diff --git a/Cargo.lock b/Cargo.lock
index c420660..2a2e825 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -436,7 +436,7 @@ dependencies = [
 
 [[package]]
 name = "dosh"
-version = "1.0.0-rc25"
+version = "1.0.0-rc26"
 dependencies = [
  "anyhow",
  "base64",
diff --git a/Cargo.toml b/Cargo.toml
index 225a906..f01a788 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "dosh"
-version = "1.0.0-rc25"
+version = "1.0.0-rc26"
 edition = "2024"
 license = "MIT"
 
diff --git a/src/bin/dosh-client.rs b/src/bin/dosh-client.rs
index 011a7ef..539dd8b 100644
--- a/src/bin/dosh-client.rs
+++ b/src/bin/dosh-client.rs
@@ -2266,8 +2266,11 @@ fn upload_file(
         overwrite: opts.overwrite,
         resume: opts.resume,
     })?;
-    let offset = match client.recv()? {
-        FileResponse::Resume { offset } => offset.min(size),
+    let (offset, prefix_sha256) = match client.recv()? {
+        FileResponse::Resume {
+            offset,
+            prefix_sha256,
+        } => (offset.min(size), prefix_sha256),
         FileResponse::Error { message } => return Err(anyhow!(message)),
         other => bail!("unexpected upload start response: {other:?}"),
     };
@@ -2275,6 +2278,7 @@ fn upload_file(
     let mut hasher = Sha256::new();
     if offset > 0 {
         hash_prefix(&mut file, offset, &mut hasher)?;
+        ensure_resume_prefix_matches(prefix_sha256, Some(digest_prefix(&hasher)), offset, remote)?;
     }
     file.seek(std::io::SeekFrom::Start(offset))?;
     let mut sent = offset;
@@ -2415,8 +2419,11 @@ fn copy_remote_file(
         overwrite: opts.overwrite,
         resume: opts.resume,
     })?;
-    let offset = match dst_client.recv()? {
-        FileResponse::Resume { offset } => offset.min(meta.len),
+    let (offset, dst_prefix_sha256) = match dst_client.recv()? {
+        FileResponse::Resume {
+            offset,
+            prefix_sha256,
+        } => (offset.min(meta.len), prefix_sha256),
         FileResponse::Error { message } => return Err(anyhow!(message)),
         other => bail!("unexpected remote copy start response: {other:?}"),
     };
@@ -2425,7 +2432,17 @@ fn copy_remote_file(
         offset,
     })?;
     match src_client.recv()? {
-        FileResponse::Start { .. } => {}
+        FileResponse::Start {
+            offset: start_offset,
+            prefix_sha256,
+            ..
+        } => {
+            anyhow::ensure!(
+                start_offset == offset,
+                "remote copy source offset mismatch: got {start_offset}, expected {offset}"
+            );
+            ensure_matching_resume_prefixes(dst_prefix_sha256, prefix_sha256, offset, dst)?;
+        }
         FileResponse::Error { message } => return Err(anyhow!(message)),
         other => bail!("unexpected remote copy source start response: {other:?}"),
     }
@@ -2498,20 +2515,22 @@ fn download_file(
     {
         fs::create_dir_all(parent).with_context(|| format!("create {}", parent.display()))?;
     }
-    let offset = if opts.resume && local.exists() {
-        fs::metadata(local)?.len().min(meta.len)
+    let original_len = if local.exists() {
+        Some(fs::metadata(local)?.len())
+    } else {
+        None
+    };
+    let offset = if opts.resume {
+        original_len.unwrap_or(0).min(meta.len)
     } else {
         0
     };
     let mut file = if offset > 0 {
-        let file = fs::OpenOptions::new()
+        fs::OpenOptions::new()
             .read(true)
             .append(true)
             .open(local)
-            .with_context(|| format!("open {}", local.display()))?;
-        file.set_len(offset)
-            .with_context(|| format!("truncate {}", local.display()))?;
-        file
+            .with_context(|| format!("open {}", local.display()))?
