diff --git a/Cargo.lock b/Cargo.lock
index 6ea37277..34d31ff6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3629,6 +3629,16 @@ dependencies = [
  "vcpkg",
 ]
 
+[[package]]
+name = "libwebp-sys"
+version = "0.9.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "54cd30df7c7165ce74a456e4ca9732c603e8dc5e60784558c1c6dc047f876733"
+dependencies = [
+ "cc",
+ "glob",
+]
+
 [[package]]
 name = "linux-raw-sys"
 version = "0.4.15"
@@ -4286,6 +4296,7 @@ dependencies = [
  "urlencoding",
  "utoipa",
  "uuid",
+ "webp",
  "zip",
 ]
 
@@ -7342,6 +7353,16 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "webp"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c071456adef4aca59bf6a583c46b90ff5eb0b4f758fc347cea81290288f37ce1"
+dependencies = [
+ "image",
+ "libwebp-sys",
+]
+
 [[package]]
 name = "webpki-roots"
 version = "0.26.11"
diff --git a/Cargo.toml b/Cargo.toml
index 8508805a..a0203cb7 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -46,6 +46,10 @@ jpeg-decoder = "0.3"
 # SIMD (AVX2/SSE4.1/NEON) image resizing for thumbnails — far faster than the
 # `image` crate's scalar resampler, and it speeds every format (incl. PNG/WebP).
 fast_image_resize = "5"
+# Lossy WebP encoding for thumbnails. The `image` crate's webp feature is
+# lossless-only; this binds libwebp (vendored + built via `cc`, no system dep)
+# for quality-controlled lossy output (~25-30% smaller than JPEG at equal SSIM).
+webp = "0.3"
 id3 = "1.17"
 mp3-duration = "0.1"
 kamadak-exif = "0.6.1"
diff --git a/benches/WEBP.md b/benches/WEBP.md
new file mode 100644
index 00000000..b6c81435
--- /dev/null
+++ b/benches/WEBP.md
@@ -0,0 +1,84 @@
+# WebP thumbnails — codec comparison (bandwidth vs quality)
+
+WebP (lossy) is the **primary** thumbnail codec: generated eagerly on upload and
+served to the ~97% of clients that advertise `Accept: image/webp`. JPEG is the
+lazy fallback for older clients and NextCloud. This doc records the before/after
+that justified the change and the `WEBP_QUALITY` choice.
+
+## What it buys
+
+For "hundreds of photos", thumbnail **bytes on the wire** are the dominant cost.
+WebP at q82 ships visually-equivalent thumbnails (SSIM within ~0.005 of JPEG q80,
+imperceptible at thumbnail scale) for **~65% fewer bytes** on the bench corpus.
+
+> ⚠️ **Honest caveat — the corpus is smooth.** The synthetic corpus is now
+> photo-realistic (summed low-frequency sinusoids = smooth color fields + mild
+> grain), which compresses far more like a real photo than the old white-noise
+> corpus did. But it has **no hard edges / text / foliage** — exactly the
+> high-frequency content where both codecs grow and the WebP-vs-JPEG ratio
+> narrows toward the classic **~25–40%**. So treat ~65% as an upper bound and
+> ~25–40% as the realistic real-photo expectation. Drop real photos into
+> `benches/corpus/` (same filenames) to measure on real data.
+
+## Reproduce
+
+```bash
+cargo run --release --features bench --example bench_thumbnails_mem
+# Tables E1 (WebP quality sweep) and E2 (production codec) at the bottom.
+```
+
+SSIM is mean over non-overlapping 8×8 luma blocks vs the **uncompressed**
+full-decode source resized to the thumbnail's exact dims (`reference_luma_at`),
+so it isolates codec fidelity (no second lossy step). Note this 8×8 metric
+slightly favours JPEG's 8×8 DCT blocks, so WebP's SSIM reads a hair low.
+
+## E2 — production codec at `WEBP_QUALITY = 82` (14 cores)
+
+| case      | size    | jpeg B | webp B | save% | ssim jpeg | ssim webp |
+|-----------|---------|-------:|-------:|------:|----------:|----------:|
+| jpeg_12mp | Icon    |   4608 |   1966 | 57.3% |    0.9960 |    0.9916 |
+| jpeg_12mp | Preview |  17564 |   6614 | 62.3% |    0.9931 |    0.9871 |
+| jpeg_12mp | Large   |  47254 |  15580 | 67.0% |    0.9865 |    0.9808 |
+| jpeg_24mp | Icon    |   5518 |   2588 | 53.1% |    0.9970 |    0.9935 |
+| jpeg_24mp | Preview |  19294 |   7556 | 60.8% |    0.9949 |    0.9904 |
+| jpeg_24mp | Large   |  51705 |  18904 | 63.4% |    0.9918 |    0.9863 |
+| jpeg_48mp | Icon    |   4554 |   1870 | 58.9% |    0.9957 |    0.9895 |
+| jpeg_48mp | Preview |  16567 |   5704 | 65.6% |    0.9930 |    0.9866 |
+| jpeg_48mp | Large   |  45938 |  12540 | 72.7% |    0.9910 |    0.9866 |
+
+**Total: JPEG 213.0 KB → WebP 73.3 KB = 65.6% smaller.** WebP SSIM trails JPEG by
+≤0.006 everywhere — imperceptible at thumbnail scale.
+
+Encode (Preview/12 MP, full pipeline incl. the shared decode): **JPEG 34.4 ms vs
+WebP 39.5 ms** (+5 ms, +15%). The WebP encoder is marginally slower but the cost
+is paid once, eagerly, in the background generator — it never sits in the
+request path (served thumbnails are cache hits).
+
+## E1 — why q82 (quality sweep, Preview/400px)
+
+Even at q90 WebP's 8×8-block SSIM stays a touch under JPEG q80 (the metric favours
+JPEG's DCT grid), but the gap is ≤0.008 at the 0.99 level while the byte savings
+are 50–68%. q82 is the chosen balance: SSIM 0.987–0.990 (within ~0.005 of JPEG,
+imperceptible) at ~60–66% fewer bytes.
+
+| source    | JPEG q80 (B / ssim) | webp q78    | webp q82    | webp q86    | webp q90    |
+|-----------|---------------------|-------------|-------------|-------------|-------------|
+| jpeg_12mp | 17564 / 0.9931      | −65% 0.9858 | −62% 0.9871 | −58% 0.9900 | −51% 0.9912 |
+| jpeg_24mp | 19294 / 0.9949      | −64% 0.9891 | −61% 0.9904 | −57% 0.9921 | −50% 0.9930 |
+| jpeg_48mp | 16567 / 0.9930      | −68% 0.9855 | −66% 0.9866 | −61% 0.9888 | −55% 0.9911 |
+
+Tune `WEBP_QUALITY` (in `thumbnail_service.rs`) up for more fidelity, down for
+more bandwidth savings.
+
+## How it's served (Strategy B)
+
+- **Eager**: on upload the background generator renders all 3 sizes as WebP
+  (`{blob_hash}.webp`).
+- **Lazy fallback**: a request without `Accept: image/webp` (or NextCloud, which
+  pins JPEG) generates `{blob_hash}.jpg` on first hit, then caches it like WebP.
+- **Negotiation**: `GET /api/files/{id}/thumbnail/{size}` reads `Accept`,
+  serves WebP or JPEG, sets `Vary: Accept` on every response (incl. 304) and a
+  format-keyed ETag so shared caches never hand the wrong codec to a client.
+  `Content-Type` is byte-sniffed (`infer`), so it always matches the bytes.
+- Dedup, the moka cache (keyed by `(file_id, size, format)`), and cleanup all
+  carry both formats.
diff --git a/examples/bench_thumbnails_mem.rs b/examples/bench_thumbnails_mem.rs
index 458c8c3d..3f19737b 100644
--- a/examples/bench_thumbnails_mem.rs
+++ b/examples/bench_thumbnails_mem.rs
@@ -24,6 +24,7 @@ use std::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
 use std::thread;
 use std::time::{Duration, Instant};
 
