diff --git a/Sources/SaplingEditor/DocumentSessionStore.swift b/Sources/SaplingEditor/DocumentSessionStore.swift
new file mode 100644
index 0000000..ed6d343
--- /dev/null
+++ b/Sources/SaplingEditor/DocumentSessionStore.swift
@@ -0,0 +1,72 @@
+import Foundation
+import SaplingCore
+
+@MainActor
+public final class MarkdownDocumentSession: Identifiable, ObservableObject {
+    public let id: UUID
+    public let documentURL: URL
+    public let viewModel: HybridMarkdownEditorViewModel
+
+    public init(
+        id: UUID = UUID(),
+        documentURL: URL,
+        viewModel: HybridMarkdownEditorViewModel
+    ) {
+        self.id = id
+        self.documentURL = documentURL
+        self.viewModel = viewModel
+    }
+}
+
+@MainActor
+public final class DocumentSessionStore: ObservableObject {
+    @Published public private(set) var sessions: [MarkdownDocumentSession]
+    @Published public private(set) var activeSession: MarkdownDocumentSession?
+
+    public init(sessions: [MarkdownDocumentSession] = []) {
+        self.sessions = sessions
+        self.activeSession = sessions.first
+    }
+
+    @discardableResult
+    public func openDocument(
+        at url: URL,
+        loadDocument: @MainActor (URL) throws -> MarkdownDocument = HybridMarkdownEditorViewModel.loadDocument(at:)
+    ) throws -> MarkdownDocumentSession {
+        let key = sessionKey(for: url)
+        if let existingSession = sessions.first(where: { sessionKey(for: $0.documentURL) == key }) {
+            activeSession = existingSession
+            return existingSession
+        }
+
+        let document = try loadDocument(url)
+        let session = MarkdownDocumentSession(
+            documentURL: url,
+            viewModel: HybridMarkdownEditorViewModel(document: document)
+        )
+        sessions.append(session)
+        activeSession = session
+        return session
+    }
+
+    @discardableResult
+    public func activateDocument(at url: URL) -> Bool {
+        let key = sessionKey(for: url)
+        guard let session = sessions.first(where: { sessionKey(for: $0.documentURL) == key }) else {
+            return false
+        }
+
+        activeSession = session
+        return true
+    }
+
+    public func updateActiveSessionFromViewModel() {
+        guard let activeSession else { return }
+        objectWillChange.send()
+        activeSession.objectWillChange.send()
+    }
+
+    private func sessionKey(for url: URL) -> String {
+        url.standardizedFileURL.path
+    }
+}
diff --git a/Tests/SaplingEditorTests/DocumentSessionStoreTests.swift b/Tests/SaplingEditorTests/DocumentSessionStoreTests.swift
new file mode 100644
index 0000000..077280d
--- /dev/null
+++ b/Tests/SaplingEditorTests/DocumentSessionStoreTests.swift
@@ -0,0 +1,55 @@
+import Foundation
+import SaplingCore
+import SaplingEditor
+import XCTest
+
+@MainActor
+final class DocumentSessionStoreTests: XCTestCase {
+    func testOpeningSameFileReusesExistingSession() throws {
+        let url = URL(fileURLWithPath: "/tmp/Sapling/Notes.md")
+        var loadCount = 0
+        let store = DocumentSessionStore()
+
+        let first = try store.openDocument(at: url) { url in
+            loadCount += 1
+            return MarkdownDocument(url: url, title: "Notes", content: "# Notes")
+        }
+
+        let second = try store.openDocument(at: url.standardizedFileURL) { url in
+            loadCount += 1
+            return MarkdownDocument(url: url, title: "Notes", content: "# Reloaded")
+        }
+
+        XCTAssertIdentical(first, second)
+        XCTAssertIdentical(store.activeSession, first)
+        XCTAssertEqual(store.sessions.count, 1)
+        XCTAssertEqual(loadCount, 1)
+        XCTAssertEqual(first.viewModel.document.content, "# Notes")
+    }
+
+    func testOpeningDifferentFilesCreatesSeparateSessionsAndActivatesLatest() throws {
+        let firstURL = URL(fileURLWithPath: "/tmp/Sapling/One.md")
+        let secondURL = URL(fileURLWithPath: "/tmp/Sapling/Two.md")
+        let store = DocumentSessionStore()
+
+        let first = try store.openDocument(at: firstURL) { url in
+            MarkdownDocument(url: url, title: "One", content: "# One")
+        }
+        let second = try store.openDocument(at: secondURL) { url in
+            MarkdownDocument(url: url, title: "Two", content: "# Two")
+        }
+
+        XCTAssertEqual(store.sessions.count, 2)
+        XCTAssertIdentical(store.activeSession, second)
+
+        XCTAssertTrue(store.activateDocument(at: firstURL))
+        XCTAssertIdentical(store.activeSession, first)
+    }
+
+    func testActivateMissingDocumentReturnsFalse() {
+        let store = DocumentSessionStore()
+
+        XCTAssertFalse(store.activateDocument(at: URL(fileURLWithPath: "/tmp/missing.md")))
+        XCTAssertNil(store.activeSession)
+    }
+}