diff --git a/Cargo.toml b/Cargo.toml
index a4e682b..0a50608 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,6 +5,7 @@ members = [
     "crates/ely_design_system",
     "crates/ely_domain",
     "crates/ely_servo_host",
+    "crates/ely_sync_client",
 ]
 default-members = ["crates/ely_app"]
 resolver = "2"
diff --git a/crates/ely_sync_client/Cargo.toml b/crates/ely_sync_client/Cargo.toml
new file mode 100644
index 0000000..5be8b37
--- /dev/null
+++ b/crates/ely_sync_client/Cargo.toml
@@ -0,0 +1,18 @@
+[package]
+name = "ely_sync_client"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+
+[dependencies]
+ely_domain = { path = "../ely_domain" }
+serde = { workspace = true }
+serde_json = { workspace = true }
+sha2 = { workspace = true }
+thiserror = { workspace = true }
+ureq = { workspace = true, features = ["json"] }
+uuid = { workspace = true }
+
+[lints]
+workspace = true
diff --git a/crates/ely_sync_client/src/auth.rs b/crates/ely_sync_client/src/auth.rs
new file mode 100644
index 0000000..3f198ff
--- /dev/null
+++ b/crates/ely_sync_client/src/auth.rs
@@ -0,0 +1,134 @@
+use std::{
+    fs,
+    io::{self, ErrorKind},
+    path::{Path, PathBuf},
+};
+
+use crate::error::SyncClientError;
+
+/// Better Auth bearer token issued by `https://<base>/api/auth/*`. The
+/// token grants access to the per-user `withAuthenticatedApiControls`
+/// routes and, once the device is bound to the session, to the
+/// `withApprovedDeviceApiControls` routes used by sync.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct BearerToken(String);
+
+impl BearerToken {
+    /// Construct a token from an existing string. Trims whitespace and
+    /// enforces the same character envelope the worker validates so
+    /// obviously-malformed tokens fail before we hit the network.
+    pub fn new(value: impl Into<String>) -> Result<Self, SyncClientError> {
+        let value = value.into();
+        let trimmed = value.trim();
+        if trimmed.is_empty() || !is_better_auth_bearer(trimmed) {
+            return Err(SyncClientError::TokenStorage(
+                "bearer token is not a Better Auth session token".to_string(),
+            ));
+        }
+        Ok(Self(trimmed.to_string()))
+    }
+
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+fn is_better_auth_bearer(token: &str) -> bool {
+    let length_ok = (32..=4096).contains(&token.len());
+    let charset_ok = token
+        .as_bytes()
+        .iter()
+        .all(|byte| matches!(byte, b'A'..=b'Z' | b'a'..=b'z' | b'0'..=b'9' | b'.' | b'_' | b'~' | b'+' | b'/' | b'=' | b'-'));
+    length_ok && charset_ok
+}
+
+/// File-backed bearer token store. Lives in the per-profile data
+/// directory so a private window never inherits the standard
+/// profile's session — same isolation the rest of the runtime
+/// enforces. Writes go through a temp-rename so a partial write
+/// can't corrupt the persisted token.