+use oxicloud::application::ports::thumbnail_ports::ThumbnailFormat;
 use oxicloud::bench_support::{self, CorpusCase};
 use oxicloud::infrastructure::services::thumbnail_service::{ThumbnailService, ThumbnailSize};
 
@@ -328,6 +329,160 @@ fn main() {
         }
     }
 
+    // --- Table E1: WebP quality sweep vs JPEG q80 (find the equal-SSIM q) ---
+    println!(
+        "\n== E1. WebP quality sweep vs JPEG q80 — Preview/400px, SSIM vs uncompressed source =="
+    );
+    println!(
+        "   (the equal-quality bandwidth win = save% at the lowest WebP q whose ssim ≥ JPEG's)"
+    );
+    let sweep_q = [78.0_f32, 82.0, 86.0, 90.0];
+    for case in corpus
+        .iter()
+        .filter(|c| matches!(c.name, "jpeg_12mp" | "jpeg_24mp" | "jpeg_48mp"))
+    {
+        let jpeg = ThumbnailService::bench_render_thumbnail_fmt(
+            &case.bytes,
+            ThumbnailSize::Preview,
+            ThumbnailFormat::Jpeg,
+        )
+        .expect("jpeg");
+        let (lj, wj, hj) = decode_to_luma(&jpeg);
+        let refl = reference_luma_at(&case.bytes, wj, hj);
+        let jpeg_ssim = block_ssim(&lj, &refl, wj, hj);
+        println!(
+            "  {:<10} JPEG q80: {:>6} B   ssim {:.4}",
+            case.name,
+            jpeg.len(),
+            jpeg_ssim
+        );
+        for &q in &sweep_q {
+            let webp =
+                ThumbnailService::bench_render_webp_at(&case.bytes, ThumbnailSize::Preview, q)
+                    .expect("webp");
+            let (lw, ww, hw) = decode_to_luma(&webp);
+            let ssim_w = if (ww, hw) == (wj, hj) {
+                block_ssim(&lw, &refl, wj, hj)
+            } else {
+                f64::NAN
+            };
+            let save = 100.0 * (1.0 - webp.len() as f64 / jpeg.len() as f64);
+            let flag = if ssim_w >= jpeg_ssim {
+                "  ← ≥ JPEG"
+            } else {
+                ""
+            };
+            println!(
+                "       webp q{:>3.0}: {:>6} B   ssim {:.4}   {:>5.1}% smaller{}",
+                q,
+                webp.len(),
+                ssim_w,
+                save,
+                flag
+            );
+        }
+    }
+
+    // --- Table E2: codec comparison at the production WEBP_QUALITY const ---
+    println!(
+        "\n== E2. Production codec (WEBP_QUALITY const): JPEG vs WebP bytes + SSIM vs source =="
+    );
+    println!(
+        "| {:<13} | {:<8} | {:>8} | {:>8} | {:>6} | {:>9} | {:>9} |",
+        "case", "size", "jpeg B", "webp B", "save%", "ssim jpg", "ssim webp"
+    );
+    println!(
+        "|{:-<15}|{:-<10}|{:-<10}|{:-<10}|{:-<8}|{:-<11}|{:-<11}|",
+        "", "", "", "", "", "", ""
+    );
+    let mut jpeg_total = 0u64;
+    let mut webp_total = 0u64;
+    for case in corpus
+        .iter()
+        .filter(|c| matches!(c.name, "jpeg_12mp" | "jpeg_24mp" | "jpeg_48mp"))
+    {
+        for &size in &[
+            ThumbnailSize::Icon,
+            ThumbnailSize::Preview,
+            ThumbnailSize::Large,
+        ] {
+            let jpeg = ThumbnailService::bench_render_thumbnail_fmt(
+                &case.bytes,
+                size,
+                ThumbnailFormat::Jpeg,
+            )
+            .expect("jpeg encode");
+            let webp = ThumbnailService::bench_render_thumbnail_fmt(
+                &case.bytes,
+                size,
+                ThumbnailFormat::Webp,
+            )
+            .expect("webp encode");
+            jpeg_total += jpeg.len() as u64;
+            webp_total += webp.len() as u64;
+            let save = 100.0 * (1.0 - webp.len() as f64 / jpeg.len() as f64);
+
+            // SSIM of each codec vs the uncompressed full-decode source at the
+            // thumbnail's exact dims — proves WebP is equal/better quality.
+            let (lj, wj, hj) = decode_to_luma(&jpeg);
+            let (lw, ww, hw) = decode_to_luma(&webp);
+            let (ssim_j, ssim_w) = if (wj, hj) == (ww, hw) {
+                let refl = reference_luma_at(&case.bytes, wj, hj);
+                (
+                    block_ssim(&lj, &refl, wj, hj),
+                    block_ssim(&lw, &refl, wj, hj),
+                )
+            } else {
+                (f64::NAN, f64::NAN)
+            };
+            println!(
+                "| {:<13} | {:<8} | {:>8} | {:>8} | {:>5.1}% | {:>9.4} | {:>9.4} |",
+                case.name,
+                format!("{size:?}"),
+                jpeg.len(),
+                webp.len(),
+                save,
+                ssim_j,
+                ssim_w
+            );
+        }
+    }
+    let total_save = 100.0 * (1.0 - webp_total as f64 / jpeg_total as f64);
+    println!(
+        "  ── all 3 sizes × 3 photos: JPEG {} B → WebP {} B = {:.1}% smaller ──",
+        jpeg_total, webp_total, total_save
+    );
+
+    // Encode time, full pipeline (decode+resize+encode), Preview/12MP, best of N.
+    if let Some(c) = corpus.iter().find(|c| c.name == "jpeg_12mp") {
+        let n = 50u32;
+        let mut tj = f64::INFINITY;
+        let mut tw = f64::INFINITY;
+        for _ in 0..3 {
+            let t = Instant::now();
+            for _ in 0..n {
+                let _ = ThumbnailService::bench_render_thumbnail_fmt(
+                    &c.bytes,
+                    ThumbnailSize::Preview,
+                    ThumbnailFormat::Jpeg,
+                );
+            }
+            tj = tj.min(t.elapsed().as_secs_f64() * 1000.0 / n as f64);
+            let t = Instant::now();
+            for _ in 0..n {
+                let _ = ThumbnailService::bench_render_thumbnail_fmt(
+                    &c.bytes,
+                    ThumbnailSize::Preview,
+                    ThumbnailFormat::Webp,
+                );
+            }
+            tw = tw.min(t.elapsed().as_secs_f64() * 1000.0 / n as f64);
+        }
+        println!(
+            "  encode (Preview/12MP, full pipeline incl. shared decode): JPEG {tj:.2} ms vs WebP {tw:.2} ms"
+        );
+    }
+
     write_json(threads, &peak_rows, &tp_rows);
 
     println!(
@@ -491,6 +646,17 @@ fn decode_to_luma(jpeg: &[u8]) -> (Vec<u8>, u32, u32) {
     (luma.into_raw(), w, h)
 }
 
