diff --git a/platforms/ios/app/src/main/swift/Models/PerGameShaderSelection.swift b/platforms/ios/app/src/main/swift/Models/PerGameShaderSelection.swift
index b78a0f7181..3931c73034 100644
--- a/platforms/ios/app/src/main/swift/Models/PerGameShaderSelection.swift
+++ b/platforms/ios/app/src/main/swift/Models/PerGameShaderSelection.swift
@@ -8,15 +8,13 @@ import Foundation
 enum PerGameShaderSelection {
     static let section = "EmuCore/GS"
 
-    /// Written together and cleared together. The token is the durable identity and the absolute
-    /// is a cache of it, because both preset roots sit under a container UUID that moves.
+    /// Written and cleared together: the token is the identity, the absolute is its cache.
     static let keys = (
         enabled: "ShaderChainEnabled",
         presetRef: "ShaderChainPresetRef",
         presetPath: "ShaderChainPreset")
 
-    /// Re-roots a game's token against the container this launch got. Writes nothing for a game
-    /// that chose no shader, and nothing again when the absolute already agrees.
+    /// Re-roots a game's token against this launch's container, and pushes what it resolves to.
     static func repair(forISO isoName: String) {
         let token = string(keys.presetRef, useCurrent: false, iso: isoName)
         guard !token.isEmpty else { return }
@@ -26,11 +24,13 @@ enum PerGameShaderSelection {
             setBool(keys.enabled, false, useCurrent: false, iso: isoName)
             return
         }
+        // Nothing else pushes this one: the settings store never reads the per-game file.
+        ShaderParams.pushStored(token: token)
         guard string(keys.presetPath, useCurrent: false, iso: isoName) != url.path else { return }
         setString(keys.presetPath, url.path, useCurrent: false, iso: isoName)
     }
 
-    /// -1 use global, 0 off, 1 on, on the sentinel every other per-game control already uses.
+    /// -1 use global, 0 off, 1 on -- the sentinel every other per-game control uses.
     static func loadedChain(useCurrent: Bool, iso: String) -> Int {
         guard has(keys.enabled, useCurrent: useCurrent, iso: iso) else { return -1 }
         return bool(keys.enabled, useCurrent: useCurrent, iso: iso) ? 1 : 0
@@ -42,9 +42,12 @@ enum PerGameShaderSelection {
 
     /// Off keeps the enabled key and drops the preset; absence is the answer that inherits.
     static func write(chain: Int, presetRef: String, useCurrent: Bool, iso: String) {
-        setBool(keys.enabled, chain == 1, useCurrent: useCurrent, iso: iso)
-        guard chain == 1, !presetRef.isEmpty,
-              let url = ShaderPresetLibrary.resolve(presetRef) else {
+        // Enabled true with no preset key reads the GLOBAL preset out of the base layer.
+        let resolved = chain == 1 && !presetRef.isEmpty
+            ? ShaderPresetLibrary.resolve(presetRef)
+            : nil
+        setBool(keys.enabled, resolved != nil, useCurrent: useCurrent, iso: iso)
+        guard let url = resolved else {
             delete(keys.presetRef, useCurrent: useCurrent, iso: iso)
             delete(keys.presetPath, useCurrent: useCurrent, iso: iso)
             return
diff --git a/platforms/ios/app/src/main/swift/Models/ShaderParams.swift b/platforms/ios/app/src/main/swift/Models/ShaderParams.swift
index 3126ab6c4b..329bef839c 100644
--- a/platforms/ios/app/src/main/swift/Models/ShaderParams.swift
+++ b/platforms/ios/app/src/main/swift/Models/ShaderParams.swift
@@ -6,6 +6,7 @@ import Foundation
 enum ShaderParamsError: LocalizedError {
     case noName
     case noSavedRoot
+    case wouldOverwriteBase
 