+#[derive(Clone, Debug)]
+pub struct BearerTokenStore {
+    path: PathBuf,
+}
+
+impl BearerTokenStore {
+    pub fn new(path: PathBuf) -> Self {
+        Self { path }
+    }
+
+    pub fn path(&self) -> &Path {
+        &self.path
+    }
+
+    pub fn load(&self) -> Result<Option<BearerToken>, SyncClientError> {
+        match fs::read_to_string(&self.path) {
+            Ok(contents) => {
+                if contents.trim().is_empty() {
+                    return Ok(None);
+                }
+                BearerToken::new(contents).map(Some)
+            }
+            Err(error) if error.kind() == ErrorKind::NotFound => Ok(None),
+            Err(error) => Err(SyncClientError::TokenStorage(error.to_string())),
+        }
+    }
+
+    pub fn save(&self, token: &BearerToken) -> Result<(), SyncClientError> {
+        if let Some(parent) = self.path.parent() {
+            fs::create_dir_all(parent).map_err(io_err)?;
+        }
+        let tmp = self.path.with_extension("tmp");
+        fs::write(&tmp, token.as_str()).map_err(io_err)?;
+        fs::rename(&tmp, &self.path).map_err(io_err)?;
+        Ok(())
+    }
+
+    pub fn clear(&self) -> Result<(), SyncClientError> {
+        match fs::remove_file(&self.path) {
+            Ok(()) => Ok(()),
+            Err(error) if error.kind() == ErrorKind::NotFound => Ok(()),
+            Err(error) => Err(SyncClientError::TokenStorage(error.to_string())),
+        }
+    }
+}
+
+fn io_err(error: io::Error) -> SyncClientError {
+    SyncClientError::TokenStorage(error.to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::env::temp_dir;
+
+    #[test]
+    fn rejects_obviously_broken_tokens() {
+        assert!(BearerToken::new("").is_err());
+        assert!(BearerToken::new("   ").is_err());
+        assert!(BearerToken::new("short").is_err());
+        // Spaces are not in the Better Auth bearer charset.
+        assert!(BearerToken::new(format!("{}aaa bb", "a".repeat(40))).is_err());
+    }
+
+    #[test]
+    fn accepts_better_auth_shape() -> Result<(), SyncClientError> {
+        let token = format!("{}-{}_{}", "a".repeat(20), "b".repeat(20), "c".repeat(20));
+        BearerToken::new(token).map(|_| ())
+    }
+
+    #[test]
+    fn token_store_round_trips() -> Result<(), SyncClientError> {
+        let dir = temp_dir().join(format!("ely-token-{}", uuid::Uuid::now_v7().simple()));
+        let store = BearerTokenStore::new(dir.join("token"));
+        let token = BearerToken::new("a".repeat(64))?;
+        assert_eq!(store.load()?, None);
+
+        store.save(&token)?;
+        assert_eq!(store.load()?, Some(token.clone()));
+
+        store.clear()?;
+        assert_eq!(store.load()?, None);
+        Ok(())
+    }
+}
diff --git a/crates/ely_sync_client/src/client.rs b/crates/ely_sync_client/src/client.rs
new file mode 100644
index 0000000..835f902
--- /dev/null
+++ b/crates/ely_sync_client/src/client.rs
@@ -0,0 +1,190 @@
+use std::time::Duration;
+
+use serde::de::DeserializeOwned;
+use ureq::{Agent, AgentBuilder};
+
+use crate::{
+    auth::BearerToken,
+    device::{DeviceIdentity, DeviceListResponse, DeviceRegistration},
+    error::SyncClientError,
+    snapshot::{SnapshotDownload, SnapshotUploadRequest},
+};
+
+const REQUEST_TIMEOUT: Duration = Duration::from_secs(30);
+
+const USER_AGENT: &str = concat!("ELY Browser/", env!("CARGO_PKG_VERSION"));
+
+/// Resolved configuration for talking to a worker deployment.
+#[derive(Clone, Debug)]
+pub struct ApiClientConfig {
+    base_url: String,
+    region: String,
+}
+
+impl ApiClientConfig {
+    /// Build the config used by the production deployment. The base URL
+    /// matches `wrangler.toml`'s `ELY_AUTH_BASE_URL`.
+    pub fn production() -> Self {
+        Self {
+            base_url: "https://ely-browser-cloud.zhangyanghaha0407.workers.dev".to_string(),
+            region: "auto".to_string(),
+        }
+    }
+
+    pub fn custom(base_url: impl Into<String>, region: impl Into<String>) -> Self {
+        Self { base_url: base_url.into(), region: region.into() }
+    }
+
+    pub fn base_url(&self) -> &str {
+        &self.base_url
+    }
+
+    pub fn region(&self) -> &str {
+        &self.region
+    }
+}
+
+/// Bearer-token authenticated HTTP client targeting `ely-browser-cloud`.