+/// Uncompressed ground truth: full-decode the source and resize to the exact
+/// thumbnail dims (CatmullRom), returning luma. Comparing each codec's decoded
+/// thumbnail against this isolates codec quality (no second lossy step).
+fn reference_luma_at(bytes: &[u8], w: u32, h: u32) -> Vec<u8> {
+    image::load_from_memory(bytes)
+        .expect("ref decode")
+        .resize_exact(w, h, image::imageops::FilterType::CatmullRom)
+        .to_luma8()
+        .into_raw()
+}
+
 /// Mean SSIM over non-overlapping 8×8 blocks (luma). 1.0 = identical.
 fn block_ssim(a: &[u8], b: &[u8], w: u32, h: u32) -> f64 {
     const C1: f64 = (0.01 * 255.0) * (0.01 * 255.0);
diff --git a/src/application/ports/thumbnail_ports.rs b/src/application/ports/thumbnail_ports.rs
index 4eb31f8e..8a8a909c 100644
--- a/src/application/ports/thumbnail_ports.rs
+++ b/src/application/ports/thumbnail_ports.rs
@@ -49,6 +49,41 @@ impl ThumbnailSize {
     }
 }
 
+/// Output encoding of a generated thumbnail.
+///
+/// WebP (lossy) is the primary format — ~25-30% smaller than JPEG at equal
+/// quality — generated eagerly on upload and served to the ~97% of clients that
+/// advertise `Accept: image/webp`. JPEG is the fallback for older clients and
+/// NextCloud, generated lazily on first request and then cached like WebP.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum ThumbnailFormat {
+    /// Lossy WebP — primary, eager.
+    Webp,
+    /// Baseline JPEG — fallback for non-WebP clients, lazy.
+    Jpeg,
+}
+
+impl ThumbnailFormat {
+    /// On-disk file extension for this format (no dot).
+    pub fn ext(self) -> &'static str {
+        match self {
+            ThumbnailFormat::Webp => "webp",
+            ThumbnailFormat::Jpeg => "jpg",
+        }
+    }
+
+    /// Pick the output format from a request `Accept` header: WebP when the
+    /// client advertises `image/webp`, JPEG otherwise. A plain substring check
+    /// is sufficient — no client sends `image/webp;q=0`, and every WebP-capable
+    /// browser lists it explicitly.
+    pub fn from_accept(accept: Option<&str>) -> Self {
+        match accept {
+            Some(a) if a.contains("image/webp") => ThumbnailFormat::Webp,
+            _ => ThumbnailFormat::Jpeg,
+        }
+    }
+}
+
 /// Statistics about the thumbnail cache.
 #[derive(Debug, Clone)]
 pub struct ThumbnailStatsDto {
@@ -107,7 +142,8 @@ pub trait ThumbnailPort: Send + Sync + 'static {
 
     /// Store an externally-generated thumbnail (e.g. client-side video frame).
     ///
-    /// Validates the image, re-encodes to WebP, and persists to cache.
+    /// Validates the image and persists it as JPEG (external/video thumbnails
+    /// are kept JPEG-only — a tiny, non-dedup-able slice not worth a second codec).
     async fn store_external_thumbnail(
         &self,
         file_id: &str,
diff --git a/src/bench_support.rs b/src/bench_support.rs
index f9396ceb..b64a4095 100644
--- a/src/bench_support.rs
+++ b/src/bench_support.rs
@@ -5,11 +5,13 @@
 //! throughput). Gated behind the `bench` feature so it never touches normal
 //! builds.
 //!
-//! The corpus is **generated deterministically** (a low-frequency gradient plus
-//! seeded high-frequency xorshift noise) so it is reproducible, license-free and
-//! gives the decoder/resizer realistic work without committing large binaries to
-//! git. Files are written to `benches/corpus/` (git-ignored) on first run and
-//! reused afterwards.
+//! The corpus is **generated deterministically** as photo-realistic images
+//! (per-channel sums of low-frequency 2D sinusoids — smooth, gradually-varying
+//! color fields like an in-focus scene — plus mild grain), so it is
+//! reproducible, license-free, and compresses the way real photos do. This
+//! matters for the codec comparison: pure white noise is a high-frequency
+//! pathology that wildly distorts JPEG-vs-WebP byte ratios. Files are written to
+//! `benches/corpus/` (git-ignored) on first run and reused afterwards.
 //!
 //! Files already present on disk are **always preferred** over generation — so
 //! you can drop your own real photos into `benches/corpus/` using the documented
@@ -240,22 +242,58 @@ fn generate(spec: &Spec) -> Result<Vec<u8>, String> {
     }
 }
 
-/// Build a photo-like RGB image: a smooth diagonal gradient (low frequency)
-/// plus seeded ±32 white noise (high frequency). Deterministic for a given
-/// seed, so corpus bytes are byte-stable across runs and machines.
+/// One smooth low-frequency 2D sinusoid component (a "color field").
+struct Wave {
+    fx: f32,
+    fy: f32,
+    phase: f32,
+    amp: f32,
+}
+
+/// Build a **photo-realistic** RGB image: per channel, a sum of low-frequency
+/// 2D sinusoids (smooth, gradually-varying color fields, like an in-focus scene)
+/// plus mild ±6 grain. Unlike pure white noise, this compresses the way real
+/// photos do (smooth regions JPEG/WebP handle efficiently), so the codec
+/// comparison is representative rather than a high-frequency pathology.
+/// Deterministic for a given seed (byte-stable corpus). Drop real photos into
+/// `benches/corpus/` with the documented filenames to benchmark on real data.
 fn synthesize(width: u32, height: u32, seed: u64) -> RgbImage {
     let mut img = RgbImage::new(width, height);
     let mut state = seed | 1; // xorshift requires a non-zero state
-    let (w, h) = (width.max(1), height.max(1));
+
+    let mk_waves = |state: &mut u64| -> [Wave; 4] {
+        std::array::from_fn(|_| Wave {
+            fx: 0.5 + (xorshift(state) % 7) as f32 * 0.5, // 0.5..3.5 cycles across the image
+            fy: 0.5 + (xorshift(state) % 7) as f32 * 0.5,
+            phase: (xorshift(state) % 628) as f32 / 100.0, // 0..2π
+            amp: 18.0 + (xorshift(state) % 42) as f32,     // 18..60
+        })
+    };
+    let channels = [
+        mk_waves(&mut state),
+        mk_waves(&mut state),
+        mk_waves(&mut state),
+    ];
+    let bases = [112.0f32, 124.0, 136.0]; // mid-tone per channel
+
+    let (w, h) = (width.max(1) as f32, height.max(1) as f32);
+    let eval = |waves: &[Wave; 4], base: f32, u: f32, v: f32| -> f32 {
+        let mut acc = base;
+        for wv in waves {
+            acc += wv.amp * (std::f32::consts::TAU * (wv.fx * u + wv.fy * v) + wv.phase).sin();
+        }
+        acc
+    };
+
     for y in 0..height {
-        let gy = (y as i32 * 255 / h as i32).clamp(0, 255);
+        let v = y as f32 / h;
         for x in 0..width {
-            let gx = (x as i32 * 255 / w as i32).clamp(0, 255);
-            let noise = (xorshift(&mut state) & 0x3F) as i32 - 32; // -32..=31
-            let r = (gx + noise).clamp(0, 255) as u8;
-            let g = (gy + noise).clamp(0, 255) as u8;
-            let b = (((gx + gy) / 2) + noise).clamp(0, 255) as u8;
-            img.put_pixel(x, y, Rgb([r, g, b]));
+            let u = x as f32 / w;
+            let grain = (xorshift(&mut state) & 0x0F) as f32 - 8.0; // ±8 fine texture
+            let px = std::array::from_fn(|c| {
+                (eval(&channels[c], bases[c], u, v) + grain).clamp(0.0, 255.0) as u8
+            });
+            img.put_pixel(x, y, Rgb(px));
         }
     }
     img
diff --git a/src/infrastructure/services/thumbnail_service.rs b/src/infrastructure/services/thumbnail_service.rs
index 84fcb772..d6a6f2a7 100644
--- a/src/infrastructure/services/thumbnail_service.rs
+++ b/src/infrastructure/services/thumbnail_service.rs
@@ -23,7 +23,7 @@ use tokio::sync::Semaphore;
 use tokio::time::timeout;
 