     } else {
         fs::OpenOptions::new()
             .create(true)
@@ -2521,16 +2540,29 @@ fn download_file(
             .with_context(|| format!("create {}", local.display()))?
     };
     let mut hasher = Sha256::new();
-    if offset > 0 {
+    let local_prefix_sha256 = if offset > 0 {
         let mut prefix = fs::File::open(local)?;
         hash_prefix(&mut prefix, offset, &mut hasher)?;
-    }
+        Some(digest_prefix(&hasher))
+    } else {
+        None
+    };
     client.send(FileRequest::Get {
         path: remote.to_string(),
         offset,
     })?;
     match client.recv()? {
-        FileResponse::Start { .. } => {}
+        FileResponse::Start {
+            offset: start_offset,
+            prefix_sha256,
+            ..
+        } => {
+            anyhow::ensure!(
+                start_offset == offset,
+                "download offset mismatch: got {start_offset}, expected {offset}"
+            );
+            ensure_resume_prefix_matches(prefix_sha256, local_prefix_sha256, offset, remote)?;
+        }
         FileResponse::Error { message } => return Err(anyhow!(message)),
         other => bail!("unexpected download start response: {other:?}"),
     }
@@ -2549,8 +2581,16 @@ fn download_file(
             FileResponse::Done { sha256, bytes } => {
                 file.flush()?;
                 let digest: [u8; 32] = hasher.finalize().into();
+                if (sha256 != digest || bytes != meta.len)
+                    && opts.resume
+                    && let Some(original_len) = original_len
+                {
+                    let _ = file.set_len(original_len);
+                }
                 anyhow::ensure!(sha256 == digest, "download checksum mismatch for {remote}");
                 anyhow::ensure!(bytes == meta.len, "download size mismatch for {remote}");
+                file.set_len(meta.len)
+                    .with_context(|| format!("truncate {}", local.display()))?;
                 set_local_mode(local, meta.mode, opts.preserve)?;
                 set_local_mtime(local, meta.modified_secs, opts.preserve)?;
                 if opts.progress {
@@ -2585,6 +2625,46 @@ fn hash_prefix(file: &mut fs::File, bytes: u64, hasher: &mut Sha256) -> Result<(
     Ok(())
 }
 
+fn digest_prefix(hasher: &Sha256) -> [u8; 32] {
+    hasher.clone().finalize().into()
+}
+
+fn ensure_resume_prefix_matches(
+    remote_prefix_sha256: Option<[u8; 32]>,
+    local_prefix_sha256: Option<[u8; 32]>,
+    offset: u64,
+    path: &str,
+) -> Result<()> {
+    if offset == 0 {
+        return Ok(());
+    }
+    let Some(remote_prefix_sha256) = remote_prefix_sha256 else {
+        bail!("resume prefix digest missing for {path}");
+    };
+    let Some(local_prefix_sha256) = local_prefix_sha256 else {
+        bail!("local resume prefix digest missing for {path}");
+    };
+    anyhow::ensure!(
+        remote_prefix_sha256 == local_prefix_sha256,
+        "resume prefix mismatch for {path}; retry without --resume or remove the partial destination"
+    );
+    Ok(())
+}
+
+fn ensure_matching_resume_prefixes(
+    destination_prefix_sha256: Option<[u8; 32]>,
+    source_prefix_sha256: Option<[u8; 32]>,
+    offset: u64,
+    path: &str,
+) -> Result<()> {
+    ensure_resume_prefix_matches(
+        source_prefix_sha256,
+        destination_prefix_sha256,
+        offset,
+        path,
+    )
+}
+
 fn path_ends_separator(path: &Path) -> bool {
     let raw = path.as_os_str().to_string_lossy();
     raw.ends_with('/') || raw.ends_with('\\')
diff --git a/src/bin/dosh-server.rs b/src/bin/dosh-server.rs
index 6f76014..cadd561 100644
--- a/src/bin/dosh-server.rs
+++ b/src/bin/dosh-server.rs
@@ -2566,8 +2566,10 @@ struct UploadState {
     hasher: Sha256,
     written: u64,
     expected_size: u64,
+    mode: Option<u32>,
     modified_secs: Option<u64>,
     atomic: bool,
+    original_len: Option<u64>,
 }
 
 async fn run_file_stream_service(
