diff --git a/src-tauri/src/app_auto_updater.rs b/src-tauri/src/app_auto_updater.rs
index 9cc77bd..399f499 100644
--- a/src-tauri/src/app_auto_updater.rs
+++ b/src-tauri/src/app_auto_updater.rs
@@ -768,42 +768,6 @@ impl AppAutoUpdater {
       .map(|a| a.browser_download_url.clone())
   }
 
-  /// Extract the hex digest for `filename` from standard `sha256sum` output
-  /// (`<hex>  <name>`, optionally with the `*` binary-mode marker).
-  fn find_checksum_for_file(checksums_text: &str, filename: &str) -> Option<String> {
-    checksums_text.lines().find_map(|line| {
-      let (hash, rest) = line.split_once(char::is_whitespace)?;
-      let name = rest.trim_start().trim_start_matches('*');
-      if name == filename && hash.len() == 64 && hash.bytes().all(|b| b.is_ascii_hexdigit()) {
-        Some(hash.to_ascii_lowercase())
-      } else {
-        None
-      }
-    })
-  }
-
-  fn sha256_file(path: &Path) -> Result<String, Box<dyn std::error::Error + Send + Sync>> {
-    use sha2::{Digest, Sha256};
-    use std::io::Read;
-    let mut file = fs::File::open(path)?;
-    let mut hasher = Sha256::new();
-    let mut buf = vec![0u8; 1024 * 1024];
-    loop {
-      let n = file.read(&mut buf)?;
-      if n == 0 {
-        break;
-      }
-      hasher.update(&buf[..n]);
-    }
-    let digest = hasher.finalize();
-    let mut hex = String::with_capacity(digest.len() * 2);
-    for byte in digest {
-      use std::fmt::Write;
-      let _ = write!(hex, "{byte:02x}");
-    }
-    Ok(hex)
-  }
-
   /// Fetch the release's SHA256SUMS.txt and return the expected digest for
   /// `filename`. Called BEFORE the (large) asset download so an unverifiable
   /// release is rejected without wasting the transfer. Every failure mode
@@ -857,7 +821,7 @@ impl AppAutoUpdater {
       }
     };
 
-    let Some(expected) = Self::find_checksum_for_file(&checksums_text, filename) else {
+    let Some(expected) = crate::checksum::find_checksum_for_file(&checksums_text, filename) else {
       log::warn!(
         "No checksum entry for {filename} in {}",
         Self::CHECKSUMS_ASSET_NAME
@@ -877,7 +841,7 @@ impl AppAutoUpdater {
     expected: &str,
     asset_digest: Option<&str>,
   ) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
-    let actual = Self::sha256_file(file_path)?;
+    let actual = crate::checksum::sha256_file(file_path)?;
 
     let mut mismatch = !actual.eq_ignore_ascii_case(expected);
 
@@ -2226,48 +2190,6 @@ mod tests {
     );
   }
 
-  #[test]
-  fn test_find_checksum_for_file() {
-    let sums = "\
-0e5a4601745092b7d1c93c1e7e1c30d923be3d1e916b661bd53d1c0c9c7f0a11  Donut_0.29.0_aarch64.dmg
-ABCDEF01745092B7D1C93C1E7E1C30D923BE3D1E916B661BD53D1C0C9C7F0A22 *Donut_0.29.0_x64.dmg
-not-a-hash  Donut_0.29.0_amd64.deb
-";
-
-    // Plain entry.
-    assert_eq!(
-      AppAutoUpdater::find_checksum_for_file(sums, "Donut_0.29.0_aarch64.dmg").as_deref(),
-      Some("0e5a4601745092b7d1c93c1e7e1c30d923be3d1e916b661bd53d1c0c9c7f0a11")
-    );
-    // Binary-mode marker is stripped; hash is normalized to lowercase.
-    assert_eq!(
-      AppAutoUpdater::find_checksum_for_file(sums, "Donut_0.29.0_x64.dmg").as_deref(),
-      Some("abcdef01745092b7d1c93c1e7e1c30d923be3d1e916b661bd53d1c0c9c7f0a22")
-    );
-    // Entries with malformed hashes are rejected rather than trusted.
-    assert_eq!(
-      AppAutoUpdater::find_checksum_for_file(sums, "Donut_0.29.0_amd64.deb"),
-      None
-    );
-    // Missing file.
-    assert_eq!(
-      AppAutoUpdater::find_checksum_for_file(sums, "Donut_0.29.0_arm64.deb"),
-      None
-    );
-  }
-
-  #[test]
-  fn test_sha256_file_matches_known_digest() {
-    let temp_dir = tempfile::TempDir::new().unwrap();
-    let path = temp_dir.path().join("data.bin");
-    std::fs::write(&path, b"hello world").unwrap();
-    assert_eq!(
-      AppAutoUpdater::sha256_file(&path).unwrap(),
-      // sha256 of "hello world"
-      "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9"
-    );
-  }
-
   #[test]
   fn test_find_checksums_url() {
     let assets = vec![
diff --git a/src-tauri/src/browser.rs b/src-tauri/src/browser.rs
index 9f998c6..b22e1bb 100644
--- a/src-tauri/src/browser.rs
+++ b/src-tauri/src/browser.rs
@@ -25,6 +25,14 @@ impl BrowserType {
     }
   }
 
+  /// Brand name for user-facing strings. `as_str` is the internal id and is
+  /// the wrong thing to put in a message the user reads.
+  pub fn display_name(&self) -> &'static str {
+    match self {
+      BrowserType::Wayfern => "Wayfern",
+    }
+  }
+
   pub fn from_str(s: &str) -> Result<Self, String> {
     match s {
       "wayfern" => Ok(BrowserType::Wayfern),
diff --git a/src-tauri/src/checksum.rs b/src-tauri/src/checksum.rs
new file mode 100644
index 0000000..802b4fc
--- /dev/null
+++ b/src-tauri/src/checksum.rs
@@ -0,0 +1,178 @@
+//! SHA256 helpers shared by the app self-updater and the browser downloader.
