diff --git a/specs/smart-fit.md b/specs/smart-fit.md
new file mode 100644
index 0000000..9f9a575
--- /dev/null
+++ b/specs/smart-fit.md
@@ -0,0 +1,37 @@
+# Smart Fit — Technical Spec
+
+## Problem
+
+Default `cover` fit crops media when its aspect ratio differs from the project canvas (e.g. 16:9 photo in a 9:16 project). Users expect the full image to remain visible unless they explicitly choose crop.
+
+## Behavior
+
+- **`smart-fit`** becomes the default global `fitMode` (themes included).
+- At plan time, `smart-fit` resolves to a concrete fit per slide:
+  - **Orientation mismatch** (landscape ↔ portrait) or **square ↔ non-square** → `blur-fill` (full media visible, blurred background).
+  - **Compatible orientation** (both landscape, both portrait, or both square) → `cover` (Ken Burns-friendly crop-fill).
+  - **Missing media dimensions** → `contain` (safe fallback, no crop).
+- **Videos** remain always `contain` (unchanged).
+- **Per-slide overrides** (`cover`, `contain`, `blur-fill`, `smart-fit`) still win via the settings cascade.
+- **Persisted `fitMode: 'cover'`** in existing projects is unchanged until the user switches.
+
+## Architecture
+
+| Layer | Responsibility |
+| --- | --- |
+| `timeline-core/smartFit.ts` | Pure `resolveSmartFit(mediaW, mediaH, canvasW, canvasH)` |
+| `timeline-core/settings.ts` | Add `'smart-fit'` to `FitMode`, default + themes |
+| `project-store/media-loader.ts` | Extract `width`/`height` at import (image bitmap, video track) |
+| `sequence-planner/planner.ts` | Accept `aspectRatio`, resolve `smart-fit` using slide dims + canvas |
+| `composition/` | No changes — receives resolved fit modes only |
+| Editor thumbnails | `object-contain` + intrinsic `aspect-ratio` from slide dimensions (fallback 16:9) |
+
+## Thumbnails
+
+- Timeline blocks: media centered with `object-contain` inside the proportional-width block (full image visible, letterboxed in block).
+- Filmstrip cards: container `aspect-ratio` derived from slide `width`/`height`; falls back to 16:9 when unknown.
+
+## Tests
+
+- `smartFit.test.ts`: orientation pairs, square cases, missing/zero dimensions.
+- `planner.test.ts`: smart-fit resolution with metadata + aspect ratio; overrides still win.
diff --git a/src/editor-shell/App.tsx b/src/editor-shell/App.tsx
index c1272be..193d254 100644
--- a/src/editor-shell/App.tsx
+++ b/src/editor-shell/App.tsx
@@ -148,8 +148,9 @@ export function App() {
       planAudioClips.length > 0 ? planAudioClips : undefined,
       beatGrid.effectiveBeatGrid,
       planBeatTimes,
+      aspectRatio,
     ),
-    [beatGrid.effectiveBeatGrid, deferredGlobalSettings, deferredSlides, planAudioClips, planBeatTimes],
+    [aspectRatio, beatGrid.effectiveBeatGrid, deferredGlobalSettings, deferredSlides, planAudioClips, planBeatTimes],
   )
   const totalFrames = renderPlan.totalFrames > 0 ? renderPlan.totalFrames : FPS
   const canvas = dimensionsForAspectRatio(aspectRatio)
diff --git a/src/editor-shell/GlobalSettingsPanel.tsx b/src/editor-shell/GlobalSettingsPanel.tsx
index 7de8407..267fa28 100644
--- a/src/editor-shell/GlobalSettingsPanel.tsx
+++ b/src/editor-shell/GlobalSettingsPanel.tsx
@@ -170,6 +170,7 @@ export function GlobalSettingsPanel({ aspectRatio, onAspectRatioChange, settings
             <SelectValue />
           </SelectTrigger>
           <SelectContent>
+            <SelectItem value="smart-fit">Smart fit</SelectItem>
             <SelectItem value="cover">Cover (crop)</SelectItem>
             <SelectItem value="contain">Letterbox</SelectItem>
             <SelectItem value="blur-fill">Blur fill</SelectItem>
diff --git a/src/editor-shell/SlideSettingsDialog.tsx b/src/editor-shell/SlideSettingsDialog.tsx
index 5f813cc..cec501a 100644
--- a/src/editor-shell/SlideSettingsDialog.tsx
+++ b/src/editor-shell/SlideSettingsDialog.tsx
@@ -137,6 +137,7 @@ export function SlideSettingsDialog({ globalSettings, onClose, onOverride, slide
                   <SelectValue />
                 </SelectTrigger>
                 <SelectContent>
+                  <SelectItem value="smart-fit">Smart fit</SelectItem>
                   <SelectItem value="cover">Cover (crop)</SelectItem>
                   <SelectItem value="contain">Letterbox</SelectItem>
                   <SelectItem value="blur-fill">Blur fill</SelectItem>
diff --git a/src/editor-shell/StoryboardFilmstrip.tsx b/src/editor-shell/StoryboardFilmstrip.tsx
index 2f96012..410ac3d 100644
--- a/src/editor-shell/StoryboardFilmstrip.tsx
+++ b/src/editor-shell/StoryboardFilmstrip.tsx
@@ -1,6 +1,7 @@
 import { useRef } from 'react'
 import { ScrollArea, ScrollBar } from '@/components/ui/scroll-area'
 import { cn } from '@/lib/utils'
+import { mediaAspectRatioCss } from '../timeline-core/aspect'
 import type { Slide } from '../timeline-core/types'
 import { isTitleSlide } from '../timeline-core/types'
 
