diff --git a/Packages/ICCeryCore/Sources/ICCeryCore/Files/ArtefactProbe.swift b/Packages/ICCeryCore/Sources/ICCeryCore/Files/ArtefactProbe.swift
index a090b31..94f62b0 100644
--- a/Packages/ICCeryCore/Sources/ICCeryCore/Files/ArtefactProbe.swift
+++ b/Packages/ICCeryCore/Sources/ICCeryCore/Files/ArtefactProbe.swift
@@ -12,6 +12,20 @@ public struct StageArtefacts: Sendable, Equatable {
     public var stage4Complete = false
     /// Absolute path of the profile file when present.
     public var profilePath: URL?
+
+    public init(
+        stage1Complete: Bool = false,
+        stage2Complete: Bool = false,
+        stage3Complete: Bool = false,
+        stage4Complete: Bool = false,
+        profilePath: URL? = nil
+    ) {
+        self.stage1Complete = stage1Complete
+        self.stage2Complete = stage2Complete
+        self.stage3Complete = stage3Complete
+        self.stage4Complete = stage4Complete
+        self.profilePath = profilePath
+    }
 }
 
 /// Filesystem probing for wizard artefacts (docs/02 §Working directory,
diff --git a/Packages/ICCeryCore/Sources/ICCeryCore/Wizard/WizardGating.swift b/Packages/ICCeryCore/Sources/ICCeryCore/Wizard/WizardGating.swift
new file mode 100644
index 0000000..91973d0
--- /dev/null
+++ b/Packages/ICCeryCore/Sources/ICCeryCore/Wizard/WizardGating.swift
@@ -0,0 +1,59 @@
+import Foundation
+
+/// Artefact-driven stage gating (issue #4, docs/06 §Stages).
+///
+/// Navigation is *disk*, not buttons: a stage unlocks only when its
+/// predecessor artefacts exist. Forward moves are gated; backward is
+/// always allowed. Gating is re-evaluated on window focus and on stage
+/// entry (#151 — files can disappear in Finder).
+public enum WizardGating {
+
+    /// Whether `stage` is reachable given the probed artefacts.
+    ///
+    /// - Stage 0 (calibrate): always — it is out-of-band, not gated.
+    /// - Stage 1: always.
+    /// - Stage 2: `.ti1` exists.
+    /// - Stage 3: `.ti1` **and** `.ti2`.
+    /// - Stage 4: `.ti3` exists (accepted measurement only — a `.ti2`
+    ///   alone never unlocks it; #109/#110).
+    /// - Stage 5: `.ti3` **and** `.icc`/`.icm`.
+    public static func isUnlocked(
+        _ stage: WizardStage,
+        artefacts: StageArtefacts
+    ) -> Bool {
+        switch stage {
+        case .calibrate:     return true
+        case .generate:      return true
+        case .layOutPrint:   return artefacts.stage1Complete
+        case .measure:       return artefacts.stage1Complete && artefacts.stage2Complete
+        case .buildProfile:  return artefacts.stage3Complete
+        case .verifyInstall: return artefacts.stage3Complete && artefacts.stage4Complete
+        }
+    }
+
+    /// Whether `go(to:)` may proceed. Backward moves and the current
+    /// stage are always allowed; forward moves must be unlocked.
+    public static func canNavigate(
+        to target: WizardStage,
+        from current: WizardStage,
+        artefacts: StageArtefacts
+    ) -> Bool {
+        if target == current { return true }
+        if target == .calibrate || current == .calibrate {
+            // Stage 0 is a side-trip, not stepper navigation.
+            return true
+        }
+        if target.rawValue < current.rawValue { return true }
+        return isUnlocked(target, artefacts: artefacts)
+    }
+
+    /// The deepest unlocked stepper stage — used when revalidation
+    /// locks the current stage (#151).
+    public static func deepestUnlocked(artefacts: StageArtefacts) -> WizardStage {
+        for stage in WizardStage.stepperStages.reversed()
+        where isUnlocked(stage, artefacts: artefacts) {
+            return stage
+        }
+        return .generate
+    }
+}
diff --git a/Packages/ICCeryCore/Sources/ICCeryCore/Wizard/WizardState.swift b/Packages/ICCeryCore/Sources/ICCeryCore/Wizard/WizardState.swift
new file mode 100644
index 0000000..85da63e
--- /dev/null
+++ b/Packages/ICCeryCore/Sources/ICCeryCore/Wizard/WizardState.swift
@@ -0,0 +1,72 @@
+import Foundation
+
+/// Session mode (docs/06 §wizardState). `"calibration"` is set while
+/// Stage 0 is driving a `CAL_` chart through the same pipeline.
+public enum SessionMode: String, Codable, Sendable {
+    case profile
+    case calibration
+}
+
+/// Persisted wizard state (docs/06 §wizardState fields) —
+/// `wizard_state.json` in app data.
+public struct WizardState: Codable, Equatable, Sendable {
+    /// 0–5 (`WizardStage.rawValue`).
+    public var currentStage: Int
+    /// Run name without extension — never invented (#60).
+    public var basename: String
+    /// Working directory for artefacts; empty → `resolveSafeCwd` (#59).
+    public var cwd: String
+    /// Last spooled printer, for calibration drift history.
+    public var printerName: String?
+    public var sessionMode: SessionMode
+    /// May differ from `basename` after a `.ti3` import (#94).
+    public var profileBasename: String?
+
+    public init(
+        currentStage: Int = WizardStage.generate.rawValue,
+        basename: String = "",
+        cwd: String = "",
+        printerName: String? = nil,
+        sessionMode: SessionMode = .profile,
+        profileBasename: String? = nil
+    ) {
+        self.currentStage = currentStage
+        self.basename = basename
+        self.cwd = cwd
+        self.printerName = printerName
+        self.sessionMode = sessionMode
+        self.profileBasename = profileBasename
+    }
+
+    public static let `default` = WizardState()
+
+    /// The stage a saved `currentStage` resolves to, clamped to a valid
+    /// value (corrupt ints fall back to Stage 1).
+    public var stage: WizardStage {
+        WizardStage(rawValue: currentStage) ?? .generate
+    }
+}
+
+/// Atomic JSON persistence for `WizardState` (issue #4).
+public final class WizardStateStore: Sendable {
+    public let fileURL: URL
+
+    public init(
+        fileURL: URL = AppPaths.appDataDir.appendingPathComponent("wizard_state.json")
+    ) {
+        self.fileURL = fileURL
+    }
+
+    public func load() -> WizardState {
+        guard let data = try? Data(contentsOf: fileURL),
+              let state = try? JSONDecoder().decode(WizardState.self, from: data)
+        else { return .default }
+        return state
+    }
+
+    public func save(_ state: WizardState) throws {
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
+        try AtomicFileWriter.write(encoder.encode(state), to: fileURL)
+    }
+}
diff --git a/Sources/ICCery/RootView.swift b/Sources/ICCery/RootView.swift
index 59f5d9f..3a38f5c 100644
--- a/Sources/ICCery/RootView.swift
+++ b/Sources/ICCery/RootView.swift
@@ -1,3 +1,4 @@
+import AppKit
 import SwiftUI
 
