diff --git a/.gitignore b/.gitignore
index 0d7b9636..905f8a95 100644
--- a/.gitignore
+++ b/.gitignore
@@ -100,3 +100,4 @@ tests/e2e/playwright/.auth/
 
 # Test fixtures generated on-the-fly by tests/api/run.sh
 tests/fixtures/chunk-over-cap-*.bin
+wasm/oxicloud-hash/target/
diff --git a/biome.json b/biome.json
index 5abc8fe5..9a5abf93 100644
--- a/biome.json
+++ b/biome.json
@@ -7,7 +7,7 @@
         }
     },
     "files": {
-        "includes": ["static/**/*.js", "static/**/*.css", "static/**/*.json", "!static/js/vendors/"]
+        "includes": ["static/**/*.js", "static/**/*.css", "static/**/*.json", "!static/js/vendors"]
     },
     "formatter": {
         "enabled": true,
diff --git a/scripts/build-wasm.sh b/scripts/build-wasm.sh
new file mode 100755
index 00000000..9a64cff7
--- /dev/null
+++ b/scripts/build-wasm.sh
@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+# Rebuild the vendored BLAKE3 WASM module (static/js/vendors/hash-wasm/).
+#
+# The generated artifacts ARE committed — like the other vendored modules
+# (pdf.js) — so regular frontend/backend builds never need the wasm
+# toolchain. Re-run this script only when wasm/oxicloud-hash/ changes
+# (e.g. bumping the blake3 crate, or adding FastCDC for the delta-sync
+# client) and commit the regenerated files.
+#
+# Requirements (one-time):
+#   rustup target add wasm32-unknown-unknown
+#   cargo install wasm-bindgen-cli --locked
+#
+# wasm-bindgen-cli's version must match the crate's `wasm-bindgen`
+# dependency; cargo prints a clear error when they drift.
+
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+CRATE=wasm/oxicloud-hash
+OUT=static/js/vendors/hash-wasm
+
+# SIMD128 is baseline in every evergreen browser (Chrome 91+, Firefox 89+,
+# Safari 16.4+) and is worth ~3-4× in hashing throughput. Browsers without
+# it fail instantiation; the frontend detects that and falls back to a
+# plain byte upload.
+RUSTFLAGS="-C target-feature=+simd128" \
+    cargo build \
+    --manifest-path "$CRATE/Cargo.toml" \
+    --target wasm32-unknown-unknown \
+    --release
+
+wasm-bindgen \
+    --target web \
+    --no-typescript \
+    --out-dir "$OUT" \
+    "$CRATE/target/wasm32-unknown-unknown/release/oxicloud_hash_wasm.wasm"
+
+echo "Vendored: $(ls -la "$OUT" | tail -n +2 | awk '{print $9, "("$5" bytes)"}' | xargs)"
diff --git a/src/application/services/file_upload_service.rs b/src/application/services/file_upload_service.rs
index 49f00a95..02ac99a0 100644
--- a/src/application/services/file_upload_service.rs
+++ b/src/application/services/file_upload_service.rs
@@ -1,14 +1,19 @@
 use std::sync::Arc;
+use uuid::Uuid;
 
 use crate::application::dtos::file_dto::FileDto;
+use crate::application::ports::authorization_ports::AuthorizationEngine;
 use crate::application::ports::file_lifecycle::FileLifecycleHook;
 use crate::application::ports::file_ports::{FileUploadUseCase, StoredBlob};
-use crate::application::ports::storage_ports::{FileReadPort, FileWritePort};
+use crate::application::ports::storage_ports::{FileReadPort, FileWritePort, StorageUsagePort};
 use crate::application::services::storage_usage_service::StorageUsageService;
 use crate::common::errors::DomainError;
+use crate::domain::services::authorization::{Permission, Resource, Subject};
 use crate::infrastructure::repositories::pg::FileBlobReadRepository;
 use crate::infrastructure::repositories::pg::FileBlobWriteRepository;
+use crate::infrastructure::services::dedup_service::DedupService;
 use crate::infrastructure::services::file_content_cache::FileContentCache;
+use crate::infrastructure::services::pg_acl_engine::PgAclEngine;
 use tracing::{debug, info, warn};
 
 /// Helper function to extract username from folder path string.
@@ -49,6 +54,18 @@ pub struct FileUploadService {
     content_cache: Option<Arc<FileContentCache>>,
     /// Single lifecycle dispatcher — fires on_file_created / on_file_updated.
     file_lifecycle_hook: Option<Arc<dyn FileLifecycleHook>>,
+    /// Dependencies of the instant-upload path
+    /// (`create_file_from_owned_blob_with_perms`); `None` in minimal test
+    /// wiring.
+    instant_upload: Option<InstantUploadDeps>,
+}
+
+/// Everything the instant-upload path needs beyond the upload service's own
+/// ports: permission checks, the dedup index, and quota enforcement.
+struct InstantUploadDeps {
+    authz: Arc<PgAclEngine>,
+    dedup: Arc<DedupService>,
+    quota: Arc<StorageUsageService>,
 }
 
 impl FileUploadService {
@@ -60,6 +77,7 @@ impl FileUploadService {
             storage_usage_service: None,
             content_cache: None,
             file_lifecycle_hook: None,
+            instant_upload: None,
         }
     }
 
@@ -74,9 +92,26 @@ impl FileUploadService {
             storage_usage_service: None,
             content_cache: None,
             file_lifecycle_hook: None,
+            instant_upload: None,
         }
     }
 
+    /// Wires the authorization engine, dedup index and quota service that
+    /// power the instant-upload path.
+    pub fn with_instant_upload(
+        mut self,
+        authz: Arc<PgAclEngine>,
+        dedup: Arc<DedupService>,
+        quota: Arc<StorageUsageService>,
+    ) -> Self {
+        self.instant_upload = Some(InstantUploadDeps {
+            authz,
+            dedup,
+            quota,
+        });
+        self
+    }
+
     /// Configures the content cache for invalidation on file updates.
     pub fn with_content_cache(mut self, cache: Arc<FileContentCache>) -> Self {
         self.content_cache = Some(cache);
@@ -98,6 +133,111 @@ impl FileUploadService {
         self
     }
 