@@ -64,7 +65,14 @@ export function StoryboardFilmstrip({
               onDragEnd={() => { dragIndexRef.current = null }}
               onClick={() => onSlideClick(slide.id)}
             >
-              <div className="relative aspect-video w-full overflow-hidden rounded-sm">
+              <div
+                className="relative flex w-full items-center justify-center overflow-hidden rounded-sm bg-black"
+                style={{
+                  aspectRatio: isTitleSlide(slide)
+                    ? '16 / 9'
+                    : mediaAspectRatioCss(slide.width, slide.height),
+                }}
+              >
                 {isTitleSlide(slide) ? (
                   <div
                     className="flex h-full w-full items-center justify-center px-1 text-xs font-medium"
@@ -77,17 +85,17 @@ export function StoryboardFilmstrip({
                   </div>
                 ) : slide.type === 'video' ? (
                   <video
-                    src={slide.blobUrl}
-                    className="h-full w-full object-cover"
-                    muted
+                    className="max-h-full max-w-full object-contain"
                     draggable={false}
+                    muted
+                    src={slide.blobUrl}
                   />
                 ) : (
                   <img
-                    src={slide.blobUrl}
                     alt={slide.filename}
-                    className="h-full w-full object-cover"
+                    className="max-h-full max-w-full object-contain"
                     draggable={false}
+                    src={slide.blobUrl}
                   />
                 )}
                 {!isTitleSlide(slide) && slide.type === 'video' && (
diff --git a/src/editor-shell/TimelineMediaBlock.tsx b/src/editor-shell/TimelineMediaBlock.tsx
index b3db102..7e33ba8 100644
--- a/src/editor-shell/TimelineMediaBlock.tsx
+++ b/src/editor-shell/TimelineMediaBlock.tsx
@@ -60,7 +60,7 @@ export function TimelineMediaBlock({
       }}
       style={{ left: leftPx, width: widthPx }}
     >
-      <div className="relative min-h-0 flex-1 overflow-hidden rounded-sm">
+      <div className="relative flex min-h-0 flex-1 items-center justify-center overflow-hidden rounded-sm bg-black">
         {isTitleSlide(slide) ? (
           <div
             className="flex h-full w-full items-center justify-center px-1 text-xs font-medium"
@@ -73,7 +73,7 @@ export function TimelineMediaBlock({
           </div>
         ) : slide.type === 'video' ? (
           <video
-            className="h-full w-full object-cover"
+            className="max-h-full max-w-full object-contain"
             draggable={false}
             muted
             src={slide.blobUrl}
@@ -81,7 +81,7 @@ export function TimelineMediaBlock({
         ) : (
           <img
             alt={slide.filename}
-            className="h-full w-full object-cover"
+            className="max-h-full max-w-full object-contain"
             draggable={false}
             src={slide.blobUrl}
           />
diff --git a/src/project-store/media-loader.ts b/src/project-store/media-loader.ts
index c77872a..9fc23a2 100644
--- a/src/project-store/media-loader.ts
+++ b/src/project-store/media-loader.ts
@@ -25,6 +25,31 @@ async function getVideoDurationFrames(file: File): Promise<number> {
   }
 }
 
