diff --git a/Sources/Core/Persistence/ScrollbackCache.swift b/Sources/Core/Persistence/ScrollbackCache.swift
new file mode 100644
index 0000000..47e800f
--- /dev/null
+++ b/Sources/Core/Persistence/ScrollbackCache.swift
@@ -0,0 +1,133 @@
+// ScrollbackCache.swift
+// Rolling on-disk cache of raw ANSI bytes per session.
+//
+// Design:
+//   • Storage: <caches>/pi-remote/scrollback/<sessionId>.bin
+//   • Cap: 5 MB. When an append would exceed the cap the oldest bytes are
+//     dropped from the front (slice off the head) so the file stays below cap.
+//   • Thread safety: a serial DispatchQueue guards all reads and writes.
+//     No async/await — this is called from both main and background contexts.
+
+import Foundation
+
+/// Rolling on-disk ANSI scrollback cache for a single session.
+///
+/// - `append(_:)` is O(n) on the file size only when the 5 MB cap is hit
+///   (the head-trim path). In the common case (data fits) it is a simple
+///   `FileHandle.seekToEnd` + `write`.
+/// - All public methods are safe to call from any thread or queue.
+public final class ScrollbackCache: @unchecked Sendable {
+
+    // MARK: - Constants
+
+    private static let maxBytes = 5 * 1024 * 1024   // 5 MB
+
+    // MARK: - State
+
+    private let fileURL: URL
+    private let queue = DispatchQueue(label: "pi.scrollback", qos: .utility)
+
+    // Cached file size tracked in memory to avoid repeated stat() calls.
+    private var _sizeBytes: Int = 0
+
+    // MARK: - Init
+
+    /// Creates (or reopens) a cache for `sessionId` stored at
+    /// `<caches>/pi-remote/scrollback/<sessionId>.bin`.
+    public init(sessionId: String) {
+        let cachesDir = FileManager.default.urls(
+            for: .cachesDirectory, in: .userDomainMask
+        ).first ?? URL(fileURLWithPath: NSTemporaryDirectory())
+
+        let dir = cachesDir
+            .appendingPathComponent("pi-remote", isDirectory: true)
+            .appendingPathComponent("scrollback", isDirectory: true)
+
+        // Best-effort directory creation — ignore errors, writes will surface
+        // any real problem.
+        try? FileManager.default.createDirectory(
+            at: dir, withIntermediateDirectories: true
+        )
+
+        fileURL = dir.appendingPathComponent("\(sessionId).bin")
+
+        // Seed the in-memory size counter from the existing file (if any).
+        _sizeBytes = (try? fileURL.resourceValues(forKeys: [.fileSizeKey]))
+            .flatMap { $0.fileSize } ?? 0
+    }
+
+    // MARK: - Public API
+
+    /// Appends `data` to the cache, trimming the head when the 5 MB cap
+    /// would be exceeded.
+    public func append(_ data: Data) {
+        guard !data.isEmpty else { return }
+        queue.sync {
+            _append(data)
+        }
+    }
+
+    /// Returns the full current cache contents (may be empty).
+    public func read() -> Data {
+        queue.sync {
+            (try? Data(contentsOf: fileURL)) ?? Data()
+        }
+    }
+
+    /// Deletes the cache file and resets the in-memory size counter.
+    public func clear() {
+        queue.sync {
+            try? FileManager.default.removeItem(at: fileURL)
+            _sizeBytes = 0
+        }
+    }
+
+    /// Current cache size in bytes (in-memory approximation, always accurate
+    /// after any `append` / `clear` call).
+    public var sizeBytes: Int {
+        queue.sync { _sizeBytes }
+    }
+
+    // MARK: - Private (always called on `queue`)
+
+    private func _append(_ data: Data) {
+        let newSize = _sizeBytes + data.count
+
+        if newSize <= Self.maxBytes {
+            // Fast path: just append.
+            _write(data)
+        } else {
+            // Slow path: we need to drop bytes from the head.
+            // Read the existing content, combine with new data, then keep
+            // only the last `maxBytes` bytes so the result fits within cap.
+            let existing = (try? Data(contentsOf: fileURL)) ?? Data()
+            var combined = existing
+            combined.append(data)
+
+            let excess = combined.count - Self.maxBytes
+            if excess > 0 {
+                combined = combined.dropFirst(excess).withUnsafeBytes { Data($0) }
+            }
+
+            // Overwrite the file with the trimmed data.
+            try? combined.write(to: fileURL, options: .atomic)
+            _sizeBytes = combined.count
+        }
+    }
+
+    private func _write(_ data: Data) {
+        if FileManager.default.fileExists(atPath: fileURL.path) {
+            // Append to existing file via FileHandle.
+            if let handle = try? FileHandle(forWritingTo: fileURL) {
+                defer { try? handle.close() }
+                handle.seekToEndOfFile()
+                handle.write(data)
+                _sizeBytes += data.count
+            }
+        } else {
+            // Create new file.
+            try? data.write(to: fileURL, options: .atomic)
+            _sizeBytes = data.count
+        }
+    }
+}
diff --git a/Sources/Core/Sessions/SessionConnection.swift b/Sources/Core/Sessions/SessionConnection.swift
new file mode 100644
index 0000000..e504f31
--- /dev/null
+++ b/Sources/Core/Sessions/SessionConnection.swift
@@ -0,0 +1,163 @@
+// SessionConnection.swift
+// IC-2.1 — one WebSocket connection per session.