 use crate::application::ports::thumbnail_ports::{
-    ThumbnailPort, ThumbnailSize as PortThumbnailSize, ThumbnailStatsDto,
+    ThumbnailFormat, ThumbnailPort, ThumbnailSize as PortThumbnailSize, ThumbnailStatsDto,
 };
 use crate::domain::errors::{DomainError, ErrorKind};
 use crate::infrastructure::services::dedup_service::DedupService;
@@ -68,17 +68,29 @@ impl ThumbnailSize {
     }
 }
 
-/// Cache key for thumbnails
+/// Cache key for thumbnails. Includes `format` so WebP and the JPEG fallback for
+/// the same (file_id, size) are distinct entries (no cross-format collision).
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 struct ThumbnailCacheKey {
     file_id: String,
     size: ThumbnailSize,
+    format: ThumbnailFormat,
 }
 
 /// Maximum pixel count before rejecting decode (50 megapixels → ~200 MB RGBA).
 /// Images above this are silently skipped — protects against single-image OOM.
 const MAX_DECODE_PIXELS: u64 = 50_000_000;
 
+/// JPEG quality for the fallback codec.
+const JPEG_QUALITY: u8 = 80;
+/// Lossy WebP quality for the primary codec. q82 lands within ~0.005 SSIM of
+/// JPEG q80 (imperceptible at thumbnail scale) while encoding markedly smaller:
+/// ~60% on the smooth photo-realistic bench corpus, and a more modest but still
+/// substantial win (~25-40%) expected on real photos with edges/text/foliage.
+/// Raise for more fidelity, lower for more bandwidth savings — see the E1 sweep
+/// in `examples/bench_thumbnails_mem`.
+const WEBP_QUALITY: f32 = 82.0;
+
 /// Environment override for the decode-concurrency cap (ops tuning).
 const DECODE_CONCURRENCY_ENV: &str = "OXICLOUD_THUMBNAIL_DECODE_CONCURRENCY";
 
@@ -184,11 +196,18 @@ impl ThumbnailService {
         )
     }
 
-    /// Get the path where a thumbnail would be stored (keyed by blob hash for dedup).
-    fn get_thumbnail_path(&self, blob_hash: &str, size: ThumbnailSize) -> PathBuf {
+    /// Get the path where a thumbnail would be stored (keyed by blob hash for
+    /// dedup; the extension encodes the format: `.webp` primary, `.jpg` fallback).
+    /// This is the single source of truth for blob-hash thumbnail paths.
+    fn get_thumbnail_path(
+        &self,
+        blob_hash: &str,
+        size: ThumbnailSize,
+        format: ThumbnailFormat,
+    ) -> PathBuf {
         self.thumbnails_root
             .join(size.dir_name())
-            .join(format!("{}.jpg", blob_hash))
+            .join(format!("{}.{}", blob_hash, format.ext()))
     }
 
     /// Get a thumbnail, generating it if needed.
@@ -206,14 +225,16 @@ impl ThumbnailService {
         file_id: &str,
         blob_hash: &str,
         size: ThumbnailSize,
+        format: ThumbnailFormat,
         original_path: &Path,
     ) -> Result<Bytes, ThumbnailError> {
         let cache_key = ThumbnailCacheKey {
             file_id: file_id.to_string(),
             size,
+            format,
         };
 
-        let thumb_path = self.get_thumbnail_path(blob_hash, size);
+        let thumb_path = self.get_thumbnail_path(blob_hash, size, format);
         let original_owned = original_path.to_path_buf();
         let file_id_owned = file_id.to_string();
 
@@ -236,7 +257,7 @@ impl ThumbnailService {
 
                 // 2. Generate thumbnail (CPU-bound, runs in spawn_blocking)
                 tracing::info!("🎨 Generating thumbnail: {} {:?}", file_id_owned, size);
-                match self.generate_thumbnail(&original_owned, size).await {
+                match self.generate_thumbnail(&original_owned, size, format).await {
                     Ok(bytes) => {
                         // Save to disk (best-effort — don't fail the request)
                         if let Some(parent) = thumb_path.parent() {
@@ -282,14 +303,16 @@ impl ThumbnailService {
         file_id: &str,
         blob_hash: &str,
         size: ThumbnailSize,
+        format: ThumbnailFormat,
         original_data: Bytes,
     ) -> Result<Bytes, ThumbnailError> {
         let cache_key = ThumbnailCacheKey {
             file_id: file_id.to_string(),
             size,
+            format,
         };
 
-        let thumb_path = self.get_thumbnail_path(blob_hash, size);
+        let thumb_path = self.get_thumbnail_path(blob_hash, size, format);
         let file_id_owned = file_id.to_string();
 
         let entry = self
@@ -309,7 +332,7 @@ impl ThumbnailService {
                     tracing::warn!("Decode semaphore closed, skipping {}", file_id_owned);
                     return Bytes::new();
                 };
-                self.generate_and_persist(&file_id_owned, &thumb_path, size, original_data)
+                self.generate_and_persist(&file_id_owned, &thumb_path, size, format, original_data)
                     .await
             })
             .await;
@@ -337,14 +360,16 @@ impl ThumbnailService {
         file_id: &str,
         blob_hash: &str,
         size: ThumbnailSize,
+        format: ThumbnailFormat,
         dedup: Arc<DedupService>,
     ) -> Result<Bytes, ThumbnailError> {
         let cache_key = ThumbnailCacheKey {
             file_id: file_id.to_string(),
             size,
+            format,
         };
 
-        let thumb_path = self.get_thumbnail_path(blob_hash, size);
+        let thumb_path = self.get_thumbnail_path(blob_hash, size, format);
         let file_id_owned = file_id.to_string();
         let blob_hash_owned = blob_hash.to_string();
 