@@ -2716,31 +2718,41 @@ async fn handle_file_request(
                 bail!("destination exists: {}", final_path.display());
             }
             let mut hasher = Sha256::new();
-            let (file, temp_path, written, atomic) = if resume && final_path.exists() {
-                let mut file = fs::OpenOptions::new()
-                    .read(true)
-                    .append(true)
-                    .open(&final_path)
-                    .with_context(|| format!("open {}", final_path.display()))?;
-                let written = file.metadata()?.len().min(size);
-                file.set_len(written)
-                    .with_context(|| format!("truncate {}", final_path.display()))?;
-                if written > 0 {
-                    let mut prefix = fs::File::open(&final_path)?;
-                    hash_exact_prefix(&mut prefix, written, &mut hasher)?;
-                }
-                file.seek(SeekFrom::Start(written))?;
-                (file, None, written, false)
-            } else {
-                let temp_path = upload_temp_path(&final_path, stream_id);
-                let file = fs::OpenOptions::new()
-                    .create(true)
-                    .truncate(true)
-                    .write(true)
-                    .open(&temp_path)
-                    .with_context(|| format!("create {}", temp_path.display()))?;
-                (file, Some(temp_path), 0, true)
-            };
+            let (file, temp_path, written, atomic, original_len, prefix_sha256) =
+                if resume && final_path.exists() {
+                    let original_len = fs::metadata(&final_path)
+                        .with_context(|| format!("stat {}", final_path.display()))?
+                        .len();
+                    let written = original_len.min(size);
+                    if written > 0 {
+                        let mut prefix = fs::File::open(&final_path)?;
+                        hash_exact_prefix(&mut prefix, written, &mut hasher)?;
+                    }
+                    let prefix_sha256 = (written > 0).then(|| hasher.clone().finalize().into());
+                    let mut file = fs::OpenOptions::new()
+                        .read(true)
+                        .append(true)
+                        .open(&final_path)
+                        .with_context(|| format!("open {}", final_path.display()))?;
+                    file.seek(SeekFrom::Start(written))?;
+                    (
+                        file,
+                        None,
+                        written,
+                        false,
+                        Some(original_len),
+                        prefix_sha256,
+                    )
+                } else {
+                    let temp_path = upload_temp_path(&final_path, stream_id);
+                    let file = fs::OpenOptions::new()
+                        .create(true)
+                        .truncate(true)
+                        .write(true)
+                        .open(&temp_path)
+                        .with_context(|| format!("create {}", temp_path.display()))?;
+                    (file, Some(temp_path), 0, true, None, None)
+                };
             *upload = Some(UploadState {
                 final_path,
                 temp_path,
@@ -2748,26 +2760,20 @@ async fn handle_file_request(
                 hasher,
                 written,
                 expected_size: size,
+                mode,
                 modified_secs,
                 atomic,
+                original_len,
             });
-            if let Some(mode) = mode {
-                let Some(active_upload) = upload.as_ref() else {
-                    return Err(anyhow!("upload state missing after start"));
-                };
-                let target = active_upload
-                    .temp_path
-                    .as_ref()
-                    .unwrap_or(&active_upload.final_path)
-                    .clone();