 /// Root layout: 270 pt sidebar + main stage area with the notification
@@ -28,6 +29,13 @@ struct RootView: View {
         }
         .frame(minWidth: 1100, minHeight: 700)
         .background(Theme.background)
+        // #151: re-probe artefacts when the window regains focus —
+        // files deleted in Finder must re-lock stages.
+        .onReceive(
+            NotificationCenter.default.publisher(
+                for: NSWindow.didBecomeKeyNotification
+            )
+        ) { _ in model.windowDidBecomeKey() }
         .sheet(isPresented: $showingSettings) {
             SettingsView()
         }
diff --git a/Sources/ICCery/SidebarView.swift b/Sources/ICCery/SidebarView.swift
index 0cefc64..7774bed 100644
--- a/Sources/ICCery/SidebarView.swift
+++ b/Sources/ICCery/SidebarView.swift
@@ -60,8 +60,8 @@ struct SidebarView: View {
                     StepperRow(
                         stage: stage,
                         isActive: model.stage == stage,
-                        // Only Stage 1 until artefact gating lands in #4.
-                        isEnabled: stage == .generate
+                        // Artefact gating (issue #4) — disk is truth.
+                        isEnabled: model.isUnlocked(stage)
                     ) {
                         model.go(to: stage)
                     }
diff --git a/Sources/ICCery/WizardViewModel.swift b/Sources/ICCery/WizardViewModel.swift
index 4c405bf..2b0b9dc 100644
--- a/Sources/ICCery/WizardViewModel.swift
+++ b/Sources/ICCery/WizardViewModel.swift
@@ -2,44 +2,142 @@ import Foundation
 import Observation
 import ICCeryCore
 