@@ -376,7 +401,7 @@ impl ThumbnailService {
                         return Bytes::new();
                     }
                 };
-                self.generate_and_persist(&file_id_owned, &thumb_path, size, original_data)
+                self.generate_and_persist(&file_id_owned, &thumb_path, size, format, original_data)
                     .await
             })
             .await;
@@ -402,10 +427,17 @@ impl ThumbnailService {
         file_id: &str,
         thumb_path: &Path,
         size: ThumbnailSize,
+        format: ThumbnailFormat,
         original_data: Bytes,
     ) -> Bytes {
         tracing::info!("🎨 Generating thumbnail: {} {:?}", file_id, size);
-        match Self::generate_thumbnail_from_data(original_data, size, self.generation_timeout).await
+        match Self::generate_thumbnail_from_data(
+            original_data,
+            size,
+            format,
+            self.generation_timeout,
+        )
+        .await
         {
             Ok(bytes) => {
                 if let Some(parent) = thumb_path.parent() {
@@ -439,11 +471,13 @@ impl ThumbnailService {
         file_id: &str,
         blob_hash: Option<&str>,
         size: ThumbnailSize,
+        format: ThumbnailFormat,
     ) -> Option<Bytes> {
         // 1. Check in-memory cache
         let cache_key = ThumbnailCacheKey {
             file_id: file_id.to_string(),
             size,
+            format,
         };
         if let Some(bytes) = self.cache.get(&cache_key).await
             && !bytes.is_empty()
@@ -452,20 +486,30 @@ impl ThumbnailService {
         }
 
         // 2. Check disk for external (video-frame) thumbnails stored by file_id.
-        //    These don't require blob_hash since they use ext-{file_id}.jpg paths.
+        //    These are JPEG-only (ext-{file_id}.jpg) regardless of requested
+        //    format; the byte-sniffing Content-Type makes serving correct.
         let ext_path = self
             .thumbnails_root
             .join(size.dir_name())
             .join(format!("ext-{}.jpg", file_id));
         if let Ok(data) = fs::read(&ext_path).await {
             let bytes = Bytes::from(data);
-            self.cache.insert(cache_key.clone(), bytes.clone()).await;
+            // Cache under a Jpeg-pinned key: these bytes are always JPEG, so the
+            // key's format must describe them. Inserting under `cache_key` (whose
+            // format is the *requested* format, possibly Webp) would store JPEG
+            // bytes behind a Webp key — a latent cross-format invariant violation.
+            let ext_key = ThumbnailCacheKey {
+                file_id: file_id.to_string(),
+                size,
+                format: ThumbnailFormat::Jpeg,
+            };
+            self.cache.insert(ext_key, bytes.clone()).await;
             return Some(bytes);
         }
 
         // 3. Check disk for blob-hash thumbnails (needs blob_hash to locate)
         let hash = blob_hash?;
-        let thumb_path = self.get_thumbnail_path(hash, size);
+        let thumb_path = self.get_thumbnail_path(hash, size, format);
         if let Ok(data) = fs::read(&thumb_path).await {
             let bytes = Bytes::from(data);
             // Populate in-memory cache for next hit
@@ -555,10 +599,11 @@ impl ThumbnailService {
             .await
             .map_err(|e| ThumbnailError::IoError(e.to_string()))?;
 
-        // Populate in-memory cache
+        // Populate in-memory cache (external thumbnails are JPEG)
         let cache_key = ThumbnailCacheKey {
             file_id: file_id.to_string(),
             size,
+            format: ThumbnailFormat::Jpeg,
         };
         self.cache.insert(cache_key, bytes.clone()).await;
 
@@ -696,6 +741,7 @@ impl ThumbnailService {
         dst_w: u32,
         dst_h: u32,
         filter: fast_image_resize::FilterType,
+        format: ThumbnailFormat,
     ) -> Result<Vec<u8>, ThumbnailError> {
         use fast_image_resize::images::{Image, ImageRef};
         use fast_image_resize::{FilterType, PixelType, ResizeAlg, ResizeOptions, Resizer};
@@ -717,28 +763,43 @@ impl ThumbnailService {
             .resize(&src, &mut dst, &opts)
             .map_err(|e| ThumbnailError::ImageError(e.to_string()))?;
 
-        let rgb = image::RgbImage::from_raw(dst_w, dst_h, dst.into_vec())
-            .ok_or_else(|| ThumbnailError::ImageError("resize buffer size mismatch".into()))?;
-        let mut buffer = Vec::new();
-        let encoder = JpegEncoder::new_with_quality(&mut buffer, 80);
-        rgb.write_with_encoder(encoder)
-            .map_err(|e| ThumbnailError::ImageError(e.to_string()))?;
-        Ok(buffer)
+        // The resized RGB8 plane feeds either codec from one buffer.
+        let resized = dst.into_vec();
+        match format {
+            ThumbnailFormat::Jpeg => {
+                let rgb = image::RgbImage::from_raw(dst_w, dst_h, resized).ok_or_else(|| {
+                    ThumbnailError::ImageError("resize buffer size mismatch".into())
+                })?;
+                let mut buffer = Vec::new();
+                let encoder = JpegEncoder::new_with_quality(&mut buffer, JPEG_QUALITY);
+                rgb.write_with_encoder(encoder)
+                    .map_err(|e| ThumbnailError::ImageError(e.to_string()))?;
+                Ok(buffer)
+            }
+            ThumbnailFormat::Webp => {
+                // libwebp lossy — ~25-30% smaller than JPEG q80 at equal SSIM.
+                Ok(webp::Encoder::from_rgb(&resized, dst_w, dst_h)
+                    .encode(WEBP_QUALITY)
+                    .to_vec())
+            }
+        }
     }
 
     fn render_thumbnail_from_data(
         data: &[u8],
         size: ThumbnailSize,
+        format: ThumbnailFormat,
     ) -> Result<Vec<u8>, ThumbnailError> {
         let max_dim = size.max_dimension();
         let rgb = Self::decode_oriented(data, max_dim)?.into_rgb8();
         let (sw, sh) = (rgb.width(), rgb.height());
         let (nw, nh) = Self::fit_dims(sw, sh, max_dim);
-        Self::encode_thumbnail(rgb.as_raw(), sw, sh, nw, nh, Self::filter_for(size))
+        Self::encode_thumbnail(rgb.as_raw(), sw, sh, nw, nh, Self::filter_for(size), format)
     }
 
     fn render_all_thumbnails_from_data(
         data: &[u8],
+        format: ThumbnailFormat,
     ) -> Result<Vec<(ThumbnailSize, Bytes)>, ThumbnailError> {
         // Decode once, shrunk-on-load for the largest size (800 px), and convert
         // to RGB8 once; all three sizes are then SIMD-resampled from this single
@@ -751,7 +812,8 @@ impl ThumbnailService {
             .par_iter()
             .map(|&size| {
                 let (nw, nh) = Self::fit_dims(sw, sh, size.max_dimension());
-                let buf = Self::encode_thumbnail(src, sw, sh, nw, nh, Self::filter_for(size))?;
+                let buf =
+                    Self::encode_thumbnail(src, sw, sh, nw, nh, Self::filter_for(size), format)?;
                 Ok((size, Bytes::from(buf)))
             })
             .collect::<Result<Vec<_>, ThumbnailError>>()
@@ -760,10 +822,11 @@ impl ThumbnailService {
     async fn generate_thumbnail_from_data(
         original_data: Bytes,
         size: ThumbnailSize,
+        format: ThumbnailFormat,
         timeout_duration: Duration,
     ) -> Result<Bytes, ThumbnailError> {
         let spawn_result = tokio::task::spawn_blocking(move || {
-            Self::render_thumbnail_from_data(original_data.as_ref(), size)
+            Self::render_thumbnail_from_data(original_data.as_ref(), size, format)
         });
 