+pub struct SyncApiClient {
+    agent: Agent,
+    config: ApiClientConfig,
+    bearer: BearerToken,
+}
+
+impl SyncApiClient {
+    pub fn new(config: ApiClientConfig, bearer: BearerToken) -> Result<Self, SyncClientError> {
+        if !config.base_url.starts_with("https://") && !config.base_url.starts_with("http://") {
+            return Err(SyncClientError::InvalidBaseUrl { url: config.base_url.clone() });
+        }
+        let agent = AgentBuilder::new().timeout(REQUEST_TIMEOUT).user_agent(USER_AGENT).build();
+        Ok(Self { agent, config, bearer })
+    }
+
+    pub fn config(&self) -> &ApiClientConfig {
+        &self.config
+    }
+
+    /// `POST /api/devices/register` — bind a freshly-generated device
+    /// identity to the current session. The response carries the
+    /// canonical device record from the worker's `user_devices` table,
+    /// including the `approval_status` that gates `/api/sync/*`.
+    pub fn register_device(
+        &self,
+        identity: &DeviceIdentity,
+        idempotency_key: &str,
+    ) -> Result<DeviceRecordDocument, SyncClientError> {
+        let registration = DeviceRegistration {
+            device_id: &identity.device_id,
+            public_key: &identity.public_key,
+            device_name: &identity.device_name,
+            platform: &identity.platform,
+            idempotency_key,
+        };
+        let endpoint = self.endpoint("/api/devices/register");
+        let response = self
+            .agent
+            .post(&endpoint)
+            .set("Authorization", &format!("Bearer {}", self.bearer.as_str()))
+            .set("Content-Type", "application/json")
+            .send_json(serde_json::to_value(&registration).map_err(|error| {
+                SyncClientError::Json { endpoint: endpoint.clone(), source: error }
+            })?);
+        let body = read_json_response::<DeviceRecordDocument>(&endpoint, response)?;
+        Ok(body)
+    }
+
+    /// `GET /api/devices` — return every device bound to the user,
+    /// approved or otherwise. Used by the UI to render the pending
+    /// device-approval list.
+    pub fn list_devices(&self) -> Result<DeviceListResponse, SyncClientError> {
+        let endpoint = self.endpoint("/api/devices");
+        let response = self
+            .agent
+            .get(&endpoint)
+            .set("Authorization", &format!("Bearer {}", self.bearer.as_str()))
+            .call();
+        read_json_response::<DeviceListResponse>(&endpoint, response)
+    }
+
+    /// `POST /api/sync/snapshot` — push the full per-user state. The
+    /// worker enforces logical-clock monotonicity, so callers must
+    /// pass a value strictly greater than the last accepted snapshot.
+    pub fn upload_snapshot(
+        &self,
+        request: &SnapshotUploadRequest<'_>,
+    ) -> Result<SnapshotUploadDocument, SyncClientError> {
+        let endpoint = self.endpoint("/api/sync/snapshot");
+        let response =
+            self.agent
+                .post(&endpoint)
+                .set("Authorization", &format!("Bearer {}", self.bearer.as_str()))
+                .set("Content-Type", "application/json")
+                .send_json(serde_json::to_value(request).map_err(|error| {
+                    SyncClientError::Json { endpoint: endpoint.clone(), source: error }
+                })?);
+        read_json_response::<SnapshotUploadDocument>(&endpoint, response)
+    }
+
+    /// `GET /api/sync/snapshot?snapshot_id=…` — fetch the snapshot for
+    /// the named id. Returns the encoded payload plus the snapshot
+    /// metadata; callers should verify the payload hash before trusting
+    /// the bytes.
