diff --git a/Sources/BlurtEngine/Injection/SystemClipboard.swift b/Sources/BlurtEngine/Injection/SystemClipboard.swift
index fb390b97..ff9b367c 100644
--- a/Sources/BlurtEngine/Injection/SystemClipboard.swift
+++ b/Sources/BlurtEngine/Injection/SystemClipboard.swift
@@ -55,11 +55,27 @@
   /// comparison that gates the deferred restore lives here, behind the seam, so a
   /// fake never re-implements it.
   struct SystemClipboard: ClipboardAccess {
+    /// The pasteboard to use. Always `.general` in the app; tests pass a private,
+    /// uniquely named pasteboard so they never touch the user's clipboard.
+    var pasteboardName: NSPasteboard.Name = .general
+
+    private var pasteboard: NSPasteboard { NSPasteboard(name: pasteboardName) }
+
+    /// Markers from the nspasteboard.org conventions that clipboard history apps
+    /// (Raycast, Maccy, Alfred, Paste ...) read. A write that carries `transient`
+    /// is not recorded; `source` names the app that wrote the data, instead of the
+    /// app that happens to be in front.
+    static let transientType = NSPasteboard.PasteboardType("org.nspasteboard.TransientType")
+    static let autoGeneratedType = NSPasteboard.PasteboardType("org.nspasteboard.AutoGeneratedType")
+    static let sourceType = NSPasteboard.PasteboardType("org.nspasteboard.source")
+
     func write(_ text: String) { setString(text) }
 
     func writeAndPrepareRestore(_ text: String) -> @Sendable () -> Void {
       let saved = snapshot()
-      setString(text)
+      // The transcript sits on the clipboard only for the paste, so history apps
+      // should not keep it (it may also be text dictated into a password field).
+      setString(text, transient: true)
       // Snapshot the change count our own write produced. If anything else writes
       // to the pasteboard before the restore fires (e.g. the user copies
       // something), the count moves and the restore leaves their newer contents
@@ -85,7 +101,7 @@
 
     // MARK: - NSPasteboard building blocks (also exercised directly by SystemClipboardTests)
 
-    var changeCount: Int { NSPasteboard.general.changeCount }
+    var changeCount: Int { pasteboard.changeCount }
 
     /// Counts the writes Blurt itself makes to the pasteboard, bumped *before*
     /// each one, so a reader that sees the pasteboard's change count move can
@@ -101,7 +117,6 @@
     /// (`pasteboardItems` is documented to return nil on error). Callers must treat
     /// nil as "don't restore" — never as an empty clipboard.
     func snapshot() -> PasteboardSnapshot? {
-      let pasteboard = NSPasteboard.general
       guard let items = pasteboard.pasteboardItems else { return nil }
       let captured = items.map { item in
         var dataMap: [NSPasteboard.PasteboardType: Data] = [:]
@@ -117,16 +132,28 @@
       return PasteboardSnapshot(items: captured, plainText: pasteboard.string(forType: .string))
     }
 
-    func setString(_ text: String) {
+    /// Writes `text` as the only item. `transient` marks it so clipboard history
+    /// apps skip it; the source marker is set on every write Blurt makes.
+    func setString(_ text: String, transient: Bool = false) {
       noteOwnWrite()
-      let pasteboard = NSPasteboard.general
+      let pasteboard = pasteboard
       pasteboard.clearContents()
       pasteboard.setString(text, forType: .string)
+      if transient {
+        pasteboard.setData(Data(), forType: Self.transientType)
+        pasteboard.setData(Data(), forType: Self.autoGeneratedType)
+      }
+      if let bundleID = Bundle.main.bundleIdentifier {
+        pasteboard.setString(bundleID, forType: Self.sourceType)
+      }
     }
 
+    /// Puts `saved` back. The restore is marked transient: the user's contents are
+    /// already in any clipboard history, so recording them again would only add a
+    /// duplicate entry on top after every dictation.
     func restore(_ saved: PasteboardSnapshot) {
       noteOwnWrite()
-      let pasteboard = NSPasteboard.general
+      let pasteboard = pasteboard
 