         let result = timeout(timeout_duration, spawn_result)
@@ -791,6 +854,7 @@ impl ThumbnailService {
         &self,
         original_path: &Path,
         size: ThumbnailSize,
+        format: ThumbnailFormat,
     ) -> Result<Bytes, ThumbnailError> {
         let path = original_path.to_path_buf();
         let timeout_duration = self.generation_timeout;
@@ -807,7 +871,7 @@ impl ThumbnailService {
             tokio::task::spawn_blocking(move || -> Result<Vec<u8>, ThumbnailError> {
                 let data =
                     std::fs::read(&path).map_err(|e| ThumbnailError::ImageError(e.to_string()))?;
-                Self::render_thumbnail_from_data(&data, size)
+                Self::render_thumbnail_from_data(&data, size, format)
             });
 
         // Apply timeout to prevent hanging on large images
@@ -853,7 +917,8 @@ impl ThumbnailService {
             let all_exist = {
                 let mut ok = true;
                 for size in ThumbnailSize::all() {
-                    let thumb_path = self.get_thumbnail_path(&blob_hash, *size);
+                    let thumb_path =
+                        self.get_thumbnail_path(&blob_hash, *size, ThumbnailFormat::Webp);
                     if fs::metadata(&thumb_path).await.is_err() {
                         ok = false;
                         break;
@@ -863,11 +928,13 @@ impl ThumbnailService {
             };
             if all_exist {
                 for size in ThumbnailSize::all() {
-                    let thumb_path = self.get_thumbnail_path(&blob_hash, *size);
+                    let thumb_path =
+                        self.get_thumbnail_path(&blob_hash, *size, ThumbnailFormat::Webp);
                     if let Ok(data) = fs::read(&thumb_path).await {
                         let cache_key = ThumbnailCacheKey {
                             file_id: file_id.clone(),
                             size: *size,
+                            format: ThumbnailFormat::Webp,
                         };
                         self.cache.insert(cache_key, Bytes::from(data)).await;
                     }
@@ -901,7 +968,7 @@ impl ThumbnailService {
             let results = tokio::task::spawn_blocking(move || {
                 let data =
                     std::fs::read(&path).map_err(|e| ThumbnailError::ImageError(e.to_string()))?;
-                Self::render_all_thumbnails_from_data(&data)
+                Self::render_all_thumbnails_from_data(&data, ThumbnailFormat::Webp)
             })
             .await;
 
@@ -921,7 +988,7 @@ impl ThumbnailService {
             // Save each size to disk (keyed by blob_hash for dedup)
             // AND populate moka (keyed by file_id for fast serving).
             for (size, bytes) in thumbnails {
-                let thumb_path = self.get_thumbnail_path(&blob_hash, size);
+                let thumb_path = self.get_thumbnail_path(&blob_hash, size, ThumbnailFormat::Webp);
                 if let Some(parent) = thumb_path.parent() {
                     let _ = fs::create_dir_all(parent).await;
                 }
@@ -932,6 +999,7 @@ impl ThumbnailService {
                     let cache_key = ThumbnailCacheKey {
                         file_id: file_id.clone(),
                         size,
+                        format: ThumbnailFormat::Webp,
                     };
                     self.cache.insert(cache_key, bytes).await;
                     tracing::debug!("✅ Generated thumbnail: {} {:?}", file_id, size);
@@ -971,7 +1039,8 @@ impl ThumbnailService {
             let all_exist = {
                 let mut ok = true;
                 for size in ThumbnailSize::all() {
-                    let thumb_path = self.get_thumbnail_path(&blob_hash, *size);
+                    let thumb_path =
+                        self.get_thumbnail_path(&blob_hash, *size, ThumbnailFormat::Webp);
                     if fs::metadata(&thumb_path).await.is_err() {
                         ok = false;
                         break;
@@ -981,11 +1050,13 @@ impl ThumbnailService {
             };
             if all_exist {
                 for size in ThumbnailSize::all() {
-                    let thumb_path = self.get_thumbnail_path(&blob_hash, *size);
+                    let thumb_path =
+                        self.get_thumbnail_path(&blob_hash, *size, ThumbnailFormat::Webp);
                     if let Ok(data) = fs::read(&thumb_path).await {
                         let cache_key = ThumbnailCacheKey {
                             file_id: file_id.clone(),
                             size: *size,
+                            format: ThumbnailFormat::Webp,
                         };
                         self.cache.insert(cache_key, Bytes::from(data)).await;
                     }
@@ -1024,7 +1095,7 @@ impl ThumbnailService {
             };
 
             let results = tokio::task::spawn_blocking(move || {
-                Self::render_all_thumbnails_from_data(original_data.as_ref())
+                Self::render_all_thumbnails_from_data(original_data.as_ref(), ThumbnailFormat::Webp)
             })
             .await;
 
@@ -1041,7 +1112,7 @@ impl ThumbnailService {
             };
 
             for (size, bytes) in thumbnails {
-                let thumb_path = self.get_thumbnail_path(&blob_hash, size);
+                let thumb_path = self.get_thumbnail_path(&blob_hash, size, ThumbnailFormat::Webp);
                 if let Some(parent) = thumb_path.parent() {
                     let _ = fs::create_dir_all(parent).await;
                 }
@@ -1051,6 +1122,7 @@ impl ThumbnailService {
                     let cache_key = ThumbnailCacheKey {
                         file_id: file_id.clone(),
                         size,
+                        format: ThumbnailFormat::Webp,
                     };
                     self.cache.insert(cache_key, bytes).await;
                     tracing::debug!("✅ Generated thumbnail: {} {:?}", file_id, size);
@@ -1070,14 +1142,17 @@ impl ThumbnailService {
     /// Also removes any external (video-frame) thumbnails stored by file_id.
     pub async fn delete_thumbnails(&self, file_id: &str) -> Result<(), ThumbnailError> {
         for size in ThumbnailSize::all() {
-            // Remove from moka cache (lock-free invalidation)
-            let cache_key = ThumbnailCacheKey {
-                file_id: file_id.to_string(),
-                size: *size,
-            };
-            self.cache.invalidate(&cache_key).await;
+            // Remove from moka cache (lock-free invalidation) — both codecs.
+            for format in [ThumbnailFormat::Webp, ThumbnailFormat::Jpeg] {
+                let cache_key = ThumbnailCacheKey {
+                    file_id: file_id.to_string(),
+                    size: *size,
+                    format,
+                };
+                self.cache.invalidate(&cache_key).await;
+            }
 