-                set_mode(&target, mode)?;
-            }
             send_file_response_to_client(
                 state,
                 socket,
                 client_id,
                 stream_id,
-                FileResponse::Resume { offset: written },
+                FileResponse::Resume {
+                    offset: written,
+                    prefix_sha256,
+                },
             )
             .await
         }
@@ -2789,8 +2795,6 @@ async fn handle_file_request(
             let Some(mut active) = upload.take() else {
                 bail!("no upload in progress");
             };
-            active.file.flush()?;
-            active.file.sync_all()?;
             anyhow::ensure!(
                 active.written == active.expected_size,
                 "upload size mismatch: got {}, expected {}",
@@ -2801,9 +2805,17 @@ async fn handle_file_request(
             if digest != sha256 {
                 if let Some(temp_path) = &active.temp_path {
                     let _ = fs::remove_file(temp_path);
+                } else if let Some(original_len) = active.original_len {
+                    let _ = active.file.set_len(original_len);
                 }
                 bail!("upload checksum mismatch");
             }
+            active
+                .file
+                .set_len(active.expected_size)
+                .with_context(|| format!("truncate {}", active.final_path.display()))?;
+            active.file.flush()?;
+            active.file.sync_all()?;
             drop(active.file);
             if active.atomic
                 && let Some(temp_path) = active.temp_path.take()
@@ -2816,6 +2828,9 @@ async fn handle_file_request(
                     )
                 })?;
             }
+            if let Some(mode) = active.mode {
+                set_mode(&active.final_path, mode)?;
+            }
             if let Some(modified_secs) = active.modified_secs {
                 filetime::set_file_mtime(
                     &active.final_path,
@@ -2843,9 +2858,20 @@ async fn handle_file_request(
                 "remote path is not a file: {}",
                 meta.path
             );
+            anyhow::ensure!(
+                offset <= meta.len,
+                "download offset {offset} is beyond file size {}",
+                meta.len
+            );
             let mut file =
                 fs::File::open(&path).with_context(|| format!("open {}", path.display()))?;
             file.seek(SeekFrom::Start(offset))?;
+            let mut hasher = Sha256::new();
+            if offset > 0 {
+                let mut prefix = fs::File::open(&path)?;
+                hash_exact_prefix(&mut prefix, offset, &mut hasher)?;
+            }
+            let prefix_sha256 = (offset > 0).then(|| hasher.clone().finalize().into());
             send_file_response_to_client(
                 state,
                 socket,
@@ -2854,14 +2880,10 @@ async fn handle_file_request(
                 FileResponse::Start {
                     meta: meta.clone(),
                     offset,
+                    prefix_sha256,
                 },
             )
             .await?;
-            let mut hasher = Sha256::new();
-            if offset > 0 {
-                let mut prefix = fs::File::open(&path)?;
-                hash_exact_prefix(&mut prefix, offset, &mut hasher)?;
-            }
             let mut sent = offset;
             let mut buf = vec![0u8; CHUNK_SIZE];
             loop {
diff --git a/src/file_transfer.rs b/src/file_transfer.rs
index b939585..a2433b0 100644
--- a/src/file_transfer.rs
+++ b/src/file_transfer.rs
@@ -46,13 +46,32 @@ pub enum FileRequest {
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub enum FileResponse {
     Ok,
-    Resume { offset: u64 },
-    Stat { meta: FileMeta },
-    List { entries: Vec<FileEntry> },
-    Start { meta: FileMeta, offset: u64 },
-    Chunk { offset: u64, bytes: Vec<u8> },