+async function getImageDimensions(file: File): Promise<{ height: number; width: number } | undefined> {
+  try {
+    const bitmap = await createImageBitmap(file)
+    const dimensions = { height: bitmap.height, width: bitmap.width }
+    bitmap.close()
+    return dimensions
+  } catch {
+    return undefined
+  }
+}
+
+async function getVideoDimensions(file: File): Promise<{ height: number; width: number } | undefined> {
+  try {
+    const input = new Input({
+      formats: ALL_FORMATS,
+      source: new BlobSource(file),
+    })
+    const videoTrack = await input.getPrimaryVideoTrack()
+    if (!videoTrack) return undefined
+    return { height: videoTrack.displayHeight, width: videoTrack.displayWidth }
+  } catch {
+    return undefined
+  }
+}
+
 export async function createMediaSlideFromFile(file: File): Promise<MediaSlide> {
   const filename = file.name
   const type = getMediaType(filename)
@@ -33,12 +58,16 @@ export async function createMediaSlideFromFile(file: File): Promise<MediaSlide>
     type === 'video'
       ? await getVideoDurationFrames(file)
       : IMAGE_DURATION_FRAMES
+  const dimensions = type === 'video'
+    ? await getVideoDimensions(file)
+    : await getImageDimensions(file)
 
   return {
     blobUrl,
     durationInFrames,
     excluded: false,
     filename,
+    ...(dimensions ? { height: dimensions.height, width: dimensions.width } : {}),
     id: `${filename}-${file.lastModified}`,
     type,
   }
diff --git a/src/sequence-planner/planner.test.ts b/src/sequence-planner/planner.test.ts
index 0193dcb..20d38a0 100644
--- a/src/sequence-planner/planner.test.ts
+++ b/src/sequence-planner/planner.test.ts
@@ -261,6 +261,13 @@ describe('plan — Ken Burns vectors', () => {
 describe('plan — fit mode resolution', () => {
   const IMG = makeSlide('i', 'image', 90)
   const VID = makeSlide('v', 'video', 120)
+  const LANDSCAPE_IMG: MediaSlide = {
+    ...IMG,
+    filename: 'landscape.jpg',
+    height: 1080,
+    width: 1920,
+  }
+  const PORTRAIT_CANVAS = '9:16' as const
 
   it('videos always get fitMode=contain regardless of global setting', () => {
     const settings: GlobalSettings = { ...DEFAULT_GLOBAL_SETTINGS, fitMode: 'cover' }
@@ -269,7 +276,7 @@ describe('plan — fit mode resolution', () => {
     expect(result.entries[1].fitMode).toBe('contain')
   })
 
-  it('images inherit global fitMode', () => {
+  it('images inherit global fitMode when not smart-fit', () => {
     const cover: GlobalSettings = { ...DEFAULT_GLOBAL_SETTINGS, fitMode: 'cover' }
     const contain: GlobalSettings = { ...DEFAULT_GLOBAL_SETTINGS, fitMode: 'contain' }
     const blur: GlobalSettings = { ...DEFAULT_GLOBAL_SETTINGS, fitMode: 'blur-fill' }
@@ -278,6 +285,24 @@ describe('plan — fit mode resolution', () => {
     expect(plan([IMG], contain).entries[0].fitMode).toBe('contain')
     expect(plan([IMG], blur).entries[0].fitMode).toBe('blur-fill')
   })
+
+  it('smart-fit uses blur-fill for landscape media in a portrait canvas', () => {
+    const settings: GlobalSettings = { ...DEFAULT_GLOBAL_SETTINGS, fitMode: 'smart-fit' }
+    const result = plan([LANDSCAPE_IMG], settings, undefined, undefined, undefined, undefined, PORTRAIT_CANVAS)
+    expect(result.entries[0].fitMode).toBe('blur-fill')
+  })
+
+  it('smart-fit uses cover for matching landscape media in a landscape canvas', () => {
+    const settings: GlobalSettings = { ...DEFAULT_GLOBAL_SETTINGS, fitMode: 'smart-fit' }
+    const result = plan([LANDSCAPE_IMG], settings)
+    expect(result.entries[0].fitMode).toBe('cover')
+  })
+
+  it('smart-fit falls back to contain when media dimensions are unknown', () => {
+    const settings: GlobalSettings = { ...DEFAULT_GLOBAL_SETTINGS, fitMode: 'smart-fit' }
+    const result = plan([IMG], settings)
+    expect(result.entries[0].fitMode).toBe('contain')
+  })
 })
 