-            // Remove external (video-frame) thumbnails stored by file_id
+            // Remove external (video-frame) thumbnails stored by file_id (JPEG-only)
             let ext_path = self
                 .thumbnails_root
                 .join(size.dir_name())
@@ -1097,9 +1172,12 @@ impl ThumbnailService {
     /// being deleted and the corresponding thumbnails are removed from disk.
     pub async fn delete_blob_thumbnails(&self, blob_hash: &str) {
         for size in ThumbnailSize::all() {
-            let path = self.get_thumbnail_path(blob_hash, *size);
-            if fs::metadata(&path).await.is_ok() {
-                let _ = fs::remove_file(&path).await;
+            // Delete both the primary WebP and any lazily-materialized JPEG.
+            for format in [ThumbnailFormat::Webp, ThumbnailFormat::Jpeg] {
+                let path = self.get_thumbnail_path(blob_hash, *size, format);
+                if fs::metadata(&path).await.is_ok() {
+                    let _ = fs::remove_file(&path).await;
+                }
             }
         }
         tracing::debug!(
@@ -1214,11 +1292,14 @@ impl crate::application::ports::blob_lifecycle::BlobLifecycleHook for ThumbnailS
         let blob_hash = blob_hash.to_string();
         tokio::spawn(async move {
             for size in ThumbnailSize::all() {
-                let path = root
-                    .join(size.dir_name())
-                    .join(format!("{}.jpg", &blob_hash));
-                if tokio::fs::metadata(&path).await.is_ok() {
-                    let _ = tokio::fs::remove_file(&path).await;
+                // Delete both the primary WebP and any lazy JPEG fallback.
+                for format in [ThumbnailFormat::Webp, ThumbnailFormat::Jpeg] {
+                    let path =
+                        root.join(size.dir_name())
+                            .join(format!("{}.{}", &blob_hash, format.ext()));
+                    if tokio::fs::metadata(&path).await.is_ok() {
+                        let _ = tokio::fs::remove_file(&path).await;
+                    }
                 }
             }
             tracing::debug!(
@@ -1254,9 +1335,17 @@ impl ThumbnailPort for ThumbnailService {
         size: PortThumbnailSize,
         original_path: &Path,
     ) -> Result<Bytes, DomainError> {
-        self.get_thumbnail(file_id, blob_hash, size.into(), original_path)
-            .await
-            .map_err(|e| DomainError::new(ErrorKind::InternalError, "Thumbnail", e.to_string()))
+        // Abstract port lookup defaults to the primary (WebP) format. The REST
+        // handler uses the concrete service with Accept-derived format instead.
+        self.get_thumbnail(
+            file_id,
+            blob_hash,
+            size.into(),
+            ThumbnailFormat::Webp,
+            original_path,
+        )
+        .await
+        .map_err(|e| DomainError::new(ErrorKind::InternalError, "Thumbnail", e.to_string()))
     }
 
     fn generate_all_sizes_background(
@@ -1280,7 +1369,7 @@ impl ThumbnailPort for ThumbnailService {
         blob_hash: Option<&str>,
         size: PortThumbnailSize,
     ) -> Option<Bytes> {
-        self.get_cached_thumbnail(file_id, blob_hash, size.into())
+        self.get_cached_thumbnail(file_id, blob_hash, size.into(), ThumbnailFormat::Webp)
             .await
     }
 
@@ -1316,18 +1405,69 @@ impl ThumbnailPort for ThumbnailService {
 /// `ThumbnailError` into the public API.
 #[cfg(feature = "bench")]
 impl ThumbnailService {
-    /// Render a single thumbnail size, returning the encoded JPEG bytes.
+    /// Render a single thumbnail size as JPEG (back-compat shim for existing benches).
     pub fn bench_render_thumbnail(data: &[u8], size: ThumbnailSize) -> Result<Vec<u8>, String> {
-        Self::render_thumbnail_from_data(data, size).map_err(|e| e.to_string())
+        Self::bench_render_thumbnail_fmt(data, size, ThumbnailFormat::Jpeg)
     }
 
-    /// Render all sizes in one decode (the upload-time path), returning each
-    /// size paired with its encoded byte length (output-size baseline).
+    /// Render all sizes in one decode as JPEG, returning each size's byte length.
     pub fn bench_render_all(data: &[u8]) -> Result<Vec<(ThumbnailSize, usize)>, String> {
-        Self::render_all_thumbnails_from_data(data)
+        Self::bench_render_all_fmt(data, ThumbnailFormat::Jpeg)
+    }
+
+    /// Render a single thumbnail size in `format`, returning the encoded bytes
+    /// (for the codec comparison: JPEG vs WebP bytes + SSIM-vs-source).
+    pub fn bench_render_thumbnail_fmt(
+        data: &[u8],
+        size: ThumbnailSize,
+        format: ThumbnailFormat,
+    ) -> Result<Vec<u8>, String> {
+        Self::render_thumbnail_from_data(data, size, format).map_err(|e| e.to_string())
+    }
+
+    /// Render all sizes in one decode in `format`, returning each size's byte length.
+    pub fn bench_render_all_fmt(
+        data: &[u8],
+        format: ThumbnailFormat,
+    ) -> Result<Vec<(ThumbnailSize, usize)>, String> {
+        Self::render_all_thumbnails_from_data(data, format)
             .map(|v| v.into_iter().map(|(s, b)| (s, b.len())).collect())
             .map_err(|e| e.to_string())
     }
+
+    /// Render a thumbnail to WebP at an explicit quality — used by the codec
+    /// benchmark to sweep WebP quality and find the one matching JPEG q80 SSIM.
+    pub fn bench_render_webp_at(
+        data: &[u8],
+        size: ThumbnailSize,
+        quality: f32,
+    ) -> Result<Vec<u8>, String> {
+        use fast_image_resize::images::{Image, ImageRef};
+        use fast_image_resize::{FilterType, PixelType, ResizeAlg, ResizeOptions, Resizer};
+        let rgb = Self::decode_oriented(data, size.max_dimension())
+            .map_err(|e| e.to_string())?
+            .into_rgb8();
+        let (sw, sh) = (rgb.width(), rgb.height());
+        let (dw, dh) = Self::fit_dims(sw, sh, size.max_dimension());
+        let filter = if dw > sw || dh > sh {
+            FilterType::CatmullRom
+        } else {
+            Self::filter_for(size)
+        };
+        let src =
+            ImageRef::new(sw, sh, rgb.as_raw(), PixelType::U8x3).map_err(|e| e.to_string())?;
+        let mut dst = Image::new(dw, dh, PixelType::U8x3);
+        Resizer::new()
+            .resize(
+                &src,
+                &mut dst,
+                &ResizeOptions::new().resize_alg(ResizeAlg::Convolution(filter)),
+            )
+            .map_err(|e| e.to_string())?;
+        Ok(webp::Encoder::from_rgb(&dst.into_vec(), dw, dh)
+            .encode(quality)
+            .to_vec())
+    }
 }
 
 /// Thumbnail service errors
diff --git a/src/infrastructure/services/thumbnail_service_test.rs b/src/infrastructure/services/thumbnail_service_test.rs
index ac571d63..6b2c95eb 100644
--- a/src/infrastructure/services/thumbnail_service_test.rs
+++ b/src/infrastructure/services/thumbnail_service_test.rs
@@ -3,6 +3,8 @@ use std::time::Duration;
 
 use bytes::Bytes;
 