+    pub fn download_snapshot(
+        &self,
+        snapshot_id: &str,
+    ) -> Result<SnapshotDownload, SyncClientError> {
+        let endpoint = self.endpoint(&format!("/api/sync/snapshot?snapshot_id={snapshot_id}"));
+        let response = self
+            .agent
+            .get(&endpoint)
+            .set("Authorization", &format!("Bearer {}", self.bearer.as_str()))
+            .call();
+        read_json_response::<SnapshotDownload>(&endpoint, response)
+    }
+
+    fn endpoint(&self, path: &str) -> String {
+        format!("{}{}", self.config.base_url.trim_end_matches('/'), path)
+    }
+}
+
+#[derive(Clone, Debug, serde::Deserialize)]
+pub struct DeviceRecordDocument {
+    pub version: u32,
+    pub user_id: String,
+    pub device: crate::device::DeviceRecord,
+}
+
+#[derive(Clone, Debug, serde::Deserialize)]
+pub struct SnapshotUploadDocument {
+    pub version: u32,
+    pub user_id: String,
+    pub device_id: String,
+    pub snapshot: crate::snapshot::SnapshotDocument,
+}
+
+fn read_json_response<T: DeserializeOwned>(
+    endpoint: &str,
+    response: Result<ureq::Response, ureq::Error>,
+) -> Result<T, SyncClientError> {
+    match response {
+        Ok(ok) => {
+            let status = ok.status();
+            let body = ok.into_string().map_err(|error| SyncClientError::HttpStatus {
+                endpoint: endpoint.to_string(),
+                status,
+                body: error.to_string(),
+            })?;
+            serde_json::from_str::<T>(&body).map_err(|error| SyncClientError::Json {
+                endpoint: endpoint.to_string(),
+                source: error,
+            })
+        }
+        Err(ureq::Error::Status(status, raw)) => {
+            let body = raw.into_string().unwrap_or_default();
+            Err(SyncClientError::HttpStatus { endpoint: endpoint.to_string(), status, body })
+        }
+        Err(other) => {
+            Err(SyncClientError::Http { endpoint: endpoint.to_string(), source: Box::new(other) })
+        }
+    }
+}
diff --git a/crates/ely_sync_client/src/device.rs b/crates/ely_sync_client/src/device.rs
new file mode 100644
index 0000000..b363697
--- /dev/null
+++ b/crates/ely_sync_client/src/device.rs
@@ -0,0 +1,155 @@
+use std::{
+    fs,
+    io::{self, ErrorKind},
+    path::Path,
+};
+
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+use crate::error::SyncClientError;
+
+/// Locally-stable device identity. Constructed once per profile data
+/// directory and persisted so reinstalls don't trigger re-approval
+/// requests — the same `device_id` is reused across runs.
+#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
+pub struct DeviceIdentity {
+    pub device_id: String,
+    pub public_key: String,
+    pub device_name: String,
+    pub platform: String,
+}
+
+impl DeviceIdentity {
+    /// Load the persisted identity, or create-and-save a new one.
+    /// The identity file lives at `path`; callers usually pick
+    /// `<profile_data>/sync/device.json`.
+    pub fn load_or_create(
+        path: &Path,
+        device_name: impl Into<String>,
+        platform: impl Into<String>,
+    ) -> Result<Self, SyncClientError> {
+        match fs::read_to_string(path) {
+            Ok(contents) => {
+                let identity: Self = serde_json::from_str(&contents).map_err(|error| {
+                    SyncClientError::TokenStorage(format!(
+                        "device identity is corrupt at {}: {error}",
+                        path.display()
+                    ))
+                })?;
+                identity.validate()?;
+                Ok(identity)
+            }
+            Err(error) if error.kind() == ErrorKind::NotFound => {
+                let identity = Self::generate(device_name, platform);
+                identity.save(path)?;
+                Ok(identity)
+            }
+            Err(error) => Err(SyncClientError::TokenStorage(error.to_string())),
+        }
+    }
+
+    pub fn generate(device_name: impl Into<String>, platform: impl Into<String>) -> Self {
+        let device_id = format!("ely-{}", Uuid::now_v7().simple());
+        // Placeholder public key — Ed25519 device-bound signing is a
+        // backend feature still in design. The worker validates the
+        // shape but does not currently challenge it.