 // --- Per-slide overrides ---
@@ -326,7 +351,7 @@ describe('plan — per-slide overrides', () => {
   it('undefined override field falls back to global', () => {
     const img = { ...makeSlide('a', 'image', 90), overrides: { fitMode: undefined } }
     const result = plan([img], BASE)
-    expect(result.entries[0].fitMode).toBe('cover') // global default
+    expect(result.entries[0].fitMode).toBe('cover') // global default in BASE fixture
   })
 })
 
@@ -456,7 +481,11 @@ describe('plan — audio clips', () => {
 })
 
 describe('plan — audio-driven loop golden snapshot', () => {
-  const CUT_SETTINGS: GlobalSettings = { ...DEFAULT_GLOBAL_SETTINGS, transitionType: 'cut' }
+  const CUT_SETTINGS: GlobalSettings = {
+    ...DEFAULT_GLOBAL_SETTINGS,
+    fitMode: 'cover',
+    transitionType: 'cut',
+  }
   const LONG_AUDIO = [{ blobUrl: 'blob:long', durationInFrames: 250 }]
 
   it('matches golden snapshot for looped entries with a partial tail', () => {
diff --git a/src/sequence-planner/planner.ts b/src/sequence-planner/planner.ts
index fc7c6ba..3fff06d 100644
--- a/src/sequence-planner/planner.ts
+++ b/src/sequence-planner/planner.ts
@@ -1,5 +1,9 @@
 import { resolve } from '../timeline-core/settings'
 import type { FitMode, GlobalSettings, TransitionType } from '../timeline-core/settings'
+import type { AspectRatio } from '../timeline-core/aspect'
+import { dimensionsForAspectRatio } from '../timeline-core/aspect'
+import { resolveSmartFit } from '../timeline-core/smartFit'
+import type { ConcreteFitMode } from '../timeline-core/smartFit'
 import { isTitleSlide } from '../timeline-core/types'
 import type { Slide } from '../timeline-core/types'
 import type { BeatGrid } from '../beat-grid/types'
@@ -104,6 +108,7 @@ export function plan(
   audioClips?: AudioClipInput[],
   beatGrid?: BeatGrid,
   concatenatedBeatTimesSecs?: number[],
+  aspectRatio: AspectRatio = '16:9',
 ): RenderPlan {
   if (slides.length === 0) {
     const audioTotal = totalAudioFrames(audioClips)
@@ -146,9 +151,30 @@ export function plan(
     return raw
   }
 
-  function getFitMode(slide: Slide): FitMode {
+  const canvas = dimensionsForAspectRatio(aspectRatio)
+
+  function getMediaDimensions(slide: Slide): { height?: number; width?: number } {
+    if (isTitleSlide(slide)) return {}
+    const meta = mediaMetadata?.get(slide.filename)
+    return {
+      height: slide.height ?? meta?.height,
+      width: slide.width ?? meta?.width,
+    }
+  }
+
+  function resolveConcreteFitMode(slide: Slide, fitMode: FitMode): ConcreteFitMode {
+    if (fitMode !== 'smart-fit') return fitMode
+
+    const { height, width } = getMediaDimensions(slide)
+    if (width === undefined || height === undefined) return 'contain'
+
+    return resolveSmartFit(width, height, canvas.width, canvas.height)
+  }
+
+  function getFitMode(slide: Slide): ConcreteFitMode {
     if (isTitleSlide(slide)) return 'cover' // unused; TitleSlideView renders its own layout
-    return slide.type === 'video' ? 'contain' : resolved(slide).fitMode
+    if (slide.type === 'video') return 'contain'
+    return resolveConcreteFitMode(slide, resolved(slide).fitMode)
   }
 