+//!
+//! Both verify a downloaded artifact against a digest published beside it, so
+//! the hashing and the `sha256sum` parsing live here instead of in either
+//! caller.
+
+use std::path::Path;
+
+/// Stream `path` through SHA256 and return the lowercase hex digest. Reads in
+/// 1 MiB blocks so a multi-gigabyte browser archive never lands in memory.
+pub fn sha256_file(path: &Path) -> Result<String, Box<dyn std::error::Error + Send + Sync>> {
+  use sha2::{Digest, Sha256};
+  use std::io::Read;
+  let mut file = std::fs::File::open(path)?;
+  let mut hasher = Sha256::new();
+  let mut buf = vec![0u8; 1024 * 1024];
+  loop {
+    let n = file.read(&mut buf)?;
+    if n == 0 {
+      break;
+    }
+    hasher.update(&buf[..n]);
+  }
+  let digest = hasher.finalize();
+  let mut hex = String::with_capacity(digest.len() * 2);
+  for byte in digest {
+    use std::fmt::Write;
+    let _ = write!(hex, "{byte:02x}");
+  }
+  Ok(hex)
+}
+
+/// Extract the hex digest for `filename` from standard `sha256sum` output
+/// (`<hex>  <name>`, optionally with the `*` binary-mode marker).
+pub fn find_checksum_for_file(checksums_text: &str, filename: &str) -> Option<String> {
+  checksums_text.lines().find_map(|line| {
+    let (hash, rest) = line.split_once(char::is_whitespace)?;
+    let name = rest.trim_start().trim_start_matches('*');
+    if name == filename && is_sha256_hex(hash) {
+      Some(hash.to_ascii_lowercase())
+    } else {
+      None
+    }
+  })
+}
+
+/// Digest from a single-asset `<file>.sha256` sidecar.
+///
+/// Prefers the entry named `filename`, because a name binds the digest to the
+/// asset it covers. Falls back to a lone digest only when the sidecar holds
+/// exactly one: `sha256sum < file` writes `-` as the name and some publishers
+/// emit the bare hash, and neither is ambiguous when it stands alone. A
+/// sidecar listing several assets always needs the name to match.
+pub fn parse_sidecar_digest(text: &str, filename: &str) -> Option<String> {
+  if let Some(named) = find_checksum_for_file(text, filename) {
+    return Some(named);
+  }
+
+  let mut digests = text
+    .lines()
+    .filter_map(|line| line.split_whitespace().next())
+    .filter(|token| is_sha256_hex(token));
+  let only = digests.next()?;
+  if digests.next().is_some() {
+    return None;
+  }
+  Some(only.to_ascii_lowercase())
+}
+
+fn is_sha256_hex(value: &str) -> bool {
+  value.len() == 64 && value.bytes().all(|b| b.is_ascii_hexdigit())
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  const HELLO_WORLD_SHA256: &str =
+    "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9";
+
+  #[test]
+  fn test_find_checksum_for_file() {
+    let sums = "\
+0e5a4601745092b7d1c93c1e7e1c30d923be3d1e916b661bd53d1c0c9c7f0a11  Donut_0.29.0_aarch64.dmg
+ABCDEF01745092B7D1C93C1E7E1C30D923BE3D1E916B661BD53D1C0C9C7F0A22 *Donut_0.29.0_x64.dmg
+not-a-hash  Donut_0.29.0_amd64.deb
+";
+
+    // Plain entry.
+    assert_eq!(
+      find_checksum_for_file(sums, "Donut_0.29.0_aarch64.dmg").as_deref(),
+      Some("0e5a4601745092b7d1c93c1e7e1c30d923be3d1e916b661bd53d1c0c9c7f0a11")
+    );
+    // Binary-mode marker is stripped; hash is normalized to lowercase.
+    assert_eq!(
+      find_checksum_for_file(sums, "Donut_0.29.0_x64.dmg").as_deref(),
+      Some("abcdef01745092b7d1c93c1e7e1c30d923be3d1e916b661bd53d1c0c9c7f0a22")
+    );
+    // Entries with malformed hashes are rejected rather than trusted.
+    assert_eq!(find_checksum_for_file(sums, "Donut_0.29.0_amd64.deb"), None);
+    // Missing file.
+    assert_eq!(find_checksum_for_file(sums, "Donut_0.29.0_arm64.deb"), None);
+  }
+
+  #[test]
+  fn test_sha256_file_matches_known_digest() {
+    let temp_dir = tempfile::TempDir::new().unwrap();
+    let path = temp_dir.path().join("data.bin");
+    std::fs::write(&path, b"hello world").unwrap();
+    assert_eq!(sha256_file(&path).unwrap(), HELLO_WORLD_SHA256);
+  }
+
+  #[test]
+  fn test_parse_sidecar_digest_prefers_the_named_entry() {
+    // The real Wayfern sidecar shape: `<hex>  <name>`, one asset per file.
+    let sidecar = format!("{HELLO_WORLD_SHA256}  wayfern-151.0.7922.71_windows_x64.zip\n");
+    assert_eq!(
+      parse_sidecar_digest(&sidecar, "wayfern-151.0.7922.71_windows_x64.zip").as_deref(),
+      Some(HELLO_WORLD_SHA256)
+    );
+  }
+
+  #[test]
+  fn test_parse_sidecar_digest_accepts_an_unnamed_lone_digest() {
+    // `sha256sum < file` writes `-` as the name, and some publishers emit the
+    // bare hash. Both cover the one asset the sidecar sits beside.