+        let public_key = format!("ed25519:{}", Uuid::now_v7().simple());
+        Self { device_id, public_key, device_name: device_name.into(), platform: platform.into() }
+    }
+
+    pub fn save(&self, path: &Path) -> Result<(), SyncClientError> {
+        if let Some(parent) = path.parent() {
+            fs::create_dir_all(parent).map_err(io_err)?;
+        }
+        let tmp = path.with_extension("tmp");
+        let serialized = serde_json::to_string_pretty(self).map_err(|error| {
+            SyncClientError::TokenStorage(format!("device identity serialize: {error}"))
+        })?;
+        fs::write(&tmp, serialized).map_err(io_err)?;
+        fs::rename(&tmp, path).map_err(io_err)?;
+        Ok(())
+    }
+
+    fn validate(&self) -> Result<(), SyncClientError> {
+        if !is_device_id_shape(&self.device_id) {
+            return Err(SyncClientError::TokenStorage(
+                "device_id does not match the Cloudflare worker pattern".to_string(),
+            ));
+        }
+        if self.public_key.trim().is_empty() {
+            return Err(SyncClientError::TokenStorage("device public_key is empty".to_string()));
+        }
+        if self.device_name.trim().is_empty() {
+            return Err(SyncClientError::TokenStorage("device_name is empty".to_string()));
+        }
+        if self.platform.trim().is_empty() {
+            return Err(SyncClientError::TokenStorage("platform is empty".to_string()));
+        }
+        Ok(())
+    }
+}
+
+fn is_device_id_shape(value: &str) -> bool {
+    (3..=128).contains(&value.len())
+        && value
+            .as_bytes()
+            .iter()
+            .all(|byte| matches!(byte, b'A'..=b'Z' | b'a'..=b'z' | b'0'..=b'9' | b'.' | b'_' | b':' | b'-'))
+}
+
+fn io_err(error: io::Error) -> SyncClientError {
+    SyncClientError::TokenStorage(error.to_string())
+}
+
+#[derive(Clone, Debug, Serialize)]
+pub struct DeviceRegistration<'a> {
+    pub device_id: &'a str,
+    pub public_key: &'a str,
+    pub device_name: &'a str,
+    pub platform: &'a str,
+    pub idempotency_key: &'a str,
+}
+
+#[derive(Clone, Debug, Deserialize)]
+pub struct DeviceListResponse {
+    pub version: u32,
+    pub user_id: String,
+    pub devices: Vec<DeviceRecord>,
+}
+
+#[derive(Clone, Debug, Deserialize)]
+pub struct DeviceRecord {
+    pub device_id: String,
+    pub device_name: String,
+    pub platform: String,
+    pub approval_status: String,
+    pub created_at: u64,
+    pub approved_at: Option<u64>,
+    pub last_active_at: Option<u64>,
+    pub revoked_at: Option<u64>,
+}
+
+impl DeviceRecord {
+    pub fn is_approved(&self) -> bool {
+        self.approval_status == "approved" && self.revoked_at.is_none()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::env::temp_dir;
+
+    #[test]
+    fn identity_round_trips() -> Result<(), SyncClientError> {
+        let dir = temp_dir().join(format!("ely-device-{}", Uuid::now_v7().simple()));
+        let path = dir.join("device.json");
+        let identity = DeviceIdentity::load_or_create(&path, "Test", "macos")?;
+        identity.validate()?;
+
+        let again = DeviceIdentity::load_or_create(&path, "ignored", "ignored")?;
+        assert_eq!(identity, again);
+        Ok(())
+    }
+}
diff --git a/crates/ely_sync_client/src/error.rs b/crates/ely_sync_client/src/error.rs
new file mode 100644
index 0000000..3885884
--- /dev/null
+++ b/crates/ely_sync_client/src/error.rs
@@ -0,0 +1,33 @@