+//
+// Lifecycle:
+//   init → resume(from:) → [stream data] → suspend() → resume(from:) → …
+//
+// URL pattern:  ws://<host>:<port>/sessions/<id>/stream?token=<bearerToken>
+// (TLS pinning is wired in a follow-up task; plain `ws://` for now.)
+
+import Combine
+import Foundation
+
+// MARK: - SessionConnection
+
+/// Manages a single IC-1 WebSocket session stream.
+///
+/// Conforms to `ObservableObject` so SwiftUI views can react to
+/// `connectionState` changes without manually subscribing to Combine.
+///
+/// All mutable state is main-actor-isolated. Callers on background contexts
+/// must dispatch accordingly (normal for `@MainActor` types).
+@MainActor
+public final class SessionConnection: ObservableObject {
+
+    // MARK: - Identity
+
+    /// The session identifier used in the URL path and scrollback file name.
+    public let id: String
+
+    // MARK: - Combine publishers
+
+    /// Emits raw ANSI bytes in arrival order (binary frames, header stripped).
+    public let stream = PassthroughSubject<Data, Never>()
+
+    /// Emits every JSON frame received from the server.
+    public let stateEvents = PassthroughSubject<ServerToClient, Never>()
+
+    /// Tracks the WebSocket lifecycle.
+    @Published public private(set) var connectionState: WebSocketClient.ConnectionState = .disconnected
+
+    // MARK: - Scrollback
+
+    /// Persistent rolling ANSI cache for this session.
+    public private(set) var scrollback: ScrollbackCache
+
+    // MARK: - Private
+
+    private let credential: SidecarCredential
+    private var client: WebSocketClient?
+    private var cancellables = Set<AnyCancellable>()
+
+    // MARK: - Init
+
+    /// Creates a `SessionConnection` for `id` authenticated with `credential`.
+    ///
+    /// Does **not** open a WebSocket. Call `resume(from:)` to connect.
+    public init(id: String, credential: SidecarCredential) {
+        self.id = id
+        self.credential = credential
+        self.scrollback = ScrollbackCache(sessionId: id)
+    }
+
+    // MARK: - Public API
+
+    /// Opens (or re-opens) the WebSocket and sends a `resume` frame.
+    ///
+    /// - Parameter lastSeq: The last acknowledged sequence number, or `nil`
+    ///   to request replay from the beginning.
+    public func resume(from lastSeq: UInt64?) async {
+        guard let url = streamURL else {
+            #if DEBUG
+            print("[SessionConnection] Could not construct stream URL for session \(id) — aborting resume.")
+            #endif
+            return
+        }
+
+        // Tear down any existing connection cleanly before reconnecting.
+        await suspend()
+
+        let ws = WebSocketClient()
+        client = ws
+
+        // Mirror WebSocketClient's connection state into our @Published property.
+        ws.connectionState
+            .receive(on: DispatchQueue.main)
+            .sink { [weak self] state in
+                self?.connectionState = state
+            }
+            .store(in: &cancellables)
+
+        // Binary frames → scrollback + downstream `stream` subject.
+        ws.incomingBinary
+            .sink { [weak self] frame in
+                guard let self else { return }
+                self.scrollback.append(frame.data)
+                self.stream.send(frame.data)
+            }
+            .store(in: &cancellables)
+
+        // JSON frames → `stateEvents` subject.
+        ws.incomingJSON
+            .sink { [weak self] frame in
+                self?.stateEvents.send(frame)
+            }
+            .store(in: &cancellables)
+
+        // Once connected, send the resume frame.
+        ws.connectionState
+            .filter { $0 == .connected }
+            .first()
+            .sink { [weak self, weak ws, lastSeq] _ in
+                guard let self, let ws else { return }
+                Task { @MainActor [self, ws, lastSeq] in
+                    try? await ws.send(.resume(lastSeq: lastSeq))
+                    _ = self // silence unused-capture warning
+                }
+            }
+            .store(in: &cancellables)
+
+        ws.connect(url: url)
+    }
+
+    /// Sends a frame to the server.
+    ///
+    /// - Throws: `WebSocketClientError.notConnected` if there is no active
+    ///   socket, or `WebSocketClientError.encodingFailed` on serialisation
+    ///   failure.
+    public func send(_ frame: ClientToServer) async throws {
+        guard let client else {
+            throw WebSocketClientError.notConnected
+        }
+        try await client.send(frame)
+    }
+
+    /// Closes the WebSocket but keeps local state (scrollback + cursor).
+    public func suspend() async {
+        client?.disconnect()
+        client = nil
+        cancellables.removeAll()
+        connectionState = .disconnected
+    }
+
+    // MARK: - URL construction
+
+    /// Builds `ws://<host>:<port>/sessions/<id>/stream?token=<bearerToken>`.
+    ///
+    /// Returns `nil` if `URLComponents` cannot produce a valid URL (should
+    /// never happen in practice with well-formed credentials).
+    ///
+    /// Note: plain `ws://` is used for now; TLS + cert-pinning wired in
+    /// the T-2.5 follow-up task.
+    private var streamURL: URL? {
+        var components = URLComponents()
+        components.scheme = "ws"
+        components.host = credential.host
+        components.port = credential.port
+        components.path = "/sessions/\(id)/stream"
+        components.queryItems = [
+            URLQueryItem(name: "token", value: credential.bearerToken)
+        ]
+        return components.url
+    }
+}