+    // ── Instant upload (zero content bytes) ──────────────────────
+
+    /// Register a new file row pointing at a blob the caller **already
+    /// owns** — the instant-upload path: the client proved it has the
+    /// content by hash, so no bytes travel and no chunk is written. Pure
+    /// metadata: one ref_count bump + one row INSERT.
+    ///
+    /// Security model (mirrors `GET /api/dedup/check/{hash}`):
+    /// - The caller must have `Create` permission on the target folder.
+    /// - The hash is only claimable when the caller owns at least one
+    ///   non-trashed file referencing it — never a global content oracle.
+    ///   A non-owned hash returns `NotFound` (anti-enumeration: same shape
+    ///   as "no such blob") and emits an `instant_upload.rejected` audit
+    ///   event with the real reason.
+    /// - Quota is enforced on the logical size, exactly like a byte upload.
+    pub async fn create_file_from_owned_blob_with_perms(
+        &self,
+        caller_id: Uuid,
+        name: String,
+        folder_id: String,
+        hash: &str,
+    ) -> Result<FileDto, DomainError> {
+        let Some(InstantUploadDeps {
+            authz,
+            dedup,
+            quota,
+        }) = &self.instant_upload
+        else {
+            return Err(DomainError::internal_error(
+                "FileUpload",
+                "instant upload is not wired (authz/dedup/quota missing)",
+            ));
+        };
+
+        // ── AuthZ: Create on the target folder ───────────────────
+        let folder_uuid = Uuid::parse_str(&folder_id)
+            .map_err(|_| DomainError::not_found("Folder", folder_id.clone()))?;
+        authz
+            .require(
+                Subject::User(caller_id),
+                Permission::Create,
+                Resource::Folder(folder_uuid),
+            )
+            .await?;
+
+        // ── Ownership: only blobs the caller can already read ────
+        if !dedup
+            .user_owns_blob_reference(hash, &caller_id.to_string())
+            .await
+        {
+            tracing::info!(
+                target: "audit",
+                event = "instant_upload.rejected",
+                reason = "hash_not_owned",
+                caller_id = %caller_id,
+                blob_hash = %hash,
+                "👮🏻‍♂️ Instant upload rejected: caller owns no file referencing the claimed hash",
+            );
+            return Err(DomainError::not_found("Blob", hash));
+        }
+
+        let Some(metadata) = dedup.get_blob_metadata(hash).await else {
+            // Lost a race with the last-reference delete — same shape as
+            // "never existed".
+            return Err(DomainError::not_found("Blob", hash));
+        };
+
+        // ── Quota on the logical size, before taking any reference ──
+        quota.check_storage_quota(caller_id, metadata.size).await?;
+
+        // The manifest knows the original content type; fall back to the
+        // new name's extension when the stored one is generic.
+        let claimed = metadata.content_type.as_deref().unwrap_or("");
+        let content_type =
+            match crate::common::mime_detect::refine_content_type(&[], &name, claimed) {
+                ct if ct.is_empty() => "application/octet-stream".to_string(),
+                ct => ct,
+            };
+
+        // Take the reference the row registration will consume (it releases
+        // it again on any failure). A concurrent GC between the ownership
+        // check and this bump surfaces as NotFound — the client falls back
+        // to a normal byte upload.
+        dedup.add_reference(hash).await?;
+
+        let dto = self
+            .upload_file_streaming(
+                name,
+                Some(folder_id),
+                content_type,
+                StoredBlob {
+                    hash: hash.to_string(),
+                    size: metadata.size,
+                    is_new_blob: false,
+                },
+            )
+            .await?;
+
+        info!(
+            "⚡ INSTANT UPLOAD: {} ({} bytes, 0 transferred, ID: {})",
+            dto.name, metadata.size, dto.id
+        );
+        Ok(dto)
+    }
+
     // ── private helpers ──────────────────────────────────────────
 
     /// Optionally update storage usage after a successful upload.
diff --git a/src/common/di.rs b/src/common/di.rs
index 97cf013d..49e9bf87 100644
--- a/src/common/di.rs
+++ b/src/common/di.rs
@@ -439,6 +439,7 @@ impl AppServiceFactory {
         repos: &RepositoryServices,
         trash_service: Option<Arc<TrashService>>,
         authz: &Arc<PgAclEngine>,
+        storage_usage: &Arc<StorageUsageService>,
     ) -> ApplicationServices {
         // Main services
         let folder_service = Arc::new(FolderService::new(
@@ -452,7 +453,12 @@ impl AppServiceFactory {
                 repos.file_read_repository.clone(),
             )
             .with_content_cache(core.file_content_cache.clone())
-            .with_file_lifecycle_hook(core.file_lifecycle.clone()),
+            .with_file_lifecycle_hook(core.file_lifecycle.clone())
+            .with_instant_upload(
+                authz.clone(),
+                core.dedup_service.clone(),
+                storage_usage.clone(),
+            ),
         );
 
         let file_retrieval_service = Arc::new(FileRetrievalService::new_with_cache(
@@ -733,9 +739,19 @@ impl AppServiceFactory {
             .create_trash_service(&repos, &core, &authorization)
             .await;
 
+        // 3c. Storage usage / quota service (needed by the instant-upload
+        // path inside the application services, and re-exposed on AppState
+        // for the handler-side quota checks of the byte-upload paths).
+        let storage_usage = self.create_storage_usage_service(&repos, &pool, &maintenance_pool);
+
         // 4. Application services (with trash + authz already wired)
-        let mut apps =
-            self.create_application_services(&core, &repos, trash_service.clone(), &authorization);
+        let mut apps = self.create_application_services(
+            &core,
+            &repos,
+            trash_service.clone(),
+            &authorization,
+            &storage_usage,
+        );
 
         // 5. Share service
         let share_service = self.create_share_service(&repos, &pool, &authorization);
@@ -775,8 +791,7 @@ impl AppServiceFactory {
             recent_service = Some(recent.clone());
             apps.recent_service = Some(recent);
 
-            storage_usage_service =
-                Some(self.create_storage_usage_service(&repos, &pool, &maintenance_pool));
+            storage_usage_service = Some(storage_usage.clone());
 
             self.start_tree_etag_flush_job(&maintenance_pool);
 
diff --git a/src/interfaces/api/handlers/file_handler.rs b/src/interfaces/api/handlers/file_handler.rs
index 903fd87e..67c27f57 100644
--- a/src/interfaces/api/handlers/file_handler.rs
+++ b/src/interfaces/api/handlers/file_handler.rs
@@ -69,6 +69,57 @@ impl FileHandler {
         }
     }
 