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum SyncClientError {
+    #[error("API base URL is invalid: {url}")]
+    InvalidBaseUrl { url: String },
+
+    #[error("Bearer token storage is unavailable: {0}")]
+    TokenStorage(String),
+
+    #[error("HTTP request failed for {endpoint}: {source}")]
+    Http {
+        endpoint: String,
+        #[source]
+        source: Box<ureq::Error>,
+    },
+
+    #[error("HTTP {status} {endpoint}: {body}")]
+    HttpStatus { endpoint: String, status: u16, body: String },
+
+    #[error("JSON parsing failed for {endpoint}: {source}")]
+    Json {
+        endpoint: String,
+        #[source]
+        source: serde_json::Error,
+    },
+
+    #[error("Snapshot payload is too large: {bytes} bytes (max {limit})")]
+    SnapshotTooLarge { bytes: usize, limit: usize },
+
+    #[error("Snapshot base64 decode failed: {0}")]
+    SnapshotBase64(String),
+}
diff --git a/crates/ely_sync_client/src/lib.rs b/crates/ely_sync_client/src/lib.rs
new file mode 100644
index 0000000..d193278
--- /dev/null
+++ b/crates/ely_sync_client/src/lib.rs
@@ -0,0 +1,37 @@
+//! HTTP client for the `ely-browser-cloud` Cloudflare worker.
+//!
+//! The worker exposes the device-bound sync API documented in
+//! `cloudflare/src/sync_*.ts`. This crate is the renderer-side counterpart:
+//! it owns the Better Auth bearer token, the locally-generated device
+//! identity, and the JSON wire types needed to push and pull the user's
+//! browser state.
+//!
+//! Scope today:
+//! - Bearer-token authenticated requests via `ureq`.
+//! - Device registration (`POST /api/devices/register`) and listing
+//!   (`GET /api/devices`).
+//! - Sync snapshot upload (`POST /api/sync/snapshot`) and download
+//!   (`GET /api/sync/snapshot?snapshot_id=…`).
+//!
+//! Intentionally omitted (kept for follow-up work, not papered over here):
+//! - The full Better Auth handshake (email + OTP / OAuth). Callers obtain
+//!   the bearer token out-of-band and hand it to the client.
+//! - First-device approval bootstrap. The Cloudflare API rejects sync from
+//!   an unapproved device; the user must approve a freshly-registered
+//!   device from another already-approved device (or via direct D1
+//!   operation), exactly as the backend enforces.
+//! - Incremental change-log push/pull (`/api/sync/push` and `/api/sync/pull`).
+//!   The snapshot path is the simplest contract that round-trips the user's
+//!   entire state, so we start there.
+
+pub mod auth;
+pub mod client;
+pub mod device;
+pub mod error;
+pub mod snapshot;
+
+pub use auth::{BearerToken, BearerTokenStore};
+pub use client::{ApiClientConfig, SyncApiClient};
+pub use device::{DeviceIdentity, DeviceListResponse, DeviceRecord, DeviceRegistration};
+pub use error::SyncClientError;
+pub use snapshot::{SnapshotDownload, SnapshotPayload, SnapshotUploadRequest};
diff --git a/crates/ely_sync_client/src/snapshot.rs b/crates/ely_sync_client/src/snapshot.rs
new file mode 100644
index 0000000..e008665
--- /dev/null
+++ b/crates/ely_sync_client/src/snapshot.rs
@@ -0,0 +1,227 @@
+use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
+
+use crate::error::SyncClientError;
+
+/// Hard cap from `cloudflare/src/sync_snapshot.ts`: a single snapshot
+/// upload may not exceed 10 MiB. We enforce the same limit client-side
+/// so the user sees a typed error instead of a generic HTTP 400.
+pub const MAX_SNAPSHOT_BYTES: usize = 10 * 1024 * 1024;
+
+/// Validated snapshot payload, ready to ship to `/api/sync/snapshot`.
