaid',
+  '.mermaid': 'mermaid',
   '.css': 'css',
   '.scss': 'scss',
   '.less': 'less',