+    for sidecar in [
+      format!("{HELLO_WORLD_SHA256}  -\n"),
+      format!("{HELLO_WORLD_SHA256}\n"),
+      format!("  {HELLO_WORLD_SHA256}  \n"),
+    ] {
+      assert_eq!(
+        parse_sidecar_digest(&sidecar, "wayfern.zip").as_deref(),
+        Some(HELLO_WORLD_SHA256),
+        "should accept lone digest in {sidecar:?}"
+      );
+    }
+  }
+
+  #[test]
+  fn test_parse_sidecar_digest_normalizes_case() {
+    let sidecar = format!("{}  -\n", HELLO_WORLD_SHA256.to_ascii_uppercase());
+    assert_eq!(
+      parse_sidecar_digest(&sidecar, "wayfern.zip").as_deref(),
+      Some(HELLO_WORLD_SHA256)
+    );
+  }
+
+  #[test]
+  fn test_parse_sidecar_digest_rejects_an_ambiguous_multi_entry_sidecar() {
+    let sidecar = format!(
+      "{HELLO_WORLD_SHA256}  other.zip\n\
+       ABCDEF01745092B7D1C93C1E7E1C30D923BE3D1E916B661BD53D1C0C9C7F0A22  another.zip\n"
+    );
+    // Two candidates and neither is named `wayfern.zip`: guessing would defeat
+    // the point of the check.
+    assert_eq!(parse_sidecar_digest(&sidecar, "wayfern.zip"), None);
+  }
+
+  #[test]
+  fn test_parse_sidecar_digest_rejects_junk() {
+    assert_eq!(parse_sidecar_digest("", "wayfern.zip"), None);
+    assert_eq!(
+      parse_sidecar_digest("not-a-hash  wayfern.zip", "wayfern.zip"),
+      None
+    );
+    // An HTML error page served with HTTP 200 must not read as a digest.
+    assert_eq!(
+      parse_sidecar_digest("<!doctype html><title>404</title>", "wayfern.zip"),
+      None
+    );
+    // Right shape, wrong length.
+    assert_eq!(
+      parse_sidecar_digest("abc123  wayfern.zip", "wayfern.zip"),
+      None
+    );
+  }
+}
diff --git a/src-tauri/src/downloader.rs b/src-tauri/src/downloader.rs
index 9761b9b..da15191 100644
--- a/src-tauri/src/downloader.rs
+++ b/src-tauri/src/downloader.rs
@@ -15,6 +15,10 @@ use crate::events;
 // the UI can surface it and the caller can move on / retry.
 const STREAM_IDLE_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(60);
 
+// Sent on both the asset request and its checksum sidecar so the CDN sees one
+// consistent client for the pair.
+const DOWNLOAD_USER_AGENT: &str = "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36";
+
 // Global state to track currently downloading browser-version pairs
 lazy_static::lazy_static! {
   static ref DOWNLOADING_BROWSERS: std::sync::Arc<Mutex<std::collections::HashSet<String>>> =
@@ -233,6 +237,113 @@ impl Downloader {
       .await?;
     log::info!("Download URL resolved");
 
+    // Every browser asset is published with a `<asset>.sha256` sidecar. Fetch
+    // it before the transfer starts: an asset nobody can verify costs one small
+    // request to reject here, or a wasted gigabyte to reject later.
+    let expected_sha256 = self
+      .fetch_expected_archive_checksum(&download_url, browser_type.display_name(), version)
+      .await?;
+
+    let file_path = self
+      .stream_download(
+        browser_type.clone(),
+        version,
+        &download_url,
+        file_path,
+        cancel_token,
+      )
+      .await?;
+
+    // Hashing a multi-gigabyte archive takes seconds, so tell the UI what the
+    // pause is for instead of leaving the bar sitting at 100%.
+    let _ = events::emit(
+      "download-progress",
+      &DownloadProgress {
+        browser: browser_type.as_str().to_string(),
+        version: version.to_string(),
+        downloaded_bytes: 0,
+        total_bytes: None,
+        percentage: 100.0,
+        speed_bytes_per_sec: 0.0,
+        eta_seconds: None,
+        stage: "verifying".to_string(),
+      },
+    );
+    verify_archive_checksum(
+      &file_path,
+      &expected_sha256,
+      browser_type.display_name(),
+      version,
+    )
+    .await?;
+
+    Ok(file_path)
+  }
+
+  /// Fetch `<asset>.sha256` and return the digest it publishes for the asset.
+  /// Every failure mode maps to `BROWSER_CHECKSUM_UNAVAILABLE`; the specifics
+  /// go to the log.