     var errorDescription: String? {
         switch self {
@@ -13,6 +14,8 @@ enum ShaderParamsError: LocalizedError {
             return "That name has nothing in it that can become a filename."
         case .noSavedRoot:
             return "The Documents shader folder could not be opened."
+        case .wouldOverwriteBase:
+            return "That is the preset this one is built from. Give it a different name."
         }
     }
 }
@@ -151,7 +154,7 @@ final class ShaderParams: ObservableObject {
         let text = Self.presetText(base: base, params: params, overrides: overrides)
         do {
             let url = try await Task.detached(priority: .userInitiated) {
-                try Self.write(text, named: safe)
+                try Self.write(text, named: safe, base: base)
             }.value
             savedName = url.deletingPathExtension().lastPathComponent
         } catch {
@@ -159,9 +162,8 @@ final class ShaderParams: ObservableObject {
         }
     }
 
-    /// Sends the effective value of EVERY parameter, not just the changed ones. librashader
-    /// has no unset call, so a name dropped from the map leaves the chain on whatever was
-    /// pushed last, and a reset would never take.
+    /// Sends EVERY parameter's effective value. librashader has no unset call, so a name
+    /// dropped from the map would leave the chain on whatever was pushed last.
     private func pushEffective() {
         guard !params.isEmpty, let url = ShaderPresetLibrary.resolve(token) else { return }
         var effective: [String: NSNumber] = [:]
@@ -177,7 +179,8 @@ final class ShaderParams: ObservableObject {
     /// the overrides go down, because a chain built from the preset file already holds that
     /// preset's number for every name it is not told about, and reading the file here to say
     /// so again would put a librashader parse on the launch path.
-    static func pushStored(token: String) {
+    /// nonisolated so the per-game boot path can push before bootISO, without hopping actors.
+    nonisolated static func pushStored(token: String) {
         guard let url = ShaderPresetLibrary.resolve(token) else { return }
         var values: [String: NSNumber] = [:]
         for (name, value) in stored()[token] ?? [:] where value.isFinite {
@@ -196,7 +199,7 @@ final class ShaderParams: ObservableObject {
         ARMSX2Bridge.setINIString(Self.section, key: Self.key, value: json)
     }
 
-    private static func stored() -> [String: [String: Float]] {
+    private nonisolated static func stored() -> [String: [String: Float]] {
         let json = ARMSX2Bridge.getINIString(section, key: key, defaultValue: "")
         guard let data = json.data(using: .utf8),
               let decoded = try? JSONDecoder()
@@ -230,8 +233,7 @@ final class ShaderParams: ObservableObject {
     }
 
     /// Relative while the base sits in the same Documents root, so the pair survives the
-    /// container UUID changing. A bundled base has no such route and gets a path that a
-    /// reinstall breaks, which is why the save sheet says the reference is a path.
+    /// container UUID moving. A bundled base gets a path instead, which a reinstall breaks.
     private static func reference(to base: URL) -> String {
         let target = base.standardizedFileURL
         guard let root = ShaderPresetLibrary.userRoot?.standardizedFileURL,
@@ -245,7 +247,8 @@ final class ShaderParams: ObservableObject {
         return (up + to[shared...]).joined(separator: "/")
     }
 
-    private nonisolated static func write(_ text: String, named name: String) throws -> URL {
+    private nonisolated static func write(_ text: String, named name: String,
+                                          base: URL) throws -> URL {
         guard ShaderPresetLibrary.prepareUserRoots() != nil,
               let root = ShaderPresetLibrary.savedPresetRoot?.standardizedFileURL else {
             throw ShaderParamsError.noSavedRoot
@@ -255,6 +258,11 @@ final class ShaderParams: ObservableObject {
         guard url.deletingLastPathComponent().path == root.path else {
             throw ShaderParamsError.noName
         }
+        // Saving onto the base leaves a preset that references itself, and the sheet
+        // pre-fills the base's name, so the default is what walks into it.
+        guard url.path != base.standardizedFileURL.path else {
+            throw ShaderParamsError.wouldOverwriteBase
+        }
         try text.write(to: url, atomically: true, encoding: .utf8)
         return url
     }