   function getTransitionDur(slide: Slide): number {
diff --git a/src/sequence-planner/types.ts b/src/sequence-planner/types.ts
index 0f82097..4144ec6 100644
--- a/src/sequence-planner/types.ts
+++ b/src/sequence-planner/types.ts
@@ -1,4 +1,5 @@
-import type { FitMode, TransitionType } from '../timeline-core/settings'
+import type { TransitionType } from '../timeline-core/settings'
+import type { ConcreteFitMode } from '../timeline-core/smartFit'
 import type { Slide } from '../timeline-core/types'
 
 export type TransitionSpec = {
@@ -37,7 +38,7 @@ export type RenderPlanEntry = {
   startFrame: number
   durationInFrames: number
   transitionIn?: TransitionSpec
-  fitMode: FitMode
+  fitMode: ConcreteFitMode
   kenBurns: KenBurnsVector | null
   videoVolume: number
 }
@@ -65,4 +66,6 @@ export type RenderPlan = {
 
 export type MediaMetadata = {
   durationInFrames?: number
+  height?: number
+  width?: number
 }
diff --git a/src/timeline-core/aspect.test.ts b/src/timeline-core/aspect.test.ts
index d67bc9e..3ac5734 100644
--- a/src/timeline-core/aspect.test.ts
+++ b/src/timeline-core/aspect.test.ts
@@ -1,5 +1,11 @@
 import { describe, it, expect } from 'vitest'
-import { DEFAULT_ASPECT_RATIO, dimensionsForAspectRatio, isAspectRatio } from './aspect'
+import {
+  DEFAULT_ASPECT_RATIO,
+  DEFAULT_MEDIA_ASPECT_RATIO_CSS,
+  dimensionsForAspectRatio,
+  isAspectRatio,
+  mediaAspectRatioCss,
+} from './aspect'
 
 describe('dimensionsForAspectRatio', () => {
   it('16:9 is a 1920×1080 landscape canvas', () => {
@@ -36,3 +42,14 @@ describe('DEFAULT_ASPECT_RATIO', () => {
     expect(dimensionsForAspectRatio(DEFAULT_ASPECT_RATIO)).toEqual({ width: 1920, height: 1080 })
   })
 })
+
+describe('mediaAspectRatioCss', () => {
+  it('returns width / height when dimensions are known', () => {
+    expect(mediaAspectRatioCss(1080, 1920)).toBe('1080 / 1920')
+  })
+
+  it('falls back to 16 / 9 when dimensions are missing or invalid', () => {
+    expect(mediaAspectRatioCss()).toBe(DEFAULT_MEDIA_ASPECT_RATIO_CSS)
+    expect(mediaAspectRatioCss(0, 1080)).toBe(DEFAULT_MEDIA_ASPECT_RATIO_CSS)
+  })
+})
diff --git a/src/timeline-core/aspect.ts b/src/timeline-core/aspect.ts
index 75dbd0f..bebb76e 100644
--- a/src/timeline-core/aspect.ts
+++ b/src/timeline-core/aspect.ts
@@ -14,6 +14,15 @@ export function isAspectRatio(value: unknown): value is AspectRatio {
   return typeof value === 'string' && (ASPECT_RATIOS as readonly string[]).includes(value)
 }
 