+    /// Instant upload: create a file from a blob the caller already owns.
+    ///
+    /// Zero content bytes travel — the client proved possession of the
+    /// content by hash (it computed BLAKE3 locally and confirmed via
+    /// `GET /api/dedup/check/{hash}`), so the server only bumps the blob's
+    /// reference count and registers the metadata row.
+    ///
+    /// All authorization (folder Create permission, hash ownership with
+    /// anti-enumeration, quota) lives in the application service.
+    pub(super) async fn create_file_by_hash_impl(
+        State(state): State<GlobalState>,
+        auth_user: AuthUser,
+        Json(request): Json<CreateFileByHashRequest>,
+    ) -> impl IntoResponse {
+        // Hash shape check — same contract as /api/dedup/check/{hash}.
+        if request.hash.len() != 64 || !request.hash.chars().all(|c| c.is_ascii_hexdigit()) {
+            return AppError::bad_request(
+                "Invalid hash format. Expected BLAKE3 (64 hex characters)",
+            )
+            .into_response();
+        }
+        // Basename only — same path-traversal guard as the multipart upload.
+        let filename = request
+            .name
+            .rsplit('/')
+            .next()
+            .unwrap_or(&request.name)
+            .rsplit('\\')
+            .next()
+            .unwrap_or(&request.name)
+            .to_string();
+        if filename.is_empty() {
+            return AppError::bad_request("File name must not be empty").into_response();
+        }
+
+        match state
+            .applications
+            .file_upload_service
+            .create_file_from_owned_blob_with_perms(
+                auth_user.id,
+                filename,
+                request.folder_id,
+                &request.hash,
+            )
+            .await
+        {
+            Ok(file) => Self::created_json_response(&file).into_response(),
+            Err(err) => Self::domain_error_response(err).into_response(),
+        }
+    }
+
     /// Core upload logic shared by [`Self::upload_file`] and
     /// [`Self::upload_file_with_thumbnails`].
     ///
@@ -1039,6 +1090,39 @@ pub async fn upload_file_with_thumbnails(
     FileHandler::upload_file_with_thumbnails_impl(state, auth_user, multipart).await
 }
 
+/// Request body for the instant-upload endpoint.
+#[derive(Debug, Deserialize, ToSchema)]
+pub struct CreateFileByHashRequest {
+    /// File name to create (path components are stripped).
+    pub name: String,
+    /// Target folder ID (the caller needs Create permission on it).
+    pub folder_id: String,
+    /// BLAKE3 hash (64 hex chars) of content the caller already owns.
+    pub hash: String,
+}
+
+#[utoipa::path(
+    post,
+    path = "/api/files/by-hash",
+    request_body = CreateFileByHashRequest,
+    responses(
+        (status = 201, description = "File created from an already-owned blob — zero bytes transferred", body = FileDto),
+        (status = 400, description = "Invalid hash format or empty name"),
+        (status = 404, description = "No owned blob with this hash (anti-enumeration: same shape as unknown hash)"),
+        (status = 409, description = "A file with this name already exists in the folder"),
+        (status = 507, description = "Storage quota exceeded"),
+    ),
+    security(("bearerAuth" = [])),
+    tag = "files"
+)]
+pub async fn create_file_by_hash(
+    state: State<GlobalState>,
+    auth_user: AuthUser,
+    request: Json<CreateFileByHashRequest>,
+) -> impl IntoResponse {
+    FileHandler::create_file_by_hash_impl(state, auth_user, request).await
+}
+
 #[utoipa::path(
     get,
     path = "/api/files/{id}",
diff --git a/src/interfaces/api/mod.rs b/src/interfaces/api/mod.rs
index e725fc67..51eb8e6d 100644
--- a/src/interfaces/api/mod.rs
+++ b/src/interfaces/api/mod.rs
@@ -80,6 +80,7 @@ use crate::interfaces::api::handlers::file_handler::MoveFilePayload;
         // File handlers (free functions — see file_handler.rs for why)
         handlers::file_handler::list_files_query,
         handlers::file_handler::upload_file_with_thumbnails,
+        handlers::file_handler::create_file_by_hash,
         handlers::file_handler::download_file,
         handlers::file_handler::get_thumbnail,
         handlers::file_handler::upload_thumbnail,
diff --git a/src/interfaces/api/routes.rs b/src/interfaces/api/routes.rs
index f2ac1662..262dfa9c 100644
--- a/src/interfaces/api/routes.rs
+++ b/src/interfaces/api/routes.rs
@@ -55,8 +55,8 @@ use crate::interfaces::api::handlers::chunked_upload_handler::{
     cancel_upload, complete_upload, create_upload, get_upload_status, upload_chunk,
 };
 use crate::interfaces::api::handlers::file_handler::{
-    delete_file, download_file, get_file_metadata, get_thumbnail, list_files_query,
-    move_file_simple, rename_file, upload_file_with_thumbnails, upload_thumbnail,
+    create_file_by_hash, delete_file, download_file, get_file_metadata, get_thumbnail,
+    list_files_query, move_file_simple, rename_file, upload_file_with_thumbnails, upload_thumbnail,
 };
 #[allow(deprecated)]
 use crate::interfaces::api::handlers::folder_handler::{
@@ -229,6 +229,7 @@ pub fn create_api_routes(app_state: &Arc<AppState>) -> Router<Arc<AppState>> {
     let basic_file_router = Router::new()
         .route("/", get(list_files_query))
         .route("/upload", post(upload_file_with_thumbnails))
+        .route("/by-hash", post(create_file_by_hash))
         .route("/{id}", get(download_file))
         .route(
             "/{id}/thumbnail/{size}",
diff --git a/static/js/core/types.js b/static/js/core/types.js
index 99ba6f02..f0b49ae1 100644
--- a/static/js/core/types.js
+++ b/static/js/core/types.js
@@ -518,3 +518,26 @@
  * @typedef {{kind: 'user', id: string} | {kind: 'group', id: string}} GroupMemberItem
  */
 
+// ------------------- Instant upload (dedup)
+
+/**
+ * Response from `GET /api/dedup/check/{hash}` — user-scoped: `exists` only
+ * reflects content the CALLER already owns, never global existence.
+ * Mirrors `HashCheckResponse` on the server (`dedup_handler.rs`).
+ * @typedef {Object} HashCheckAnswer
+ * @property {boolean} exists
+ * @property {string} hash BLAKE3 echoed back (64 hex chars)
+ * @property {number} [existing_size] size in bytes, present when `exists`
+ */
+
+/**
+ * Request body for `POST /api/files/by-hash` (instant upload — registers a
+ * file from an already-owned blob, zero content bytes on the wire).
+ * Mirrors `CreateFileByHashRequest` on the server (`file_handler.rs`).
+ * The 201 response body is a {@link FileItem}.
+ * @typedef {Object} CreateFileByHash
+ * @property {string} name file name to create (basename only)
+ * @property {string} folder_id target folder (caller needs Create on it)
+ * @property {string} hash BLAKE3 of the owned content (64 hex chars)
+ */
+
diff --git a/static/js/features/files/fileOperations.js b/static/js/features/files/fileOperations.js
index 87f7578d..a4bc092d 100644
--- a/static/js/features/files/fileOperations.js
+++ b/static/js/features/files/fileOperations.js
@@ -12,6 +12,7 @@ import { i18n } from '../../core/i18n.js';
 import { notifications } from '../../core/notifications.js';
 import { invalidateFolderMeta } from '../../model/filesModel.js';
 import { triggerBrowserDownload } from '../../utils/download.js';
+import { tryInstantUpload } from './instantUpload.js';
 