+  async fn fetch_expected_archive_checksum(
+    &self,
+    download_url: &str,
+    browser: &str,
+    version: &str,
+  ) -> Result<String, Box<dyn std::error::Error + Send + Sync>> {
+    let unavailable = || -> Box<dyn std::error::Error + Send + Sync> {
+      serde_json::json!({
+        "code": "BROWSER_CHECKSUM_UNAVAILABLE",
+        "params": { "browser": browser, "version": version }
+      })
+      .to_string()
+      .into()
+    };
+
+    let sidecar_url = checksum_sidecar_url(download_url);
+    let response = match self
+      .client
+      .get(&sidecar_url)
+      .header("User-Agent", DOWNLOAD_USER_AGENT)
+      .send()
+      .await
+    {
+      Ok(response) if response.status().is_success() => response,
+      Ok(response) => {
+        log::warn!(
+          "Checksum sidecar request failed for {browser} {version}: HTTP {}",
+          response.status()
+        );
+        return Err(unavailable());
+      }
+      Err(e) => {
+        log::warn!("Checksum sidecar request failed for {browser} {version}: {e}");
+        return Err(unavailable());
+      }
+    };
+
+    let sidecar_text = match response.text().await {
+      Ok(text) => text,
+      Err(e) => {
+        log::warn!("Failed to read the checksum sidecar for {browser} {version}: {e}");
+        return Err(unavailable());
+      }
+    };
+
+    let asset_name = asset_filename_from_url(download_url);
+    let Some(expected) = crate::checksum::parse_sidecar_digest(&sidecar_text, asset_name) else {
+      log::warn!("No usable digest for {asset_name} in {sidecar_url}");
+      return Err(unavailable());
+    };
+    Ok(expected)
+  }
+
+  async fn stream_download(
+    &self,
+    browser_type: BrowserType,
+    version: &str,
+    download_url: &str,
+    file_path: PathBuf,
+    cancel_token: Option<&CancellationToken>,
+  ) -> Result<PathBuf, Box<dyn std::error::Error + Send + Sync>> {
     // In-session resume: a large (~1GB) download over a flaky connection can
     // drop mid-stream. Rather than surfacing the first stall/chunk error as a
     // terminal failure (which forces the user to re-click and risks the CDN
@@ -257,11 +368,8 @@ impl Downloader {
       for attempt in 0..=max_send_retries {
         let mut request = self
           .client
-          .get(&download_url)
-          .header(
-            "User-Agent",
-            "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36",
-          );
+          .get(download_url)
+          .header("User-Agent", DOWNLOAD_USER_AGENT);
 
         if existing_size > 0 {
           request = request.header("Range", format!("bytes={existing_size}-"));
@@ -682,7 +790,7 @@ impl Downloader {
         };
         let _ = events::emit("download-progress", &progress);
 
-        return Err(format!("Failed to download browser: {e}").into());
+        return Err(contextualize("Failed to download browser", e));
       }
     };
 
@@ -872,6 +980,74 @@ impl Downloader {
   }
 }
 
+/// Offset of a URL's query or fragment, or its length when it has neither.
+fn url_path_end(url: &str) -> usize {
+  url.find(['?', '#']).unwrap_or(url.len())
+}
+
+/// The `<asset>.sha256` published next to every browser asset. Any query or
+/// fragment stays at the end so a signed URL keeps working.
+fn checksum_sidecar_url(download_url: &str) -> String {
+  let (base, suffix) = download_url.split_at(url_path_end(download_url));
+  format!("{base}.sha256{suffix}")
+}
+
+/// Last path segment of `url`. This is the name a `sha256sum` sidecar records,
+/// and it is not the local filename: the local one is built from the running
+/// platform, while this one is whatever the publisher called the asset.
+fn asset_filename_from_url(url: &str) -> &str {
+  url[..url_path_end(url)].rsplit('/').next().unwrap_or("")
+}
+
+/// Compare the finished archive against the digest published beside it. A
+/// mismatch means the bytes on disk are not the asset the manifest promised,
+/// so the file is deleted instead of being handed to the extractor.
+async fn verify_archive_checksum(
+  file_path: &Path,
+  expected: &str,
+  browser: &str,
+  version: &str,
+) -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+  let hash_path = file_path.to_path_buf();
+  let actual = tokio::task::spawn_blocking(move || crate::checksum::sha256_file(&hash_path))
+    .await
+    .map_err(|e| -> Box<dyn std::error::Error + Send + Sync> {
+      format!("Checksum task failed: {e}").into()
+    })??;
+
+  if actual.eq_ignore_ascii_case(expected) {
+    log::info!("Checksum verified for {browser} {version}: {actual}");
+    return Ok(());
+  }
+
+  log::error!("Checksum mismatch for {browser} {version}: expected {expected}, got {actual}");
+  let _ = std::fs::remove_file(file_path);
+  Err(
+    serde_json::json!({
+      "code": "BROWSER_CHECKSUM_MISMATCH",
+      "params": { "browser": browser, "version": version }
+    })
+    .to_string()
+    .into(),
+  )
+}
+
+/// Prefix the calling context onto a bare message, but leave an already-coded
+/// backend error alone: `wrap_backend_error` only passes a payload through
+/// when it still starts with `{`, so prefixing one would strip the code and
+/// the frontend would fall back to the untranslated INTERNAL_ERROR text.
+fn contextualize(
+  context: &str,
+  e: impl std::fmt::Display,
+) -> Box<dyn std::error::Error + Send + Sync> {
+  let msg = e.to_string();
+  if msg.starts_with('{') {
+    msg.into()
+  } else {
+    format!("{context}: {msg}").into()
+  }
+}
+
 /// Check if a specific browser-version pair is currently being downloaded
 pub fn is_downloading(browser: &str, version: &str) -> bool {
   let download_key = format!("{browser}-{version}");
@@ -1040,6 +1216,183 @@ mod tests {
     assert_eq!(downloaded_content.len(), test_content.len());
   }
 
+  // Stand-in archive body for the checksum tests. Digests are computed from
+  // it rather than hardcoded, so the fixture and the assertion cannot drift.
+  const ARCHIVE_BODY: &[u8] = b"wayfern archive bytes";
+
+  fn digest_of(bytes: &[u8]) -> String {
+    let temp_dir = TempDir::new().unwrap();
+    let path = temp_dir.path().join("archive.zip");
+    std::fs::write(&path, bytes).unwrap();
+    crate::checksum::sha256_file(&path).unwrap()
+  }
+
+  #[test]
+  fn test_checksum_sidecar_url_appends_to_the_asset_path() {
+    assert_eq!(
+      checksum_sidecar_url("https://download.wayfern.com/wayfern-151_windows_x64.zip"),
+      "https://download.wayfern.com/wayfern-151_windows_x64.zip.sha256"
+    );
+    // A signed URL keeps its query, so the sidecar stays reachable.