+export const DEFAULT_MEDIA_ASPECT_RATIO_CSS = '16 / 9'
+
+export function mediaAspectRatioCss(width?: number, height?: number): string {
+  if (width !== undefined && height !== undefined && width > 0 && height > 0) {
+    return `${width} / ${height}`
+  }
+  return DEFAULT_MEDIA_ASPECT_RATIO_CSS
+}
+
 export function dimensionsForAspectRatio(ratio: AspectRatio): CanvasDimensions {
   switch (ratio) {
     case '16:9':
diff --git a/src/timeline-core/index.ts b/src/timeline-core/index.ts
index 641b8a9..fb228b7 100644
--- a/src/timeline-core/index.ts
+++ b/src/timeline-core/index.ts
@@ -1,4 +1,4 @@
-export { ASPECT_RATIOS, DEFAULT_ASPECT_RATIO, dimensionsForAspectRatio, isAspectRatio } from './aspect'
+export { ASPECT_RATIOS, DEFAULT_ASPECT_RATIO, DEFAULT_MEDIA_ASPECT_RATIO_CSS, dimensionsForAspectRatio, isAspectRatio, mediaAspectRatioCss } from './aspect'
 export type { AspectRatio, CanvasDimensions } from './aspect'
 export { getMediaType, isSupportedAudio, isSupportedMedia, sortByFilename } from './media'
 export { addAudioClip, moveAudioClip, removeAudioClip, updateAudioClipGain } from './audioClips'
@@ -16,3 +16,5 @@ export type {
 } from './settings'
 export type { AudioClip, MediaSlide, TitleSlide, Slide, MediaType } from './types'
 export { isTitleSlide } from './types'
+export { resolveSmartFit } from './smartFit'
+export type { ConcreteFitMode } from './smartFit'
diff --git a/src/timeline-core/settings.ts b/src/timeline-core/settings.ts
index b07f50b..39dd1c2 100644
--- a/src/timeline-core/settings.ts
+++ b/src/timeline-core/settings.ts
@@ -2,7 +2,7 @@ import { isTitleSlide } from './types'
 import type { Slide } from './types'
 
 export type TransitionType = 'crossfade' | 'dip-to-black' | 'cut'
-export type FitMode = 'cover' | 'contain' | 'blur-fill'
+export type FitMode = 'blur-fill' | 'contain' | 'cover' | 'smart-fit'
 export type KenBurnsMode = 'alternate' | 'zoom-in-only'
 export type ThemeName = 'classic' | 'energetic' | 'plain'
 export type Energy = 'calm' | 'medium' | 'punchy'
@@ -32,7 +32,7 @@ export const DEFAULT_GLOBAL_SETTINGS: GlobalSettings = {
   imageDurationSecs: 3,
   transitionType: 'crossfade',
   kenBurns: true,
-  fitMode: 'cover',
+  fitMode: 'smart-fit',
 }
 
 const FPS = 30
@@ -54,20 +54,20 @@ export const THEMES: Record<ThemeName, GlobalSettings> = {
     imageDurationSecs: 4,
     transitionType: 'crossfade',
     kenBurns: true,
-    fitMode: 'cover',
+    fitMode: 'smart-fit',
   },
   energetic: {
     imageDurationSecs: 2,
     transitionType: 'cut',
     kenBurns: true,
-    fitMode: 'cover',
+    fitMode: 'smart-fit',
     kenBurnsMode: 'zoom-in-only',
   },
   plain: {
     imageDurationSecs: 5,
     transitionType: 'cut',
     kenBurns: false,
-    fitMode: 'cover',
+    fitMode: 'smart-fit',
   },
 }
 