-/// Wizard shell state (issue #1). Artefact gating, persistence and the
-/// "open existing" flow land in issue #4.
+/// Wizard state machine + artefact gating (issue #4, docs/06).
+///
+/// `wizardState` fields (`currentStage`, `basename`, `cwd`,
+/// `printerName`, `sessionMode`, `profileBasename`) are persisted to
+/// `wizard_state.json`; unlocks come from `ArtefactProbe.verify` —
+/// navigation is disk, not buttons.
 @MainActor
 @Observable
 final class WizardViewModel {
-    /// Currently displayed stage.
-    var stage: WizardStage = .generate
+
+    // MARK: - wizardState fields (persisted)
+
+    var stage: WizardStage {
+        didSet { if stage != oldValue { persist() } }
+    }
+    /// `wizardState.basename` — empty until a real artefact names it (#60).
+    var basename: String {
+        didSet { if basename != oldValue { refreshGating(); persist() } }
+    }
+    /// `wizardState.cwd` — resolved via `resolveSafeCwd` (#59).
+    var workingDirectory: URL? {
+        didSet { if workingDirectory != oldValue { refreshGating(); persist() } }
+    }
+    var printerName: String? {
+        didSet { if printerName != oldValue { persist() } }
+    }
+    var sessionMode: SessionMode {
+        didSet { if sessionMode != oldValue { persist() } }
+    }
+    /// `profileBasename` may differ after a `.ti3` import (#94).
+    var profileBasename: String? {
+        didSet { if profileBasename != oldValue { persist() } }
+    }
+
+    // MARK: - Ephemeral
 
     /// Banner notice currently displayed (`#wizardNotification`).
     var notice: Notice?
+    /// Current artefact probe result; recomputed on `refreshGating()`.
+    private(set) var artefacts = StageArtefacts()
 
-    /// Target basename shared across stages (`targetBasename`).
-    var basename: String = ""
-
-    /// Working directory for all Argyll artefacts.
-    var workingDirectory: URL?
-
-    /// Printer queue selected in Stage 2; retained across stages.
-    var printerName: String?
-
+    private let stateStore: WizardStateStore
     private var noticeDismissTask: Task<Void, Never>?
 
-    /// `true` while Stage 0 (printer calibration) is shown instead of a
-    /// stepper stage.
+    init(stateStore: WizardStateStore = WizardStateStore()) {
+        self.stateStore = stateStore
+        let s = stateStore.load()
+        self.stage = s.stage
+        self.basename = s.basename
+        self.workingDirectory = s.cwd.isEmpty ? nil : URL(fileURLWithPath: s.cwd)
+        self.printerName = s.printerName
+        self.sessionMode = s.sessionMode
+        self.profileBasename = s.profileBasename
+        refreshGating()
+        // A restored stage may have been locked since (#151).
+        if !WizardGating.isUnlocked(stage, artefacts: artefacts), stage != .calibrate {
+            stage = WizardGating.deepestUnlocked(artefacts: artefacts)
+        }
+    }
+
+    // MARK: - Gating
+
+    /// `isUnlocked` for the sidebar stepper.
+    func isUnlocked(_ stage: WizardStage) -> Bool {
+        WizardGating.isUnlocked(stage, artefacts: artefacts)
+    }
+
+    /// `true` while Stage 0 (printer calibration) is shown.
     var isCalibrating: Bool { stage == .calibrate }
 