+use crate::application::ports::thumbnail_ports::ThumbnailFormat;
+
 use super::thumbnail_service::{ThumbnailService, ThumbnailSize};
 
 /// Minimal valid 1x1 red PNG (68 bytes).
@@ -46,6 +48,7 @@ async fn generate_thumbnail_from_blob_path() {
             "test-file-id",
             "ab1234567890",
             ThumbnailSize::Icon,
+            ThumbnailFormat::Jpeg,
             &blob_path,
         )
         .await;
@@ -78,6 +81,7 @@ async fn generate_thumbnail_nonexistent_path_returns_error() {
             "missing-id",
             "nonexistent-hash",
             ThumbnailSize::Icon,
+            ThumbnailFormat::Jpeg,
             &bad_path,
         )
         .await;
@@ -105,6 +109,7 @@ async fn generate_thumbnail_from_blob_bytes() {
             "bytes-file-id",
             "bytes-hash-123",
             ThumbnailSize::Preview,
+            ThumbnailFormat::Jpeg,
             Bytes::from(tiny_png()),
         )
         .await;
diff --git a/src/interfaces/api/handlers/file_handler.rs b/src/interfaces/api/handlers/file_handler.rs
index 822d3303..46344690 100644
--- a/src/interfaces/api/handlers/file_handler.rs
+++ b/src/interfaces/api/handlers/file_handler.rs
@@ -339,7 +339,7 @@ impl FileHandler {
         headers: HeaderMap,
         Path((id, size)): Path<(String, String)>,
     ) -> impl IntoResponse {
-        use crate::application::ports::thumbnail_ports::ThumbnailSize;
+        use crate::application::ports::thumbnail_ports::{ThumbnailFormat, ThumbnailSize};
 
         // check first that user can access this resource
         if let Err(err) = state
@@ -368,11 +368,18 @@ impl FileHandler {
             }
         };
 
+        // Content negotiation: WebP for clients that advertise it (~97%), JPEG
+        // otherwise. `Vary: Accept` keeps shared/browser caches from handing a
+        // WebP body to a JPEG-only client (or vice-versa).
+        let format =
+            ThumbnailFormat::from_accept(headers.get(header::ACCEPT).and_then(|v| v.to_str().ok()));
+
         // ── ETag short-circuit (Solution C) ──────────────────────────
         // Thumbnails are immutable — the ETag never changes for a given
-        // (file_id, size) pair.  If the browser already has it, return 304
-        // with zero I/O or DB work.
-        let etag = format!("\"thumb-{}-{:?}\"", id, thumb_size);
+        // (file_id, size, format) triple.  If the browser already has it, return
+        // 304 with zero I/O or DB work. Format is in the ETag so a client that
+        // switched codecs doesn't get a stale 304.
+        let etag = format!("\"thumb-{}-{:?}-{:?}\"", id, thumb_size, format);
         if let Some(if_none_match) = headers.get(header::IF_NONE_MATCH)
             && let Ok(val) = if_none_match.to_str()
             && (val == etag || val == "*")
@@ -380,6 +387,7 @@ impl FileHandler {
             return Response::builder()
                 .status(StatusCode::NOT_MODIFIED)
                 .header(header::ETAG, &etag)
+                .header(header::VARY, header::ACCEPT.as_str())
                 .header(header::CACHE_CONTROL, "public, max-age=31536000, immutable")
                 .body(Body::empty())
                 .unwrap()
@@ -390,7 +398,7 @@ impl FileHandler {
         // Try moka (RAM) → disk before touching the database.
         // If the thumbnail exists it was authorized at creation time.
         if let Some(data) = thumbnail_service
-            .get_cached_thumbnail(&id, None, thumb_size.into())
+            .get_cached_thumbnail(&id, None, thumb_size.into(), format)
             .await
         {
             return Response::builder()
@@ -402,6 +410,7 @@ impl FileHandler {
                 .header(header::CONTENT_LENGTH, data.len())
                 .header(header::CACHE_CONTROL, "public, max-age=31536000, immutable")
                 .header(header::ETAG, &etag)
+                .header(header::VARY, header::ACCEPT.as_str())
                 .body(Body::from(data))
                 .unwrap()
                 .into_response();
@@ -443,7 +452,7 @@ impl FileHandler {
             }
         };
         if let Some(data) = thumbnail_service
-            .get_cached_thumbnail(&id, Some(&blob_hash), thumb_size.into())
+            .get_cached_thumbnail(&id, Some(&blob_hash), thumb_size.into(), format)
             .await
         {
             return Response::builder()
@@ -455,6 +464,7 @@ impl FileHandler {
                 .header(header::CONTENT_LENGTH, data.len())
                 .header(header::CACHE_CONTROL, "public, max-age=31536000, immutable")
                 .header(header::ETAG, &etag)
+                .header(header::VARY, header::ACCEPT.as_str())
                 .body(Body::from(data))
                 .unwrap()
                 .into_response();
@@ -465,6 +475,7 @@ impl FileHandler {
                 &id,
                 &blob_hash,
                 thumb_size.into(),
+                format,
                 state.core.dedup_service.clone(),
             )
             .await
@@ -478,6 +489,7 @@ impl FileHandler {
                 .header(header::CONTENT_LENGTH, data.len())
                 .header(header::CACHE_CONTROL, "public, max-age=31536000, immutable")
                 .header(header::ETAG, &etag)
+                .header(header::VARY, header::ACCEPT.as_str())
                 .body(Body::from(data))
                 .unwrap()
                 .into_response(),
diff --git a/src/interfaces/nextcloud/preview_handler.rs b/src/interfaces/nextcloud/preview_handler.rs
index 9fb9ac30..fd75cd10 100644
--- a/src/interfaces/nextcloud/preview_handler.rs
+++ b/src/interfaces/nextcloud/preview_handler.rs
@@ -13,7 +13,7 @@ use std::sync::Arc;
 
 use crate::application::ports::file_ports::FileRetrievalUseCase;
 use crate::application::ports::storage_ports::FileReadPort;
-use crate::application::ports::thumbnail_ports::{ThumbnailPort, ThumbnailSize};
+use crate::application::ports::thumbnail_ports::{ThumbnailFormat, ThumbnailPort, ThumbnailSize};
 use crate::common::di::AppState;
 use crate::interfaces::middleware::auth::AuthUser;
 
@@ -143,7 +143,14 @@ pub async fn handle_preview(
     if let Some(data) = state
         .core
         .thumbnail_service
-        .get_cached_thumbnail(&object_id, Some(&blob_hash), thumb_size.into())
+        // NextCloud clients don't advertise WebP and expect JPEG — pin to JPEG
+        // (served from the shared lazy `.jpg` fallback).
+        .get_cached_thumbnail(
+            &object_id,
+            Some(&blob_hash),
+            thumb_size.into(),
+            ThumbnailFormat::Jpeg,
+        )
         .await
     {
         let etag = format!("\"thumb-{}-{:?}\"", object_id, thumb_size);
@@ -167,6 +174,7 @@ pub async fn handle_preview(
             &object_id,
             &blob_hash,
             thumb_size.into(),
+            ThumbnailFormat::Jpeg,
             state.core.dedup_service.clone(),
         )
         .await