       // The pasteboard genuinely held nothing, so restoring it means emptying it.
       // Safe to clear because the snapshot succeeded — a *failed* read is nil and
@@ -147,6 +174,7 @@
         }
         return pasteboardItem
       }
+      rebuilt.first?.setData(Data(), forType: Self.transientType)
 
       if !rebuilt.isEmpty {
         pasteboard.clearContents()
@@ -170,12 +198,13 @@
       if let plainText = saved.plainText {
         pasteboard.clearContents()
         pasteboard.setString(plainText, forType: .string)
+        pasteboard.setData(Data(), forType: Self.transientType)
       }
     }
   }
 
   /// The read-aloud copy fallback's view of the pasteboard (see `SelectionCopy`).
   extension SystemClipboard: CopyPasteboard {
-    func currentString() -> String? { NSPasteboard.general.string(forType: .string) }
+    func currentString() -> String? { pasteboard.string(forType: .string) }
   }
 #endif
diff --git a/Tests/BlurtEngineTests/SystemClipboardMarkerTests.swift b/Tests/BlurtEngineTests/SystemClipboardMarkerTests.swift
new file mode 100644
index 00000000..5d7eb81d
--- /dev/null
+++ b/Tests/BlurtEngineTests/SystemClipboardMarkerTests.swift
@@ -0,0 +1,72 @@
+#if os(macOS)
+  import AppKit
+  import Foundation
+  import Testing
+
+  @testable import BlurtEngine
+
+  /// Issue 310: clipboard history apps saved every dictation, and the restore added the
+  /// user's old clipboard again as a new entry. Blurt's own writes now carry the
+  /// nspasteboard.org transient marker. These tests use a private, uniquely named
+  /// pasteboard, so they never read or change the user's clipboard.
+  @Suite("SystemClipboard history markers", .serialized)
+  struct SystemClipboardMarkerTests {
+
+    private func withPrivatePasteboard(_ body: (SystemClipboard, NSPasteboard) throws -> Void) rethrows {
+      let name = NSPasteboard.Name("app.blurt.tests.\(UUID().uuidString)")
+      let pb = NSPasteboard(name: name)
+      defer { pb.releaseGlobally() }
+      try body(SystemClipboard(pasteboardName: name), pb)
+    }
+
+    @Test("the transcript written for a paste is marked transient")
+    func transcriptIsTransient() {
+      withPrivatePasteboard { clip, pb in
+        pb.clearContents()
+        pb.setString("original", forType: .string)
+
+        _ = clip.writeAndPrepareRestore("dictated text")
+
+        #expect(pb.string(forType: .string) == "dictated text")
+        let types = pb.types ?? []
+        #expect(types.contains(SystemClipboard.transientType))
+        #expect(types.contains(SystemClipboard.autoGeneratedType))
+      }
+    }
+
+    @Test("the restore brings the old contents back, marked transient")
+    func restoreIsTransient() {
+      withPrivatePasteboard { clip, pb in
+        pb.clearContents()
+        pb.setString("original", forType: .string)
+
+        let restore = clip.writeAndPrepareRestore("dictated text")
+        restore()
+
+        #expect(pb.string(forType: .string) == "original")
+        #expect((pb.types ?? []).contains(SystemClipboard.transientType))
+      }
+    }
+
+    @Test("a transcript left for a manual paste stays in history")
+    func manualPasteFallbackIsNotTransient() {
+      withPrivatePasteboard { clip, pb in
+        clip.write("paste me")
+        #expect(pb.string(forType: .string) == "paste me")
+        #expect(!(pb.types ?? []).contains(SystemClipboard.transientType))
+      }
+    }
+
+    @Test("Blurt names itself as the source when it has a bundle identifier")
+    func sourceMarker() {
+      withPrivatePasteboard { clip, pb in
+        clip.write("paste me")
+        if let bundleID = Bundle.main.bundleIdentifier {
+          #expect(pb.string(forType: SystemClipboard.sourceType) == bundleID)
+        } else {
+          #expect(pb.string(forType: SystemClipboard.sourceType) == nil)
+        }
+      }
+    }
+  }
+#endif