diff --git a/src/cloud-api.ts b/src/cloud-api.ts
new file mode 100644
index 0000000..8330fd8
--- /dev/null
+++ b/src/cloud-api.ts
@@ -0,0 +1,50 @@
+// cloud-api.ts · C3 Cloud API v0.1.0
+
+export type RecordKind =
+  | "project" | "session" | "event"
+  | "memory" | "preference";
+
+export interface CloudDevice { id: string; name: string; createdAt: string }
+
+export interface AuthResult {
+  userId: string;
+  device: CloudDevice;
+  deviceToken: string; // per laptop, revoked on logout
+}
+
+export interface CloudAuth {
+  signUp(input: { githubCode: string; deviceName: string }): Promise<AuthResult>;
+  logIn(input: { githubCode: string; deviceName: string }): Promise<AuthResult>;
+  logOut(): Promise<{ ok: true }>; // revokes only this device's token
+}
+
+export interface SyncChange {
+  kind: RecordKind;
+  id: string;          // stable ID. Events: `${sessionId}:${seq}`
+  version: number;
+  deleted?: boolean;
+  data: Record<string, unknown>; // never paths, secrets or raw output
+}
+
+export interface PushRequest { changes: SyncChange[] }
+export interface PushResult {
+  accepted: string[];
+  rejected: { id: string; reason: "too_large" | "forbidden_field" | "illegal_move" | "stale" }[];
+}
+
+export interface PullRequest { cursor: string | null }
+export interface PullResult {
+  changes: SyncChange[];
+  nextCursor: string;
+  hasMore: boolean;
+}
+
+export interface CloudSync {
+  push(req: PushRequest): Promise<PushResult>;
+  pull(req: PullRequest): Promise<PullResult>;
+}
+
+export interface CloudDevices {
+  listDevices(): Promise<{ devices: CloudDevice[] }>;
+  revokeDevice(id: string): Promise<{ ok: true }>;
+}
\ No newline at end of file
diff --git a/src/index.ts b/src/index.ts
index 3144a6a..93e4fe1 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -4,3 +4,4 @@
 
 export * from "./session.js";
 export * from "./runner.js";
+export * from "./cloud-api.js"
\ No newline at end of file