-    Done { sha256: [u8; 32], bytes: u64 },
-    Error { message: String },
+    Resume {
+        offset: u64,
+        prefix_sha256: Option<[u8; 32]>,
+    },
+    Stat {
+        meta: FileMeta,
+    },
+    List {
+        entries: Vec<FileEntry>,
+    },
+    Start {
+        meta: FileMeta,
+        offset: u64,
+        prefix_sha256: Option<[u8; 32]>,
+    },
+    Chunk {
+        offset: u64,
+        bytes: Vec<u8>,
+    },
+    Done {
+        sha256: [u8; 32],
+        bytes: u64,
+    },
+    Error {
+        message: String,
+    },
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
diff --git a/tests/integration_smoke.rs b/tests/integration_smoke.rs
index 04a2812..5a667ae 100644
--- a/tests/integration_smoke.rs
+++ b/tests/integration_smoke.rs
@@ -3,6 +3,7 @@
 use std::fs;
 use std::io::{Read, Write};
 use std::net::{TcpListener, TcpStream, UdpSocket};
+use std::os::unix::fs::PermissionsExt;
 use std::os::unix::net::{UnixListener, UnixStream};
 use std::path::{Path, PathBuf};
 use std::process::{Child, Command, Stdio};
@@ -1241,6 +1242,80 @@ fn native_file_copy_resume_truncates_oversized_destinations() {
     let _ = server.wait();
 }
 
+#[test]
+fn native_file_copy_resume_prefix_mismatch_preserves_destinations() {
+    let dir = tempfile::tempdir().unwrap();
+    let port = free_udp_port();
+    let config = write_server_config(&dir, port);
+    write_native_client_auth(&dir, &config);
+    let mut server = start_server(&dir, &config);
+    let client_bin = env!("CARGO_BIN_EXE_dosh-client");
+
+    let local_upload = dir.path().join("upload-prefix.txt");
+    let remote_upload = dir.path().join("remote-upload.txt");
+    fs::write(&local_upload, b"local-prefix\n").unwrap();
+    fs::write(&remote_upload, b"remote-prefix-with-tail\n").unwrap();
+    fs::set_permissions(&local_upload, fs::Permissions::from_mode(0o755)).unwrap();
+    fs::set_permissions(&remote_upload, fs::Permissions::from_mode(0o600)).unwrap();
+
+    let bad_upload_resume = Command::new(client_bin)
+        .arg("--dosh-host")
+        .arg("127.0.0.1")
+        .arg("--dosh-port")
+        .arg(port.to_string())
+        .arg("cp")
+        .arg("--resume")
+        .arg(&local_upload)
+        .arg("local:remote-upload.txt")
+        .env("HOME", dir.path())
+        .output()
+        .unwrap();
+    assert!(
+        !bad_upload_resume.status.success(),
+        "bad upload resume unexpectedly succeeded: stdout={} stderr={}",
+        String::from_utf8_lossy(&bad_upload_resume.stdout),
+        String::from_utf8_lossy(&bad_upload_resume.stderr)
+    );
+    assert_eq!(
+        fs::read_to_string(&remote_upload).unwrap(),
+        "remote-prefix-with-tail\n"
+    );
+    assert_eq!(
+        fs::metadata(&remote_upload).unwrap().permissions().mode() & 0o777,
+        0o600
+    );
+
+    fs::write(dir.path().join("remote-download.txt"), b"remote-download\n").unwrap();
+    let local_download = dir.path().join("download-prefix.txt");
+    fs::write(&local_download, b"local-download-with-tail\n").unwrap();
+
+    let bad_download_resume = Command::new(client_bin)
+        .arg("--dosh-host")
+        .arg("127.0.0.1")
+        .arg("--dosh-port")
+        .arg(port.to_string())
+        .arg("cp")
+        .arg("--resume")
+        .arg("local:remote-download.txt")
+        .arg(&local_download)
+        .env("HOME", dir.path())
+        .output()
+        .unwrap();
+    assert!(
+        !bad_download_resume.status.success(),
+        "bad download resume unexpectedly succeeded: stdout={} stderr={}",
+        String::from_utf8_lossy(&bad_download_resume.stdout),
+        String::from_utf8_lossy(&bad_download_resume.stderr)
+    );
+    assert_eq!(
+        fs::read_to_string(&local_download).unwrap(),
+        "local-download-with-tail\n"
+    );
+
+    let _ = server.kill();
+    let _ = server.wait();
+}
+
 #[test]
 fn native_exec_command_smoke() {
     let dir = tempfile::tempdir().unwrap();