 /**
  * @typedef {Object} BatchResult
@@ -404,20 +405,28 @@ const fileOps = {
                 if (quotaStop) return;
                 const file = readableFiles[idx];
 
-                const formData = new FormData();
-                if (targetFolderId) formData.append('folder_id', targetFolderId);
-                formData.append('file', file);
+                // ── Instant upload: when the server already has this exact
+                // content for this user, register it by hash — zero bytes
+                // on the wire. Any miss/failure falls back to a byte upload.
+                /** @type {UploadAnswer | null} */
+                let result = await tryInstantUpload(file, targetFolderId);
+                if (result) {
+                    if (batchId) {
+                        try {
+                            notifications.updateFile(batchId, file.name, 100, result.ok ? 'done' : 'error');
+                        } catch (_) {}
+                    }
+                } else {
+                    const formData = new FormData();
+                    if (targetFolderId) formData.append('folder_id', targetFolderId);
+                    formData.append('file', file);
 
-                console.log(`Uploading file to folder: ${targetFolderId || 'root'}`, {
-                    file: file.name,
-                    size: file.size
-                });
-
-                // Scale stall timeout with file size:
-                // base 120s + 60s per GB, so a 7 GB file gets ~540s stall limit
-                const sizeGB = file.size / (1024 * 1024 * 1024);
-                const dynamicTimeout = Math.max(120000, 120000 + Math.ceil(sizeGB) * 60000);
-                const result = await this._uploadFileXHR(formData, batchId, file.name, dynamicTimeout);
+                    // Scale stall timeout with file size:
+                    // base 120s + 60s per GB, so a 7 GB file gets ~540s stall limit
+                    const sizeGB = file.size / (1024 * 1024 * 1024);
+                    const dynamicTimeout = Math.max(120000, 120000 + Math.ceil(sizeGB) * 60000);
+                    result = await this._uploadFileXHR(formData, batchId, file.name, dynamicTimeout);
+                }
 
                 uploadedCount++;
 
@@ -663,48 +672,54 @@ const fileOps = {
                     const parentPath = parts.slice(0, -1).join('/');
                     const targetFolderId = folderMap.get(parentPath) || currentFolderId;
 