+    assert_eq!(
+      checksum_sidecar_url("https://cdn.example.com/a.zip?token=abc&exp=1"),
+      "https://cdn.example.com/a.zip.sha256?token=abc&exp=1"
+    );
+    assert_eq!(
+      checksum_sidecar_url("https://cdn.example.com/a.zip#frag"),
+      "https://cdn.example.com/a.zip.sha256#frag"
+    );
+  }
+
+  #[test]
+  fn test_asset_filename_from_url_takes_the_publisher_name() {
+    // Deliberately different from the local filename, which is built from the
+    // running platform and would never match a sidecar entry.
+    assert_eq!(
+      asset_filename_from_url("https://download.wayfern.com/wayfern-151.0.7922.71_windows_x64.zip"),
+      "wayfern-151.0.7922.71_windows_x64.zip"
+    );
+    assert_eq!(
+      asset_filename_from_url("https://cdn.example.com/dir/a.tar.xz?token=abc"),
+      "a.tar.xz"
+    );
+    assert_eq!(asset_filename_from_url("https://cdn.example.com/"), "");
+  }
+
+  #[tokio::test]
+  async fn test_fetch_expected_archive_checksum_reads_the_sidecar() {
+    let server = MockServer::start().await;
+    let downloader = Downloader::new_for_test();
+    let digest = digest_of(ARCHIVE_BODY);
+
+    Mock::given(method("GET"))
+      .and(path("/wayfern-151_windows_x64.zip.sha256"))
+      .respond_with(
+        ResponseTemplate::new(200)
+          .set_body_string(format!("{digest}  wayfern-151_windows_x64.zip\n")),
+      )
+      .mount(&server)
+      .await;
+
+    let url = format!("{}/wayfern-151_windows_x64.zip", server.uri());
+    let expected = downloader
+      .fetch_expected_archive_checksum(&url, "wayfern", "151")
+      .await
+      .expect("sidecar should resolve");
+
+    assert_eq!(expected, digest);
+  }
+
+  #[tokio::test]
+  async fn test_fetch_expected_archive_checksum_fails_when_the_sidecar_is_missing() {
+    let server = MockServer::start().await;
+    let downloader = Downloader::new_for_test();
+
+    Mock::given(method("GET"))
+      .and(path("/wayfern-151_windows_x64.zip.sha256"))
+      .respond_with(ResponseTemplate::new(404))
+      .mount(&server)
+      .await;
+
+    let url = format!("{}/wayfern-151_windows_x64.zip", server.uri());
+    let error = downloader
+      .fetch_expected_archive_checksum(&url, "wayfern", "151")
+      .await
+      .expect_err("an unverifiable asset must not be downloaded")
+      .to_string();
+
+    assert!(
+      error.contains("BROWSER_CHECKSUM_UNAVAILABLE") && error.contains("151"),
+      "expected a coded, translatable error, got: {error}"
+    );
+  }
+
+  #[tokio::test]
+  async fn test_fetch_expected_archive_checksum_rejects_a_sidecar_without_a_digest() {
+    let server = MockServer::start().await;
+    let downloader = Downloader::new_for_test();
+
+    // A CDN that answers 200 with an error page must not be read as a digest.
+    Mock::given(method("GET"))
+      .and(path("/wayfern-151_windows_x64.zip.sha256"))
+      .respond_with(ResponseTemplate::new(200).set_body_string("<!doctype html><title>404</title>"))
+      .mount(&server)
+      .await;
+
+    let url = format!("{}/wayfern-151_windows_x64.zip", server.uri());
+    let error = downloader
+      .fetch_expected_archive_checksum(&url, "wayfern", "151")
+      .await
+      .expect_err("an unparsable sidecar must not pass")
+      .to_string();
+
+    assert!(
+      error.contains("BROWSER_CHECKSUM_UNAVAILABLE"),
+      "expected a coded error, got: {error}"
+    );
+  }
+
+  #[tokio::test]
+  async fn test_verify_archive_checksum_accepts_a_matching_digest() {
+    let temp_dir = TempDir::new().unwrap();
+    let archive = temp_dir.path().join("wayfern.zip");
+    std::fs::write(&archive, ARCHIVE_BODY).unwrap();
+    let digest = crate::checksum::sha256_file(&archive).unwrap();
+
+    // Case is normalized on both sides, so an uppercase sidecar still matches.
+    verify_archive_checksum(
+      &archive,
+      &digest.to_ascii_uppercase(),
+      "wayfern",
+      "151.0.7922.71",
+    )
+    .await
+    .expect("a matching digest should verify");
+
+    assert!(archive.exists(), "a verified archive must be kept");
+  }
+
+  #[tokio::test]
+  async fn test_verify_archive_checksum_rejects_and_deletes_a_mismatch() {
+    let temp_dir = TempDir::new().unwrap();
+    let archive = temp_dir.path().join("wayfern.zip");
+    // The file on disk is the wrong asset entirely, which is what a mislinked
+    // manifest slot delivers, while the sidecar describes the right one.
+    std::fs::write(&archive, b"a macOS disk image, not a windows zip").unwrap();
+    let expected = digest_of(ARCHIVE_BODY);
+
+    let error = verify_archive_checksum(&archive, &expected, "wayfern", "151.0.7922.71")
+      .await
+      .expect_err("a mismatched digest must fail")
+      .to_string();
+
+    assert!(
+      error.contains("BROWSER_CHECKSUM_MISMATCH") && error.contains("151.0.7922.71"),
+      "expected a coded, translatable error, got: {error}"
+    );
+    assert!(
+      !archive.exists(),
+      "an archive that failed verification must be deleted, not extracted"
+    );
+  }
+
+  #[test]
+  fn test_contextualize_preserves_a_coded_backend_error() {
+    // A coded payload must survive untouched: wrap_backend_error only passes
+    // it through while it still starts with '{'.