+#[derive(Clone, Debug)]
+pub struct SnapshotPayload {
+    bytes: Vec<u8>,
+    payload_hash: String,
+}
+
+impl SnapshotPayload {
+    /// Wrap `bytes` and pre-compute the SHA-256 the worker validates
+    /// before persisting the R2 object. Rejects payloads above the
+    /// 10 MiB limit so the caller sees the failure before any
+    /// network IO.
+    pub fn new(bytes: Vec<u8>) -> Result<Self, SyncClientError> {
+        if bytes.is_empty() {
+            return Err(SyncClientError::SnapshotTooLarge { bytes: 0, limit: MAX_SNAPSHOT_BYTES });
+        }
+        if bytes.len() > MAX_SNAPSHOT_BYTES {
+            return Err(SyncClientError::SnapshotTooLarge {
+                bytes: bytes.len(),
+                limit: MAX_SNAPSHOT_BYTES,
+            });
+        }
+        let mut hasher = Sha256::new();
+        hasher.update(&bytes);
+        let digest = hasher.finalize();
+        let payload_hash = digest.iter().map(|byte| format!("{byte:02x}")).collect::<String>();
+        Ok(Self { bytes, payload_hash })
+    }
+
+    pub fn bytes(&self) -> &[u8] {
+        &self.bytes
+    }
+
+    pub fn payload_hash(&self) -> &str {
+        &self.payload_hash
+    }
+}
+
+#[derive(Clone, Debug, Serialize)]
+pub struct SnapshotUploadRequest<'a> {
+    pub version: u32,
+    pub snapshot_id: &'a str,
+    pub region: &'a str,
+    pub payload_hash: &'a str,
+    pub schema_rev: u32,
+    pub logical_clock: u64,
+    pub data_base64: String,
+}
+
+impl<'a> SnapshotUploadRequest<'a> {
+    pub fn new(
+        snapshot_id: &'a str,
+        region: &'a str,
+        schema_rev: u32,
+        logical_clock: u64,
+        payload: &'a SnapshotPayload,
+    ) -> Self {
+        Self {
+            version: 1,
+            snapshot_id,
+            region,
+            payload_hash: payload.payload_hash(),
+            schema_rev,
+            logical_clock,
+            data_base64: encode_base64(payload.bytes()),
+        }
+    }
+}
+
+#[derive(Clone, Debug, Deserialize)]
+pub struct SnapshotDownload {
+    pub version: u32,
+    pub user_id: String,
+    pub device_id: String,
+    pub snapshot: SnapshotDocument,
+    pub data_base64: String,
+}
+
+impl SnapshotDownload {
+    /// Decode + hash-verify the returned base64 payload against the
+    /// snapshot's advertised `payload_hash`. Mirrors the contract the
+    /// worker enforces on upload — we re-check on download so a
+    /// tampered storage layer doesn't silently desync the user.