-                    // ── FIFO/pipe guard (0-byte files only) ──
-                    // Named pipes (runit supervise/control) report size=0
-                    // but block on open(). Pre-read only 0-byte files into
-                    // memory; files with size>0 are always regular files and
-                    // go straight to FormData (zero extra memory copy).
-                    /** @type {Blob} */
-                    let uploadFile = file; // default: use original File
-                    if (file.size === 0) {
-                        try {
-                            const buf = await Promise.race([
-                                file.arrayBuffer(),
-                                new Promise((_, rej) => setTimeout(() => rej(new Error('read-timeout')), 2000))
-                            ]);
-                            uploadFile = new Blob([buf], {
-                                type: file.type || 'application/octet-stream'
-                            });
-                        } catch {
-                            console.warn(`[SKIP] #${idx} ${rel} — cannot read 0-byte file (FIFO/pipe?), skipping`);
-                            uploadedCount++;
-                            successCount++;
-                            if (batchId) {
-                                try {
-                                    notifications.fileCompleted(batchId, true);
-                                } catch (_) {}
+                    // ── Instant upload (zero bytes on the wire) ──
+                    // Same fallback contract as uploadFiles: a null result
+                    // means "do the byte upload". The shared accounting
+                    // after this try block handles both outcomes.
+                    const instant = await tryInstantUpload(file, targetFolderId);
+                    if (instant) {
+                        result = instant;
+                    } else {
+                        // ── FIFO/pipe guard (0-byte files only) ──
+                        // Named pipes (runit supervise/control) report size=0
+                        // but block on open(). Pre-read only 0-byte files into
+                        // memory; files with size>0 are always regular files and
+                        // go straight to FormData (zero extra memory copy).
+                        /** @type {Blob} */
+                        let uploadFile = file; // default: use original File
+                        if (file.size === 0) {
+                            try {
+                                const buf = await Promise.race([
+                                    file.arrayBuffer(),
+                                    new Promise((_, rej) => setTimeout(() => rej(new Error('read-timeout')), 2000))
+                                ]);
+                                uploadFile = new Blob([buf], {
+                                    type: file.type || 'application/octet-stream'
+                                });
+                            } catch {
+                                console.warn(`[SKIP] #${idx} ${rel} — cannot read 0-byte file (FIFO/pipe?), skipping`);
+                                uploadedCount++;
+                                successCount++;
+                                if (batchId) {
+                                    try {
+                                        notifications.fileCompleted(batchId, true);
+                                    } catch (_) {}
+                                }
+                                return;
                             }
-                            return;
                         }
+
+                        const formData = new FormData();
+                        formData.append('folder_id', targetFolderId);
+                        formData.append('file', uploadFile, file.name);
+
+                        const thisTimeout =
+                            file.size === 0
+                                ? TIMEOUT_MS_ZERO
+                                : Math.max(TIMEOUT_MIN_MS, TIMEOUT_BASE_MS + Math.ceil(file.size / (1024 * 1024)) * TIMEOUT_PER_MB_MS);
+
+                        result = await this._uploadFileFetch(formData, thisTimeout);
                     }
-
-                    const formData = new FormData();
-                    formData.append('folder_id', targetFolderId);
-                    formData.append('file', uploadFile, file.name);
-
-                    const thisTimeout =
-                        file.size === 0
-                            ? TIMEOUT_MS_ZERO
-                            : Math.max(TIMEOUT_MIN_MS, TIMEOUT_BASE_MS + Math.ceil(file.size / (1024 * 1024)) * TIMEOUT_PER_MB_MS);
-                    console.log(`[UPLOAD START] #${idx} ${rel} (${file.size} bytes, timeout=${thisTimeout}ms)`);
-
-                    result = await this._uploadFileFetch(formData, thisTimeout);
-
-                    console.log(`[UPLOAD END]   #${idx} ${rel} ok=${result.ok}${result.errorMsg ? ` err=${result.errorMsg}` : ''}`);
                 } catch (e) {
                     result = {
                         ok: false,
diff --git a/static/js/features/files/instantUpload.js b/static/js/features/files/instantUpload.js
new file mode 100644
index 00000000..bda924e1
--- /dev/null
+++ b/static/js/features/files/instantUpload.js
@@ -0,0 +1,177 @@
+/**
+ * OxiCloud - Instant upload (zero-byte dedup upload)
+ *
+ * Before transferring a file's bytes, compute its BLAKE3 locally (in a
+ * worker, off the main thread) and ask the server whether the caller
+ * already owns that exact content (`GET /api/dedup/check/{hash}` — the
+ * check is user-scoped, never a global content oracle). On a hit, a
+ * single metadata call (`POST /api/files/by-hash`) registers the file
+ * with ZERO content bytes on the wire.
+ *
+ * Performance posture:
+ * - Hashing runs in a dedicated worker with WASM SIMD128 — the UI thread
+ *   never blocks, RAM stays constant (8 MiB slices).
+ * - Files below {@link INSTANT_UPLOAD_MIN_SIZE} skip the whole dance:
+ *   two extra round-trips cost more than just uploading them.
+ * - Any failure (no WASM support, worker error, server miss, races)
+ *   falls back silently to the normal byte upload — instant upload is
+ *   an optimization, never a gate.
+ */
+
+import { getCsrfHeaders } from '../../core/csrf.js';
+
+/**
+ * Files smaller than this upload normally: hashing + two round-trips
+ * outweigh the transfer. 8 MiB matches the chunked-upload threshold's
+ * order of magnitude.
+ */
+export const INSTANT_UPLOAD_MIN_SIZE = 8 * 1024 * 1024;
+
+// Absolute URL on purpose — works in dev and in the release IIFE bundle
+// (same pattern as the pdf.js loader in thumbnail.js).
+const HASH_WORKER_URL = '/js/workers/hashWorker.js';
+
+/** Hashing budget: 60 s base + 30 s per GB (WASM SIMD does ~0.5-1 GB/s). */
+const HASH_TIMEOUT_BASE_MS = 60000;
+const HASH_TIMEOUT_PER_GB_MS = 30000;
+
+/**
+ * `false` once the environment proved unable to run the worker/WASM
+ * (old browser, blocked worker) — later files skip straight to the byte
+ * upload instead of failing the same way again. `null` = not yet known.
+ * @type {boolean | null}
+ */
+let _instantUploadUsable = null;
+
+/**
+ * Hash a file in a one-shot worker. Resolves `null` on any failure —
+ * the caller falls back to a normal upload.
+ * @param {File} file
+ * @returns {Promise<string | null>}
+ */
+function hashFileInWorker(file) {
+    return new Promise((resolve) => {
+        /** @type {Worker} */
+        let worker;
+        try {