+    let coded = r#"{"code":"BROWSER_CHECKSUM_MISMATCH","params":{"browser":"wayfern"}}"#;
+    assert_eq!(
+      contextualize("Failed to download browser", coded).to_string(),
+      coded
+    );
+
+    // A bare message still gets its context.
+    assert_eq!(
+      contextualize("Failed to download browser", "connection reset").to_string(),
+      "Failed to download browser: connection reset"
+    );
+  }
+
   #[test]
   fn test_clear_download_state_for_browser_removes_stuck_keys() {
     // Simulate a download future that was abandoned without running its own cleanup,
diff --git a/src-tauri/src/lib.rs b/src-tauri/src/lib.rs
index d3074fd..945e284 100644
--- a/src-tauri/src/lib.rs
+++ b/src-tauri/src/lib.rs
@@ -62,6 +62,7 @@ mod browser;
 mod browser_runner;
 mod browser_version_manager;
 mod cdp_target;
+mod checksum;
 mod default_browser;
 pub mod dns_blocklist;
 mod downloaded_browsers_registry;
diff --git a/src/i18n/locales/en.json b/src/i18n/locales/en.json
index 52bf16b..036e7d5 100644
--- a/src/i18n/locales/en.json
+++ b/src/i18n/locales/en.json
@@ -1890,6 +1890,8 @@
     "camoufoxImportDeprecated": "Importing this profile type is no longer supported. Please use Wayfern instead.",
     "updateChecksumsUnavailable": "The update {{version}} could not be verified because its checksum file could not be retrieved. The update was not installed; it will be retried later.",
     "updateChecksumMismatch": "The downloaded update file {{file}} failed checksum verification and was discarded. Please try again.",
+    "browserChecksumUnavailable": "{{browser}} {{version}} could not be verified because its checksum file could not be retrieved. The download was stopped; it will be retried later.",
+    "browserChecksumMismatch": "The downloaded {{browser}} {{version}} archive failed checksum verification and was discarded. Please try again.",
     "nameCannotBeEmpty": "Name cannot be empty",
     "wayfernVersionNotAvailable": "Wayfern version {{requested}} is not available for download. The current version is {{current}}.",
     "profileNameExists": "A profile named \"{{name}}\" already exists",
diff --git a/src/i18n/locales/es.json b/src/i18n/locales/es.json
index e5d1470..9531eb7 100644
--- a/src/i18n/locales/es.json
+++ b/src/i18n/locales/es.json
@@ -1896,6 +1896,8 @@
     "camoufoxImportDeprecated": "La importación de este tipo de perfil ya no es compatible. Utiliza Wayfern en su lugar.",
     "updateChecksumsUnavailable": "No se pudo verificar la actualización {{version}} porque no se pudo obtener su archivo de sumas de comprobación. La actualización no se instaló; se reintentará más tarde.",
     "updateChecksumMismatch": "El archivo de actualización descargado {{file}} no superó la verificación de suma de comprobación y fue descartado. Inténtalo de nuevo.",
+    "browserChecksumUnavailable": "No se pudo verificar {{browser}} {{version}} porque no se pudo obtener su archivo de sumas de comprobación. La descarga se detuvo; se reintentará más tarde.",
+    "browserChecksumMismatch": "El archivo descargado de {{browser}} {{version}} no superó la verificación de suma de comprobación y fue descartado. Inténtalo de nuevo.",
     "nameCannotBeEmpty": "El nombre no puede estar vacío",
     "wayfernVersionNotAvailable": "La versión {{requested}} de Wayfern no está disponible para descargar. La versión actual es {{current}}.",
     "profileNameExists": "Ya existe un perfil llamado \"{{name}}\"",
diff --git a/src/i18n/locales/fr.json b/src/i18n/locales/fr.json
index 4f56520..2b91366 100644
--- a/src/i18n/locales/fr.json
+++ b/src/i18n/locales/fr.json
@@ -1896,6 +1896,8 @@
     "camoufoxImportDeprecated": "L'importation de ce type de profil n'est plus prise en charge. Veuillez utiliser Wayfern à la place.",
     "updateChecksumsUnavailable": "La mise à jour {{version}} n'a pas pu être vérifiée car son fichier de sommes de contrôle n'a pas pu être récupéré. La mise à jour n'a pas été installée ; une nouvelle tentative aura lieu plus tard.",
     "updateChecksumMismatch": "Le fichier de mise à jour téléchargé {{file}} a échoué à la vérification de la somme de contrôle et a été supprimé. Veuillez réessayer.",
+    "browserChecksumUnavailable": "{{browser}} {{version}} n'a pas pu être vérifié car son fichier de sommes de contrôle n'a pas pu être récupéré. Le téléchargement a été interrompu ; une nouvelle tentative aura lieu plus tard.",
+    "browserChecksumMismatch": "L'archive {{browser}} {{version}} téléchargée a échoué à la vérification de la somme de contrôle et a été supprimée. Veuillez réessayer.",