-    func go(to stage: WizardStage) {
-        self.stage = stage
+    /// Re-probes the artefact directory and re-locks (#151). Called on
+    /// window focus, stage entry, and basename/cwd changes.
+    func refreshGating() {
+        guard !basename.isEmpty, let dir = effectiveWorkingDirectory else {
+            artefacts = StageArtefacts()
+            return
+        }
+        artefacts = ArtefactProbe.verify(basename: basename, cwd: dir)
+    }
+
+    /// `setTarget(basename, cwd)` — validates the basename (no `/`, `\`,
+    /// `..`; no placeholders — #60) and resolves the cwd (#59).
+    func setTarget(basename: String, workingDirectory: URL?) {
+        do {
+            self.basename = try PathSecurity.sanitizeBasename(basename)
+        } catch {
+            showNotice("Invalid target name.", kind: .error)
+            return
+        }
+        self.workingDirectory = PathSecurity.resolveSafeCwd(workingDirectory)
+    }
+
+    /// cwd never stays empty once a basename exists (#59).
+    var effectiveWorkingDirectory: URL? {
+        if let workingDirectory { return workingDirectory }
+        return basename.isEmpty ? nil : PathSecurity.resolveSafeCwd(nil)
+    }
+
+    // MARK: - Navigation
+
+    /// `navigateToStage(n)` — refuses locked forward moves with a
+    /// warning banner; backward is always allowed (docs/06).
+    func go(to target: WizardStage) {
+        guard target != .calibrate else { enterCalibration(); return }
+        if WizardGating.canNavigate(to: target, from: stage, artefacts: artefacts) {
+            stage = target
+        } else {
+            showNotice(
+                "Stage \(target.stepperIndex ?? 0) is locked — the required artefact is missing.",
+                kind: .warning
+            )
+        }
     }
 
     func enterCalibration() {
+        sessionMode = .calibration
         stage = .calibrate
     }
 
     func exitCalibration() {
+        sessionMode = .profile
         stage = .generate
     }
 