+            worker = new Worker(HASH_WORKER_URL, { type: 'module' });
+        } catch (_) {
+            _instantUploadUsable = false;
+            resolve(null);
+            return;
+        }
+
+        const sizeGB = file.size / (1024 * 1024 * 1024);
+        const timeoutMs = HASH_TIMEOUT_BASE_MS + Math.ceil(sizeGB) * HASH_TIMEOUT_PER_GB_MS;
+
+        /** @param {string | null} hash */
+        const settle = (hash) => {
+            clearTimeout(timer);
+            worker.terminate();
+            resolve(hash);
+        };
+        const timer = setTimeout(() => settle(null), timeoutMs);
+
+        worker.onmessage = (event) => {
+            const data = /** @type {{ ok: boolean, hash?: string, error?: string }} */ (event.data);
+            if (!data.ok) {
+                // The worker ran but WASM failed (e.g. no SIMD128 support):
+                // a permanent environment property, don't retry per file.
+                _instantUploadUsable = false;
+            }
+            settle(data.ok && data.hash ? data.hash : null);
+        };
+        worker.onerror = () => {
+            // Worker script failed to load/parse — permanent.
+            _instantUploadUsable = false;
+            settle(null);
+        };
+
+        worker.postMessage({ file });
+    });
+}
+
+/**
+ * Ask the server whether the caller already owns content with this hash.
+ * @param {string} hash
+ * @returns {Promise<boolean>}
+ */
+async function callerOwnsHash(hash) {
+    try {
+        const response = await fetch(`/api/dedup/check/${hash}`, {
+            headers: { 'Cache-Control': 'no-cache, no-store, must-revalidate' }
+        });
+        if (!response.ok) return false;
+        const body = /** @type {import('../../core/types.js').HashCheckAnswer} */ (await response.json());
+        return body.exists === true;
+    } catch (_) {
+        return false;
+    }
+}
+
+/**
+ * Try to register `file` as a zero-byte instant upload.
+ *
+ * Returns `null` whenever the byte upload should proceed (file too
+ * small, environment unusable, hash miss, lost race, transient errors).
+ * Returns an upload-result object compatible with the uploaders'
+ * `UploadAnswer` shape when the attempt is conclusive — success, quota
+ * exceeded, or name conflict (a byte upload would fail identically).
+ *
+ * @param {File} file
+ * @param {string | null | undefined} folderId
+ * @returns {Promise<{ ok: boolean, data?: any, errorMsg?: string, isQuotaError?: boolean } | null>}
+ */
+export async function tryInstantUpload(file, folderId) {
+    if (!folderId || file.size < INSTANT_UPLOAD_MIN_SIZE || _instantUploadUsable === false || typeof Worker === 'undefined') {
+        return null;
+    }
+
+    const hash = await hashFileInWorker(file);
+    if (!hash) return null;
+
+    if (!(await callerOwnsHash(hash))) return null;
+
+    try {
+        const response = await fetch('/api/files/by-hash', {
+            method: 'POST',
+            headers: {
+                ...getCsrfHeaders(),
+                'Content-Type': 'application/json',
+                'Cache-Control': 'no-cache, no-store, must-revalidate'
+            },
+            body: JSON.stringify(
+                /** @type {import('../../core/types.js').CreateFileByHash} */ ({
+                    name: file.name,
+                    folder_id: folderId,
+                    hash
+                })
+            )
+        });
+
+        if (response.status === 201) {
+            return { ok: true, data: await response.json() };
+        }
+
+        /** @type {string} */
+        let errorMsg = `Instant upload failed (HTTP ${response.status})`;
+        try {
+            const body = await response.json();
+            errorMsg = body.message || body.error || errorMsg;
+        } catch (_) {}
+
+        if (response.status === 507) {
+            return { ok: false, isQuotaError: true, errorMsg };
+        }
+        if (response.status === 409) {
+            // Duplicate name in the folder — a byte upload would hit the
+            // exact same conflict; surface it without transferring.
+            return { ok: false, errorMsg };
+        }
+        // 404 (ownership race with a delete+GC), 4xx/5xx: fall back to the
+        // byte upload — the server dedups it on write anyway.
+        return null;
+    } catch (_) {
+        return null;
+    }
+}
diff --git a/static/js/vendors/hash-wasm/oxicloud_hash_wasm.js b/static/js/vendors/hash-wasm/oxicloud_hash_wasm.js
new file mode 100644
index 00000000..2a341978
--- /dev/null
+++ b/static/js/vendors/hash-wasm/oxicloud_hash_wasm.js
@@ -0,0 +1,261 @@
+/**
+ * Incremental BLAKE3 hasher.
+ *
+ * ```js
+ * const h = new Blake3Hasher();
+ * h.update(chunkBytes);   // repeat per slice
+ * const hex = h.finalizeHex();
+ * ```
+ */
+export class Blake3Hasher {
+    __destroy_into_raw() {
+        const ptr = this.__wbg_ptr;
+        this.__wbg_ptr = 0;
+        Blake3HasherFinalization.unregister(this);
+        return ptr;
+    }
+    free() {
+        const ptr = this.__destroy_into_raw();
+        wasm.__wbg_blake3hasher_free(ptr, 0);
+    }
+    /**
+     * Bytes hashed so far — lets the worker report progress without
+     * tracking its own counter.
+     * @returns {number}
+     */
+    count() {
+        const ret = wasm.blake3hasher_count(this.__wbg_ptr);
+        return ret;
+    }
+    /**
+     * Finish and return the lowercase hex digest (64 chars). The hasher
+     * can keep receiving `update` calls afterwards (BLAKE3 finalization
+     * is non-destructive), but the frontend treats it as terminal.
+     * @returns {string}
+     */
+    finalizeHex() {
+        let deferred1_0;
+        let deferred1_1;
+        try {
+            const retptr = wasm.__wbindgen_add_to_stack_pointer(-16);
+            wasm.blake3hasher_finalizeHex(retptr, this.__wbg_ptr);
+            var r0 = getDataViewMemory0().getInt32(retptr + 4 * 0, true);
+            var r1 = getDataViewMemory0().getInt32(retptr + 4 * 1, true);
+            deferred1_0 = r0;
+            deferred1_1 = r1;
+            return getStringFromWasm0(r0, r1);
+        } finally {
+            wasm.__wbindgen_add_to_stack_pointer(16);
+            wasm.__wbindgen_export2(deferred1_0, deferred1_1, 1);
+        }
+    }
+    /**
+     * Create a fresh hasher.
+     */
+    constructor() {
+        const ret = wasm.blake3hasher_new();
+        this.__wbg_ptr = ret;
+        Blake3HasherFinalization.register(this, this.__wbg_ptr, this);
+        return this;
+    }
+    /**
+     * Feed one slice of the file.
+     * @param {Uint8Array} data
+     */
+    update(data) {
+        const ptr0 = passArray8ToWasm0(data, wasm.__wbindgen_export);
+        const len0 = WASM_VECTOR_LEN;
+        wasm.blake3hasher_update(this.__wbg_ptr, ptr0, len0);
+    }
+}
+if (Symbol.dispose) Blake3Hasher.prototype[Symbol.dispose] = Blake3Hasher.prototype.free;
+
+/**
+ * One-shot convenience for small buffers.
+ * @param {Uint8Array} data
+ * @returns {string}
+ */
+export function blake3Hex(data) {
+    let deferred2_0;
+    let deferred2_1;
+    try {
+        const retptr = wasm.__wbindgen_add_to_stack_pointer(-16);
+        const ptr0 = passArray8ToWasm0(data, wasm.__wbindgen_export);
+        const len0 = WASM_VECTOR_LEN;
+        wasm.blake3Hex(retptr, ptr0, len0);
+        var r0 = getDataViewMemory0().getInt32(retptr + 4 * 0, true);
+        var r1 = getDataViewMemory0().getInt32(retptr + 4 * 1, true);
+        deferred2_0 = r0;
+        deferred2_1 = r1;
+        return getStringFromWasm0(r0, r1);