     "nameCannotBeEmpty": "Le nom ne peut pas être vide",
     "wayfernVersionNotAvailable": "La version {{requested}} de Wayfern n'est pas disponible au téléchargement. La version actuelle est {{current}}.",
     "profileNameExists": "Un profil nommé « {{name}} » existe déjà",
diff --git a/src/i18n/locales/ja.json b/src/i18n/locales/ja.json
index 4f18246..15b28fd 100644
--- a/src/i18n/locales/ja.json
+++ b/src/i18n/locales/ja.json
@@ -1889,6 +1889,8 @@
     "camoufoxImportDeprecated": "このタイプのプロファイルのインポートはサポートされなくなりました。代わりにWayfernを使用してください。",
     "updateChecksumsUnavailable": "アップデート {{version}} のチェックサムファイルを取得できなかったため、検証できませんでした。アップデートはインストールされませんでした。後で再試行されます。",
     "updateChecksumMismatch": "ダウンロードしたアップデートファイル {{file}} はチェックサム検証に失敗したため破棄されました。もう一度お試しください。",
+    "browserChecksumUnavailable": "{{browser}} {{version}} のチェックサムファイルを取得できなかったため、検証できませんでした。ダウンロードは中止されました。後で再試行されます。",
+    "browserChecksumMismatch": "ダウンロードした {{browser}} {{version}} のアーカイブはチェックサム検証に失敗したため破棄されました。もう一度お試しください。",
     "nameCannotBeEmpty": "名前を空にすることはできません",
     "wayfernVersionNotAvailable": "Wayfernのバージョン{{requested}}はダウンロードできません。現在のバージョンは{{current}}です。",
     "profileNameExists": "「{{name}}」という名前のプロファイルは既に存在します",
diff --git a/src/i18n/locales/ko.json b/src/i18n/locales/ko.json
index 5d3635b..a817cfd 100644
--- a/src/i18n/locales/ko.json
+++ b/src/i18n/locales/ko.json
@@ -1889,6 +1889,8 @@
     "camoufoxImportDeprecated": "이 유형의 프로필 가져오기는 더 이상 지원되지 않습니다. 대신 Wayfern을 사용하세요.",
     "updateChecksumsUnavailable": "업데이트 {{version}}의 체크섬 파일을 가져올 수 없어 검증하지 못했습니다. 업데이트가 설치되지 않았으며 나중에 다시 시도됩니다.",
     "updateChecksumMismatch": "다운로드한 업데이트 파일 {{file}}이(가) 체크섬 검증에 실패하여 삭제되었습니다. 다시 시도해 주세요.",
+    "browserChecksumUnavailable": "{{browser}} {{version}}의 체크섬 파일을 가져올 수 없어 검증하지 못했습니다. 다운로드가 중단되었으며 나중에 다시 시도됩니다.",
+    "browserChecksumMismatch": "다운로드한 {{browser}} {{version}} 아카이브가 체크섬 검증에 실패하여 삭제되었습니다. 다시 시도해 주세요.",
     "nameCannotBeEmpty": "이름은 비워둘 수 없습니다",
     "wayfernVersionNotAvailable": "Wayfern 버전 {{requested}}은(는) 다운로드할 수 없습니다. 현재 버전은 {{current}}입니다.",
     "profileNameExists": "\"{{name}}\" 이름의 프로필이 이미 있습니다",
diff --git a/src/i18n/locales/pt.json b/src/i18n/locales/pt.json
index ddb7393..f5b3443 100644
--- a/src/i18n/locales/pt.json
+++ b/src/i18n/locales/pt.json
@@ -1896,6 +1896,8 @@
     "camoufoxImportDeprecated": "A importação deste tipo de perfil não é mais suportada. Use o Wayfern em vez disso.",
     "updateChecksumsUnavailable": "Não foi possível verificar a atualização {{version}} porque o arquivo de somas de verificação não pôde ser obtido. A atualização não foi instalada; será tentada novamente mais tarde.",
     "updateChecksumMismatch": "O arquivo de atualização baixado {{file}} falhou na verificação de soma de verificação e foi descartado. Tente novamente.",
+    "browserChecksumUnavailable": "Não foi possível verificar o {{browser}} {{version}} porque o arquivo de somas de verificação não pôde ser obtido. O download foi interrompido; será tentado novamente mais tarde.",
+    "browserChecksumMismatch": "O arquivo baixado do {{browser}} {{version}} falhou na verificação de soma de verificação e foi descartado. Tente novamente.",
     "nameCannotBeEmpty": "O nome não pode estar vazio",
     "wayfernVersionNotAvailable": "A versão {{requested}} do Wayfern não está disponível para download. A versão atual é {{current}}.",
     "profileNameExists": "Já existe um perfil chamado \"{{name}}\"",
diff --git a/src/i18n/locales/ru.json b/src/i18n/locales/ru.json
index 3a657ff..58a734a 100644
--- a/src/i18n/locales/ru.json
+++ b/src/i18n/locales/ru.json
@@ -1903,6 +1903,8 @@
     "camoufoxImportDeprecated": "Импорт профилей этого типа больше не поддерживается. Используйте Wayfern.",
     "updateChecksumsUnavailable": "Не удалось проверить обновление {{version}}: файл контрольных сумм не удалось получить. Обновление не было установлено; попытка будет повторена позже.",
     "updateChecksumMismatch": "Загруженный файл обновления {{file}} не прошёл проверку контрольной суммы и был удалён. Попробуйте ещё раз.",
+    "browserChecksumUnavailable": "Не удалось проверить {{browser}} {{version}}: файл контрольных сумм не удалось получить. Загрузка остановлена; попытка будет повторена позже.",
+    "browserChecksumMismatch": "Загруженный архив {{browser}} {{version}} не прошёл проверку контрольной суммы и был удалён. Попробуйте ещё раз.",
     "nameCannotBeEmpty": "Имя не может быть пустым",