+    /// Window-focus hook (#151): files deleted in Finder re-lock stages.
+    /// If the current stage re-locked, fall back to the deepest unlocked.
+    func windowDidBecomeKey() {
+        refreshGating()
+        if stage != .calibrate,
+           !WizardGating.isUnlocked(stage, artefacts: artefacts) {
+            stage = WizardGating.deepestUnlocked(artefacts: artefacts)
+        }
+    }
+
+    // MARK: - Notice
+
     func showNotice(_ text: String, kind: Notice.Kind = .info, autoHideAfter: TimeInterval? = 6) {
         noticeDismissTask?.cancel()
         let notice = Notice(kind: kind, text: text, autoHideAfter: autoHideAfter)
@@ -59,4 +157,18 @@ final class WizardViewModel {
         noticeDismissTask?.cancel()
         notice = nil
     }
+
+    // MARK: - Persistence
+
+    private func persist() {
+        let state = WizardState(
+            currentStage: stage.rawValue,
+            basename: basename,
+            cwd: workingDirectory?.path ?? "",
+            printerName: printerName,
+            sessionMode: sessionMode,
+            profileBasename: profileBasename
+        )
+        try? stateStore.save(state)
+    }
 }
diff --git a/Tests/ICCeryCoreTests/WizardGatingTests.swift b/Tests/ICCeryCoreTests/WizardGatingTests.swift
new file mode 100644
index 0000000..4ebf930
--- /dev/null
+++ b/Tests/ICCeryCoreTests/WizardGatingTests.swift
@@ -0,0 +1,127 @@
+import Testing
+import Foundation
+@testable import ICCeryCore
+
+private func artefacts(
+    ti1: Bool = false, ti2: Bool = false, ti3: Bool = false, profile: Bool = false
+) -> StageArtefacts {
+    var a = StageArtefacts()
+    a.stage1Complete = ti1
+    a.stage2Complete = ti2
+    a.stage3Complete = ti3
+    a.stage4Complete = profile
+    if profile {
+        a.profilePath = URL(fileURLWithPath: "/x/t.icc")
+    }
+    return a
+}
+
+@Suite("WizardGating matrix")
+struct WizardGatingTests {
+
+    @Test func emptyProjectOnlyStage1() {
+        let a = artefacts()
+        #expect(WizardGating.isUnlocked(.generate, artefacts: a))
+        #expect(WizardGating.isUnlocked(.calibrate, artefacts: a))
+        for s in [WizardStage.layOutPrint, .measure, .buildProfile, .verifyInstall] {
+            #expect(!WizardGating.isUnlocked(s, artefacts: a), "\(s) should be locked")
+        }
+    }
+
+    @Test func ti1UnlocksStage2Only() {
+        let a = artefacts(ti1: true)
+        #expect(WizardGating.isUnlocked(.layOutPrint, artefacts: a))
+        #expect(!WizardGating.isUnlocked(.measure, artefacts: a))
+        #expect(!WizardGating.isUnlocked(.buildProfile, artefacts: a))
+        #expect(!WizardGating.isUnlocked(.verifyInstall, artefacts: a))
+    }
+
+    @Test func stage3NeedsTi1AndTi2() {
+        #expect(!WizardGating.isUnlocked(.measure, artefacts: artefacts(ti2: true)))
+        #expect(WizardGating.isUnlocked(.measure, artefacts: artefacts(ti1: true, ti2: true)))
+    }
+
+    @Test func stage4NeedsTi3NotTi2() {
+        // #109/#110: .ti2 alone must never unlock Stage 4.
+        let a = artefacts(ti1: true, ti2: true)
+        #expect(!WizardGating.isUnlocked(.buildProfile, artefacts: a))
+        #expect(WizardGating.isUnlocked(.buildProfile, artefacts: artefacts(ti3: true)))
+    }
+
+    @Test func stage5NeedsTi3AndProfile() {
+        #expect(!WizardGating.isUnlocked(.verifyInstall, artefacts: artefacts(ti3: true)))
+        #expect(!WizardGating.isUnlocked(.verifyInstall, artefacts: artefacts(profile: true)))
+        #expect(WizardGating.isUnlocked(
+            .verifyInstall, artefacts: artefacts(ti3: true, profile: true)
+        ))
+    }
+
+    @Test func forwardGatedBackwardFree() {
+        let a = artefacts()
+        #expect(!WizardGating.canNavigate(to: .layOutPrint, from: .generate, artefacts: a))
+        // Backward always allowed even when artefacts vanished.
+        #expect(WizardGating.canNavigate(to: .generate, from: .measure, artefacts: a))
+        // Same stage is a no-op.
+        #expect(WizardGating.canNavigate(to: .measure, from: .measure, artefacts: a))
+        // Stage 0 is a side-trip, never gated.
+        #expect(WizardGating.canNavigate(to: .calibrate, from: .generate, artefacts: a))
+    }
+
+    @Test func deepestUnlocked() {
+        #expect(WizardGating.deepestUnlocked(artefacts: artefacts()) == .generate)
+        #expect(WizardGating.deepestUnlocked(
+            artefacts: artefacts(ti1: true, ti2: true)
+        ) == .measure)
+        #expect(WizardGating.deepestUnlocked(
+            artefacts: artefacts(ti3: true, profile: true)
+        ) == .verifyInstall)
+    }
+}
+
+@Suite("WizardStateStore")
+struct WizardStateStoreTests {
+    private func tempURL() -> URL {
+        FileManager.default.temporaryDirectory
+            .appendingPathComponent("iccery-wiz-\(UUID().uuidString)")
+            .appendingPathComponent("wizard_state.json")
+    }
+
+    @Test func roundTrip() throws {
+        let url = tempURL()
+        let store = WizardStateStore(fileURL: url)
+        var s = WizardState()
+        s.currentStage = 3
+        s.basename = "run-42"
+        s.cwd = "/tmp/charts"
+        s.sessionMode = .calibration
+        s.profileBasename = "imported"
+        try store.save(s)
+        #expect(store.load() == s)
+    }
+
+    @Test func missingFileDefaults() {
+        let s = WizardStateStore(fileURL: tempURL()).load()
+        #expect(s == .default)
+        #expect(s.stage == .generate)
+        #expect(s.sessionMode == .profile)
+    }
+
+    @Test func corruptStageFallsBackToGenerate() throws {
+        let url = tempURL()
+        try FileManager.default.createDirectory(
+            at: url.deletingLastPathComponent(), withIntermediateDirectories: true
+        )
+        try #"{"current_stage": 99, "basename": "", "cwd": "", "session_mode": "profile"}"#
+            .write(to: url, atomically: true, encoding: .utf8)
+        #expect(WizardStateStore(fileURL: url).load().stage == .generate)
+    }
+
+    @Test func sessionModeCalibrationRoundTrips() throws {
+        var s = WizardState(sessionMode: .calibration)
+        let data = try JSONEncoder().encode(s)
+        let decoded = try JSONDecoder().decode(WizardState.self, from: data)
+        #expect(decoded.sessionMode == .calibration)
+        s.sessionMode = .profile
+        #expect(s.sessionMode == .profile)
+    }
+}