+    } finally {
+        wasm.__wbindgen_add_to_stack_pointer(16);
+        wasm.__wbindgen_export2(deferred2_0, deferred2_1, 1);
+    }
+}
+function __wbg_get_imports() {
+    const import0 = {
+        __proto__: null,
+        __wbg___wbindgen_throw_bbadd78c1bac3a77: (arg0, arg1) => {
+            throw new Error(getStringFromWasm0(arg0, arg1));
+        }
+    };
+    return {
+        __proto__: null,
+        './oxicloud_hash_wasm_bg.js': import0
+    };
+}
+
+const Blake3HasherFinalization =
+    typeof FinalizationRegistry === 'undefined'
+        ? { register: () => {}, unregister: () => {} }
+        : new FinalizationRegistry((ptr) => wasm.__wbg_blake3hasher_free(ptr, 1));
+
+let cachedDataViewMemory0 = null;
+function getDataViewMemory0() {
+    if (
+        cachedDataViewMemory0 === null ||
+        cachedDataViewMemory0.buffer.detached === true ||
+        (cachedDataViewMemory0.buffer.detached === undefined && cachedDataViewMemory0.buffer !== wasm.memory.buffer)
+    ) {
+        cachedDataViewMemory0 = new DataView(wasm.memory.buffer);
+    }
+    return cachedDataViewMemory0;
+}
+
+function getStringFromWasm0(ptr, len) {
+    return decodeText(ptr >>> 0, len);
+}
+
+let cachedUint8ArrayMemory0 = null;
+function getUint8ArrayMemory0() {
+    if (cachedUint8ArrayMemory0 === null || cachedUint8ArrayMemory0.byteLength === 0) {
+        cachedUint8ArrayMemory0 = new Uint8Array(wasm.memory.buffer);
+    }
+    return cachedUint8ArrayMemory0;
+}
+
+function passArray8ToWasm0(arg, malloc) {
+    const ptr = malloc(arg.length * 1, 1) >>> 0;
+    getUint8ArrayMemory0().set(arg, ptr / 1);
+    WASM_VECTOR_LEN = arg.length;
+    return ptr;
+}
+
+let cachedTextDecoder = new TextDecoder('utf-8', { ignoreBOM: true, fatal: true });
+cachedTextDecoder.decode();
+const MAX_SAFARI_DECODE_BYTES = 2146435072;
+let numBytesDecoded = 0;
+function decodeText(ptr, len) {
+    numBytesDecoded += len;
+    if (numBytesDecoded >= MAX_SAFARI_DECODE_BYTES) {
+        cachedTextDecoder = new TextDecoder('utf-8', { ignoreBOM: true, fatal: true });
+        cachedTextDecoder.decode();
+        numBytesDecoded = len;
+    }
+    return cachedTextDecoder.decode(getUint8ArrayMemory0().subarray(ptr, ptr + len));
+}
+
+let WASM_VECTOR_LEN = 0;
+
+let wasmModule, wasmInstance, wasm;
+function __wbg_finalize_init(instance, module) {
+    wasmInstance = instance;
+    wasm = instance.exports;
+    wasmModule = module;
+    cachedDataViewMemory0 = null;
+    cachedUint8ArrayMemory0 = null;
+    return wasm;
+}
+
+async function __wbg_load(module, imports) {
+    if (typeof Response === 'function' && module instanceof Response) {
+        if (typeof WebAssembly.instantiateStreaming === 'function') {
+            try {
+                return await WebAssembly.instantiateStreaming(module, imports);
+            } catch (e) {
+                const validResponse = module.ok && expectedResponseType(module.type);
+
+                if (validResponse && module.headers.get('Content-Type') !== 'application/wasm') {
+                    console.warn(
+                        '`WebAssembly.instantiateStreaming` failed because your server does not serve Wasm with `application/wasm` MIME type. Falling back to `WebAssembly.instantiate` which is slower. Original error:\n',
+                        e
+                    );
+                } else {
+                    throw e;
+                }
+            }
+        }
+
+        const bytes = await module.arrayBuffer();
+        return await WebAssembly.instantiate(bytes, imports);
+    } else {
+        const instance = await WebAssembly.instantiate(module, imports);
+
+        if (instance instanceof WebAssembly.Instance) {
+            return { instance, module };
+        } else {
+            return instance;
+        }
+    }
+
+    function expectedResponseType(type) {
+        switch (type) {
+            case 'basic':
+            case 'cors':
+            case 'default':
+                return true;
+        }
+        return false;
+    }
+}
+
+function initSync(module) {
+    if (wasm !== undefined) return wasm;
+
+    if (module !== undefined) {
+        if (Object.getPrototypeOf(module) === Object.prototype) {
+            ({ module } = module);
+        } else {
+            console.warn('using deprecated parameters for `initSync()`; pass a single object instead');
+        }
+    }
+
+    const imports = __wbg_get_imports();
+    if (!(module instanceof WebAssembly.Module)) {
+        module = new WebAssembly.Module(module);
+    }
+    const instance = new WebAssembly.Instance(module, imports);
+    return __wbg_finalize_init(instance, module);
+}
+
+async function __wbg_init(module_or_path) {
+    if (wasm !== undefined) return wasm;
+
+    if (module_or_path !== undefined) {
+        if (Object.getPrototypeOf(module_or_path) === Object.prototype) {
+            ({ module_or_path } = module_or_path);
+        } else {
+            console.warn('using deprecated parameters for the initialization function; pass a single object instead');
+        }
+    }
+
+    if (module_or_path === undefined) {
+        module_or_path = new URL('oxicloud_hash_wasm_bg.wasm', import.meta.url);
+    }
+    const imports = __wbg_get_imports();
+
+    if (
+        typeof module_or_path === 'string' ||
+        (typeof Request === 'function' && module_or_path instanceof Request) ||
+        (typeof URL === 'function' && module_or_path instanceof URL)
+    ) {
+        module_or_path = fetch(module_or_path);
+    }
+
+    const { instance, module } = await __wbg_load(await module_or_path, imports);
+
+    return __wbg_finalize_init(instance, module);
+}
+
+export { __wbg_init as default, initSync };
diff --git a/static/js/vendors/hash-wasm/oxicloud_hash_wasm_bg.wasm b/static/js/vendors/hash-wasm/oxicloud_hash_wasm_bg.wasm
new file mode 100644
index 00000000..7ab00d46
Binary files /dev/null and b/static/js/vendors/hash-wasm/oxicloud_hash_wasm_bg.wasm differ
diff --git a/static/js/workers/hashWorker.js b/static/js/workers/hashWorker.js
new file mode 100644
index 00000000..d2061ecd
--- /dev/null
+++ b/static/js/workers/hashWorker.js
@@ -0,0 +1,79 @@
+/**
+ * OxiCloud — BLAKE3 hashing worker (instant-upload support).
+ *
+ * Hashes a File off the main thread, reading it in fixed-size slices so
+ * RAM stays constant regardless of file size. The WASM module is compiled
+ * from the exact same `blake3` crate the server uses, so the digest
+ * computed here equals the server's content address bit for bit.
+ *
+ * Protocol: receives `{ file: File }`, answers
+ * `{ ok: true, hash: string }` or `{ ok: false, error: string }`.
+ * The spawner terminates the worker after one file.
+ */
+
+// Absolute URL on purpose: vendors are served verbatim at /js/vendors/ in
+// both dev and release mode (the release IIFE bundle would break a
+// relative import) — same pattern as the pdf.js loader in thumbnail.js.
+const WASM_GLUE_URL = '/js/vendors/hash-wasm/oxicloud_hash_wasm.js';
+
+/**
+ * 8 MiB slices — large enough to amortize the per-slice Blob→ArrayBuffer
+ * round-trip, small enough that peak worker RAM stays flat for any size.
+ */
+const SLICE_BYTES = 8 * 1024 * 1024;
+
+/**
+ * Typed view of the dedicated-worker global scope. The project's
+ * jsconfig targets the DOM lib, where `self` is a Window — cast to the
+ * two members this worker actually uses.
+ * @type {{ onmessage: ((event: MessageEvent) => void) | null,
+ *          postMessage: (message: unknown) => void }}
+ */
+const workerScope = /** @type {any} */ (self);