+    pub fn payload(&self) -> Result<SnapshotPayload, SyncClientError> {
+        let bytes = decode_base64(&self.data_base64)
+            .map_err(|error| SyncClientError::SnapshotBase64(error.to_string()))?;
+        let payload = SnapshotPayload::new(bytes)?;
+        if payload.payload_hash() != self.snapshot.payload_hash {
+            return Err(SyncClientError::SnapshotBase64(
+                "downloaded snapshot hash does not match server-advertised hash".to_string(),
+            ));
+        }
+        Ok(payload)
+    }
+}
+
+#[derive(Clone, Debug, Deserialize)]
+pub struct SnapshotDocument {
+    pub snapshot_id: String,
+    pub r2_key: String,
+    pub payload_hash: String,
+    pub schema_rev: u32,
+    pub logical_clock: u64,
+    pub device_id: String,
+    pub size_bytes: u64,
+    pub created_at: u64,
+}
+
+const BASE64_CHARS: &[u8; 64] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+fn encode_base64(bytes: &[u8]) -> String {
+    let mut out = String::with_capacity(bytes.len().div_ceil(3) * 4);
+    for chunk in bytes.chunks(3) {
+        let len = chunk.len();
+        let b0 = chunk[0];
+        let b1 = if len > 1 { chunk[1] } else { 0 };
+        let b2 = if len > 2 { chunk[2] } else { 0 };
+        out.push(BASE64_CHARS[(b0 >> 2) as usize] as char);
+        out.push(BASE64_CHARS[(((b0 & 0b11) << 4) | (b1 >> 4)) as usize] as char);
+        out.push(if len > 1 {
+            BASE64_CHARS[(((b1 & 0b1111) << 2) | (b2 >> 6)) as usize] as char
+        } else {
+            '='
+        });
+        out.push(if len > 2 { BASE64_CHARS[(b2 & 0b111111) as usize] as char } else { '=' });
+    }
+    out
+}
+
+#[derive(Debug)]
+pub(crate) struct Base64DecodeError(pub String);
+
+impl std::fmt::Display for Base64DecodeError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(&self.0)
+    }
+}
+
+fn decode_base64(value: &str) -> Result<Vec<u8>, Base64DecodeError> {
+    let trimmed = value.trim();
+    if !trimmed.len().is_multiple_of(4) {
+        return Err(Base64DecodeError("input length is not a multiple of 4".to_string()));
+    }
+    let mut out = Vec::with_capacity(trimmed.len() / 4 * 3);
+    let mut buffer = [0u32; 4];
+    let mut buffer_len = 0;
+    let mut padding = 0;
+    for byte in trimmed.bytes() {
+        let value = match byte {
+            b'A'..=b'Z' => u32::from(byte - b'A'),
+            b'a'..=b'z' => u32::from(byte - b'a') + 26,
+            b'0'..=b'9' => u32::from(byte - b'0') + 52,
+            b'+' => 62,
+            b'/' => 63,
+            b'=' => {
+                padding += 1;
+                if padding > 2 {
+                    return Err(Base64DecodeError("excess padding".to_string()));
+                }
+                0
+            }
+            _ => return Err(Base64DecodeError(format!("unexpected byte {byte:#x}"))),
+        };
+        buffer[buffer_len] = value;
+        buffer_len += 1;
+        if buffer_len == 4 {
+            let combined = (buffer[0] << 18) | (buffer[1] << 12) | (buffer[2] << 6) | buffer[3];
+            out.push(((combined >> 16) & 0xFF) as u8);
+            if padding < 2 {
+                out.push(((combined >> 8) & 0xFF) as u8);
+            }
+            if padding < 1 {
+                out.push((combined & 0xFF) as u8);
+            }
+            buffer_len = 0;
+            padding = 0;
+        }
+    }
+    if buffer_len != 0 {
+        return Err(Base64DecodeError("trailing partial quartet".to_string()));
+    }
+    Ok(out)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn payload_rejects_empty_and_oversized() {
+        assert!(SnapshotPayload::new(vec![]).is_err());
+        let too_big = vec![0u8; MAX_SNAPSHOT_BYTES + 1];
+        assert!(SnapshotPayload::new(too_big).is_err());
+    }
+
+    #[test]
+    fn payload_hash_matches_sha256() -> Result<(), SyncClientError> {
+        let payload = SnapshotPayload::new(b"ely".to_vec())?;
+        // Real SHA-256 of "ely" — verified at test time so a future
+        // hashing regression is caught immediately.
+        assert_eq!(
+            payload.payload_hash(),
+            "52fe440ae87b395cc46438ee9275fcadc0f98a048161345f53d6b0038976a6f5"
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn base64_round_trip() -> Result<(), Base64DecodeError> {
+        for sample in [b"".as_slice(), b"a", b"ab", b"abc", b"ely browser"] {
+            let encoded = encode_base64(sample);
+            let decoded = decode_base64(&encoded)?;
+            assert_eq!(decoded.as_slice(), sample);
+        }
+        Ok(())
+    }
+}