     "wayfernVersionNotAvailable": "Версия Wayfern {{requested}} недоступна для загрузки. Текущая версия — {{current}}.",
     "profileNameExists": "Профиль с именем «{{name}}» уже существует",
diff --git a/src/i18n/locales/tr.json b/src/i18n/locales/tr.json
index 2b3bf31..7e8f4d2 100644
--- a/src/i18n/locales/tr.json
+++ b/src/i18n/locales/tr.json
@@ -1889,6 +1889,8 @@
     "camoufoxImportDeprecated": "Bu profil türünün içe aktarılması artık desteklenmiyor. Lütfen bunun yerine Wayfern kullanın.",
     "updateChecksumsUnavailable": "{{version}} güncellemesi doğrulanamadı çünkü sağlama toplamı dosyası alınamadı. Güncelleme yüklenmedi; daha sonra yeniden denenecek.",
     "updateChecksumMismatch": "İndirilen güncelleme dosyası {{file}} sağlama toplamı doğrulamasını geçemedi ve silindi. Lütfen yeniden deneyin.",
+    "browserChecksumUnavailable": "{{browser}} {{version}} doğrulanamadı çünkü sağlama toplamı dosyası alınamadı. İndirme durduruldu; daha sonra yeniden denenecek.",
+    "browserChecksumMismatch": "İndirilen {{browser}} {{version}} arşivi sağlama toplamı doğrulamasını geçemedi ve silindi. Lütfen yeniden deneyin.",
     "nameCannotBeEmpty": "Ad boş olamaz",
     "wayfernVersionNotAvailable": "Wayfern {{requested}} sürümü indirilemiyor. Güncel sürüm: {{current}}.",
     "profileNameExists": "\"{{name}}\" adlı bir profil zaten var",
diff --git a/src/i18n/locales/vi.json b/src/i18n/locales/vi.json
index 8345639..84b18ac 100644
--- a/src/i18n/locales/vi.json
+++ b/src/i18n/locales/vi.json
@@ -1889,6 +1889,8 @@
     "camoufoxImportDeprecated": "Việc nhập loại hồ sơ này không còn được hỗ trợ. Vui lòng sử dụng Wayfern thay thế.",
     "updateChecksumsUnavailable": "Không thể xác minh bản cập nhật {{version}} vì không thể tải tệp checksum. Bản cập nhật chưa được cài đặt; sẽ thử lại sau.",
     "updateChecksumMismatch": "Tệp cập nhật đã tải xuống {{file}} không vượt qua kiểm tra checksum và đã bị loại bỏ. Vui lòng thử lại.",
+    "browserChecksumUnavailable": "Không thể xác minh {{browser}} {{version}} vì không thể tải tệp checksum. Quá trình tải xuống đã dừng; sẽ thử lại sau.",
+    "browserChecksumMismatch": "Kho lưu trữ {{browser}} {{version}} đã tải xuống không vượt qua kiểm tra checksum và đã bị loại bỏ. Vui lòng thử lại.",
     "nameCannotBeEmpty": "Tên không được để trống",
     "wayfernVersionNotAvailable": "Phiên bản Wayfern {{requested}} không có sẵn để tải xuống. Phiên bản hiện tại là {{current}}.",
     "profileNameExists": "Hồ sơ có tên \"{{name}}\" đã tồn tại",
diff --git a/src/i18n/locales/zh.json b/src/i18n/locales/zh.json
index 75372b2..3aa68a0 100644
--- a/src/i18n/locales/zh.json
+++ b/src/i18n/locales/zh.json
@@ -1889,6 +1889,8 @@
     "camoufoxImportDeprecated": "不再支持导入此类型的配置文件。请改用 Wayfern。",
     "updateChecksumsUnavailable": "无法验证更新 {{version}}：无法获取其校验和文件。更新未安装，稍后将重试。",
     "updateChecksumMismatch": "下载的更新文件 {{file}} 未通过校验和验证，已被丢弃。请重试。",
+    "browserChecksumUnavailable": "无法验证 {{browser}} {{version}}：无法获取其校验和文件。下载已停止，稍后将重试。",
+    "browserChecksumMismatch": "下载的 {{browser}} {{version}} 压缩包未通过校验和验证，已被丢弃。请重试。",
     "nameCannotBeEmpty": "名称不能为空",
     "wayfernVersionNotAvailable": "Wayfern 版本 {{requested}} 无法下载。当前版本为 {{current}}。",
     "profileNameExists": "名为“{{name}}”的配置文件已存在",
diff --git a/src/lib/backend-errors.ts b/src/lib/backend-errors.ts
index 2327179..9c4a0f4 100644
--- a/src/lib/backend-errors.ts
+++ b/src/lib/backend-errors.ts
@@ -48,6 +48,8 @@ export type BackendErrorCode =
   | "PROXY_SIDECAR_VERSION_MISMATCH"
   | "UPDATE_CHECKSUMS_UNAVAILABLE"
   | "UPDATE_CHECKSUM_MISMATCH"
+  | "BROWSER_CHECKSUM_UNAVAILABLE"
+  | "BROWSER_CHECKSUM_MISMATCH"
   | "UPDATE_PROFILES_RUNNING"
   | "UPDATE_PREPARATION_FAILED"
   | "PROFILE_NAME_EXISTS"
@@ -287,6 +289,16 @@ export function translateBackendError(t: TFunction, err: unknown): string {
       return t("backendErrors.updateChecksumMismatch", {
         file: parsed.params?.file ?? "",
       });
+    case "BROWSER_CHECKSUM_UNAVAILABLE":
+      return t("backendErrors.browserChecksumUnavailable", {
+        browser: parsed.params?.browser ?? "",
+        version: parsed.params?.version ?? "",
+      });
+    case "BROWSER_CHECKSUM_MISMATCH":
+      return t("backendErrors.browserChecksumMismatch", {
+        browser: parsed.params?.browser ?? "",
+        version: parsed.params?.version ?? "",
+      });
     case "UPDATE_PROFILES_RUNNING":
       return t("backendErrors.updateProfilesRunning");
     case "UPDATE_PREPARATION_FAILED":