+
+/**
+ * Memoized WASM module (in-flight or settled), `default()` already run.
+ * Reset on failure so a later message can retry a transient load error.
+ * @type {Promise<any> | null}
+ */
+let _wasmPromise = null;
+
+/** @returns {Promise<any>} */
+function getWasm() {
+    if (!_wasmPromise) {
+        _wasmPromise = import(WASM_GLUE_URL)
+            .then(async (mod) => {
+                await mod.default();
+                return mod;
+            })
+            .catch((err) => {
+                _wasmPromise = null;
+                throw err;
+            });
+    }
+    return _wasmPromise;
+}
+
+workerScope.onmessage = async (event) => {
+    const file = /** @type {{ file: File }} */ (event.data).file;
+    try {
+        const wasm = await getWasm();
+        const hasher = new wasm.Blake3Hasher();
+        try {
+            for (let offset = 0; offset < file.size; offset += SLICE_BYTES) {
+                const end = Math.min(offset + SLICE_BYTES, file.size);
+                // eslint-disable-next-line no-await-in-loop -- sequential by design: constant RAM
+                const buffer = await file.slice(offset, end).arrayBuffer();
+                hasher.update(new Uint8Array(buffer));
+            }
+            workerScope.postMessage({ ok: true, hash: hasher.finalizeHex() });
+        } finally {
+            hasher.free();
+        }
+    } catch (err) {
+        workerScope.postMessage({
+            ok: false,
+            error: err instanceof Error ? err.message : String(err)
+        });
+    }
+};
diff --git a/wasm/oxicloud-hash/Cargo.lock b/wasm/oxicloud-hash/Cargo.lock
new file mode 100644
index 00000000..b053178b
--- /dev/null
+++ b/wasm/oxicloud-hash/Cargo.lock
@@ -0,0 +1,184 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "arrayref"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "76a2e8124351fda1ef8aaaa3bbd7ebbcb486bbcd4225aca0aa0d84bb2db8fecb"
+
+[[package]]
+name = "arrayvec"
+version = "0.7.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7c02d123df017efcdfbd739ef81735b36c5ba83ec3c59c80a9d7ecc718f92e50"
+
+[[package]]
+name = "blake3"
+version = "1.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0aa83c34e62843d924f905e0f5c866eb1dd6545fc4d719e803d9ba6030371fce"
+dependencies = [
+ "arrayref",
+ "arrayvec",
+ "cc",
+ "cfg-if",
+ "constant_time_eq",
+ "cpufeatures",
+]
+
+[[package]]
+name = "bumpalo"
+version = "3.20.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72f5acc6cb2ba439de613abc23857ec3d78374d8ed5ac84e9d11336e87da8649"
+
+[[package]]
+name = "cc"
+version = "1.2.63"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "556e016178bb5662a08681bbe0f00f8e17631781a4dfc8c45e466e4b185ec27f"
+dependencies = [
+ "find-msvc-tools",
+ "shlex",
+]
+
+[[package]]
+name = "cfg-if"
+version = "1.0.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
+
+[[package]]
+name = "constant_time_eq"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3d52eff69cd5e647efe296129160853a42795992097e8af39800e1060caeea9b"
+
+[[package]]
+name = "cpufeatures"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8b2a41393f66f16b0823bb79094d54ac5fbd34ab292ddafb9a0456ac9f87d201"
+dependencies = [
+ "libc",
+]
+
+[[package]]
+name = "find-msvc-tools"
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diff --git a/wasm/oxicloud-hash/Cargo.toml b/wasm/oxicloud-hash/Cargo.toml
new file mode 100644
index 00000000..0c54d4d1
--- /dev/null
+++ b/wasm/oxicloud-hash/Cargo.toml
@@ -0,0 +1,30 @@
+[package]
+name = "oxicloud-hash-wasm"
+version = "0.1.0"
+edition = "2021"
+description = "BLAKE3 hashing for the OxiCloud web frontend — compiled from the exact same crate the server uses, so client-side hashes match server-side content addressing bit for bit."
+publish = false
+
+# Standalone workspace root: this crate is built only by
+# scripts/build-wasm.sh (wasm32 target) and must not join the server
+# workspace — `cargo test --workspace` / clippy on the host have nothing
+# useful to do with a wasm cdylib.
+[workspace]
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+# Same major as the server's Cargo.toml. `wasm32_simd` enables the WASM
+# SIMD128 kernels (~3-4× over the portable implementation); every
+# evergreen browser since 2023 supports it, and the frontend falls back
+# to a plain byte upload when instantiation fails.
+blake3 = { version = "1.8.4", default-features = false, features = ["wasm32_simd"] }
+wasm-bindgen = "0.2"
+
+[profile.release]
+# Hashing throughput is the whole point of this module.
+opt-level = 3
+lto = "fat"
+codegen-units = 1
+strip = true
diff --git a/wasm/oxicloud-hash/src/lib.rs b/wasm/oxicloud-hash/src/lib.rs
new file mode 100644
index 00000000..91cb8d55
--- /dev/null
+++ b/wasm/oxicloud-hash/src/lib.rs
@@ -0,0 +1,98 @@
+//! BLAKE3 for the OxiCloud web frontend.
+//!
+//! Compiled from the same `blake3` crate the server uses, so a hash
+//! computed in the browser equals the server's content address bit for
+//! bit — the property the instant-upload path depends on.
+//!
+//! The API is incremental on purpose: the worker feeds the file in
+//! slices (`Blob.slice().arrayBuffer()`), keeping RAM constant no matter
+//! how large the file is.
+
+use wasm_bindgen::prelude::*;
+
+/// Incremental BLAKE3 hasher.
+///
+/// ```js
+/// const h = new Blake3Hasher();
+/// h.update(chunkBytes);   // repeat per slice
+/// const hex = h.finalizeHex();
+/// ```
+#[wasm_bindgen]
+pub struct Blake3Hasher {
+    inner: blake3::Hasher,
+}
+
+#[wasm_bindgen]
+impl Blake3Hasher {
+    /// Create a fresh hasher.
+    #[wasm_bindgen(constructor)]
+    pub fn new() -> Blake3Hasher {
+        Blake3Hasher {
+            inner: blake3::Hasher::new(),
+        }
+    }
+
+    /// Feed one slice of the file.
+    pub fn update(&mut self, data: &[u8]) {
+        self.inner.update(data);
+    }
+
+    /// Finish and return the lowercase hex digest (64 chars). The hasher
+    /// can keep receiving `update` calls afterwards (BLAKE3 finalization
+    /// is non-destructive), but the frontend treats it as terminal.
+    #[wasm_bindgen(js_name = finalizeHex)]
+    pub fn finalize_hex(&self) -> String {
+        self.inner.finalize().to_hex().to_string()
+    }
+
+    /// Bytes hashed so far — lets the worker report progress without
+    /// tracking its own counter.
+    pub fn count(&self) -> f64 {
+        self.inner.count() as f64
+    }
+}
+
+impl Default for Blake3Hasher {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+/// One-shot convenience for small buffers.
+#[wasm_bindgen(js_name = blake3Hex)]
+pub fn blake3_hex(data: &[u8]) -> String {
+    blake3::hash(data).to_hex().to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// The vector the frontend smoke test uses — also proves the wasm
+    /// build hashes identically to the server (same crate, same output).
+    #[test]
+    fn hello_world_vector() {
+        let hasher = {
+            let mut h = Blake3Hasher::new();
+            h.update(b"Hello, ");
+            h.update(b"World!");
+            h
+        };
+        assert_eq!(
+            hasher.finalize_hex(),
+            "288a86a79f20a3d6dccdca7713beaed178798296bdfa7913fa2a62d9727bf8f8"
+        );
+        assert_eq!(
+            blake3_hex(b"Hello, World!"),
+            "288a86a79f20a3d6dccdca7713beaed178798296bdfa7913fa2a62d9727bf8f8"
+        );
+    }
+
+    #[test]
+    fn empty_input_vector() {
+        assert_eq!(
+            blake3_hex(b""),
+            "af1349b9f5f9a1a6a0404dea36dcc9499bcb25c9adc112b7cc9a93cae41f3262"
+        );
+    }
+}