diff --git a/src/timeline-core/smartFit.test.ts b/src/timeline-core/smartFit.test.ts
new file mode 100644
index 0000000..437c23d
--- /dev/null
+++ b/src/timeline-core/smartFit.test.ts
@@ -0,0 +1,76 @@
+import { describe, expect, it } from 'vitest'
+import { resolveSmartFit } from './smartFit'
+
+const LANDSCAPE_16_9 = { height: 1080, width: 1920 }
+const PORTRAIT_9_16 = { height: 1920, width: 1080 }
+const SQUARE = { height: 1080, width: 1080 }
+
+describe('resolveSmartFit', () => {
+  it('uses cover when media and canvas share landscape orientation', () => {
+    expect(resolveSmartFit(
+      LANDSCAPE_16_9.width,
+      LANDSCAPE_16_9.height,
+      LANDSCAPE_16_9.width,
+      LANDSCAPE_16_9.height,
+    )).toBe('cover')
+  })
+
+  it('uses cover when media and canvas share portrait orientation', () => {
+    expect(resolveSmartFit(
+      PORTRAIT_9_16.width,
+      PORTRAIT_9_16.height,
+      PORTRAIT_9_16.width,
+      PORTRAIT_9_16.height,
+    )).toBe('cover')
+  })
+
+  it('uses blur-fill for landscape media in a portrait canvas', () => {
+    expect(resolveSmartFit(
+      LANDSCAPE_16_9.width,
+      LANDSCAPE_16_9.height,
+      PORTRAIT_9_16.width,
+      PORTRAIT_9_16.height,
+    )).toBe('blur-fill')
+  })
+
+  it('uses blur-fill for portrait media in a landscape canvas', () => {
+    expect(resolveSmartFit(
+      PORTRAIT_9_16.width,
+      PORTRAIT_9_16.height,
+      LANDSCAPE_16_9.width,
+      LANDSCAPE_16_9.height,
+    )).toBe('blur-fill')
+  })
+
+  it('uses blur-fill for square media in a landscape canvas', () => {
+    expect(resolveSmartFit(
+      SQUARE.width,
+      SQUARE.height,
+      LANDSCAPE_16_9.width,
+      LANDSCAPE_16_9.height,
+    )).toBe('blur-fill')
+  })
+
+  it('uses blur-fill for landscape media in a square canvas', () => {
+    expect(resolveSmartFit(
+      LANDSCAPE_16_9.width,
+      LANDSCAPE_16_9.height,
+      SQUARE.width,
+      SQUARE.height,
+    )).toBe('blur-fill')
+  })
+
+  it('uses cover for square media in a square canvas', () => {
+    expect(resolveSmartFit(
+      SQUARE.width,
+      SQUARE.height,
+      SQUARE.width,
+      SQUARE.height,
+    )).toBe('cover')
+  })
+
+  it('falls back to contain when dimensions are missing or zero', () => {
+    expect(resolveSmartFit(0, 1080, 1920, 1080)).toBe('contain')
+    expect(resolveSmartFit(1920, 0, 1920, 1080)).toBe('contain')
+  })
+})
diff --git a/src/timeline-core/smartFit.ts b/src/timeline-core/smartFit.ts
new file mode 100644
index 0000000..4af3cc7
--- /dev/null
+++ b/src/timeline-core/smartFit.ts
@@ -0,0 +1,46 @@
+import type { FitMode } from './settings'
+
+export type ConcreteFitMode = Exclude<FitMode, 'smart-fit'>
+
+const ORIENTATION_THRESHOLD = 0.05
+
+function isLandscape(aspect: number): boolean {
+  return aspect > 1 + ORIENTATION_THRESHOLD
+}
+
+function isPortrait(aspect: number): boolean {
+  return aspect < 1 - ORIENTATION_THRESHOLD
+}
+
+function isSquare(aspect: number): boolean {
+  return !isLandscape(aspect) && !isPortrait(aspect)
+}
+
+export function resolveSmartFit(
+  mediaWidth: number,
+  mediaHeight: number,
+  canvasWidth: number,
+  canvasHeight: number,
+): ConcreteFitMode {
+  if (mediaWidth <= 0 || mediaHeight <= 0 || canvasWidth <= 0 || canvasHeight <= 0) {
+    return 'contain'
+  }
+
+  const mediaAspect = mediaWidth / mediaHeight
+  const canvasAspect = canvasWidth / canvasHeight
+
+  const orientationMismatch =
+    (isLandscape(mediaAspect) && isPortrait(canvasAspect))
+    || (isPortrait(mediaAspect) && isLandscape(canvasAspect))
+
+  if (orientationMismatch) {
+    return 'blur-fill'
+  }
+
+  const shapeMismatch = isSquare(mediaAspect) !== isSquare(canvasAspect)
+  if (shapeMismatch) {
+    return 'blur-fill'
+  }
+
+  return 'cover'
+}
diff --git a/src/timeline-core/types.ts b/src/timeline-core/types.ts
index c7d47ed..d374cc8 100644
--- a/src/timeline-core/types.ts
+++ b/src/timeline-core/types.ts
@@ -13,8 +13,12 @@ export type MediaSlide = {
   durationInFrames: number
   /** excluded from timeline but still in folder */
   excluded: boolean
+  /** intrinsic pixel height when known */
+  height?: number
   /** per-slide setting overrides; absent means "use global defaults" */
   overrides?: SlideOverrides
+  /** intrinsic pixel width when known */
+  width?: number
 }
 
 export type TitleSlide = {