diff --git a/Packages/macOS/CmuxFoundation/Sources/CmuxFoundation/RemoteTmuxSessionCommandBuilder.swift b/Packages/macOS/CmuxFoundation/Sources/CmuxFoundation/RemoteTmuxSessionCommandBuilder.swift
index c6a974c595ea..a311fe78b780 100644
--- a/Packages/macOS/CmuxFoundation/Sources/CmuxFoundation/RemoteTmuxSessionCommandBuilder.swift
+++ b/Packages/macOS/CmuxFoundation/Sources/CmuxFoundation/RemoteTmuxSessionCommandBuilder.swift
@@ -35,6 +35,9 @@ public struct RemoteTmuxSessionCommandBuilder: Sendable {
         ]
         scriptLines.append(contentsOf: Self.workspaceEnvironmentRebindingLines.map { "  " + $0 })
         scriptLines += [
+            #"  cmux_default_command="$("$cmux_tmux" show-options -v -t "$cmux_session_target:" default-command 2>/dev/null || true)""#,
+            "  case \"$cmux_default_command\" in *CMUX_PERSISTENT_PTY_EXEC_HELPER*|*CMUX_SHELL_INTEGRATION_DIR*) \"$cmux_tmux\" set-option -t \"$cmux_session_target:\" default-command \"$cmux_shell_command\" >/dev/null 2>/dev/null || exit $? ;; esac",
+            "  unset cmux_default_command",
             "  exec \"$cmux_tmux\" attach-session -t \"$cmux_session_target\"",
             "fi",
             "if \"$cmux_tmux\" new-session -d \(Self.newSessionEnvironmentArguments) -s \"$cmux_session_name\" \"$cmux_shell_command\"; then",
diff --git a/Packages/macOS/CmuxFoundation/Tests/CmuxFoundationTests/RemoteTmuxSessionCommandBuilderTests.swift b/Packages/macOS/CmuxFoundation/Tests/CmuxFoundationTests/RemoteTmuxSessionCommandBuilderTests.swift
index 01723cd96b95..5e3ff8c0d828 100644
--- a/Packages/macOS/CmuxFoundation/Tests/CmuxFoundationTests/RemoteTmuxSessionCommandBuilderTests.swift
+++ b/Packages/macOS/CmuxFoundation/Tests/CmuxFoundationTests/RemoteTmuxSessionCommandBuilderTests.swift
@@ -193,8 +193,60 @@ struct RemoteTmuxSessionCommandBuilderTests {
         }
     }
 
+    @Test("existing session refreshes a stale cmux-managed default command")
+    func existingSessionRefreshesManagedDefaultCommand() throws {
+        try withFakeTmux(sessionExists: true) { directory, environment in
+            let shellCommand = #"exec "$CMUX_PERSISTENT_PTY_EXEC_HELPER" --internal-persistent-pty-exec "$SHELL" "$SHELL" --rcfile "$CMUX_SHELL_INTEGRATION_DIR/.bashrc" -i"#
+            let builder = RemoteTmuxSessionCommandBuilder(
+                sessionName: "existing-managed",
+                shellCommand: shellCommand
+            )
+            let result = try run(
+                builder.remoteShellCommand,
+                environment: environment.merging([
+                    "CMUX_TMUX_DEFAULT_COMMAND": "exec \"$CMUX_PERSISTENT_PTY_EXEC_HELPER\" --rcfile \"$HOME/.cmux/relay/old.shell/.bashrc\" -i",
+                ]) { _, current in current }
+            )
+
+            #expect(result.status == 0)
+            #expect(result.stderr.isEmpty)
+            let calls = try invocations(in: directory)
+            let setOption = try #require(calls.first { $0.first == "set-option" })
+            #expect(setOption == ["set-option", "-t", "=existing-managed:", "default-command", shellCommand])
+            let attachIndex = try #require(calls.firstIndex(of: ["attach-session", "-t", "=existing-managed"]))
+            #expect(calls.firstIndex(of: setOption)! < attachIndex)
+        }
+    }
+
+    /// A failed managed refresh must stop reconnect before it attaches, without
+    /// exposing the remote tmux diagnostic directly to the caller.
+    @Test("existing session stops before attach when managed default refresh fails")
+    func existingSessionStopsWhenManagedDefaultRefreshFails() throws {
+        try withFakeTmux(sessionExists: true, setOptionStatus: 23) { directory, environment in
+            let shellCommand = "exec integrated-shell"
+            let builder = RemoteTmuxSessionCommandBuilder(
+                sessionName: "existing-managed",
+                shellCommand: shellCommand
+            )
+            let result = try run(
+                builder.remoteShellCommand,
+                environment: environment.merging([
+                    "CMUX_TMUX_DEFAULT_COMMAND": "managed CMUX_PERSISTENT_PTY_EXEC_HELPER command",
+                ]) { _, current in current }
+            )
+
+            #expect(result.status == 23)
+            #expect(result.stderr.isEmpty)
+            let calls = try invocations(in: directory)
+            #expect(calls.contains(["set-option", "-t", "=existing-managed:", "default-command", shellCommand]))
+            #expect(!calls.contains { $0.first == "attach-session" })
+        }
+    }
+
+    /// Runs a builder command against a disposable fake tmux executable.
     private func withFakeTmux(
         sessionExists: Bool,
+        setOptionStatus: Int = 0,
         operation: (URL, [String: String]) throws -> Void
     ) throws {
         let directory = FileManager.default.temporaryDirectory
@@ -212,9 +264,18 @@ struct RemoteTmuxSessionCommandBuilderTests {
           has-session)
             [ -f "$CMUX_TMUX_SESSION_STATE" ]
             ;;
+          show-options)
+            printf '%s\\n' "${CMUX_TMUX_DEFAULT_COMMAND:-}"
+            ;;
           new-session)
             : > "$CMUX_TMUX_SESSION_STATE"
             ;;
+          set-option)
+            if [ "${CMUX_TMUX_SET_OPTION_STATUS:-0}" -ne 0 ]; then
+              printf '%s\n' 'tmux: private session diagnostic' >&2
+            fi
+            exit "${CMUX_TMUX_SET_OPTION_STATUS:-0}"
+            ;;
         esac
         """.write(to: executable, atomically: true, encoding: .utf8)
         try FileManager.default.setAttributes(
@@ -230,6 +291,7 @@ struct RemoteTmuxSessionCommandBuilderTests {
             "PATH": "/usr/bin:/bin",
             "CMUX_TMUX_LOG": directory.appendingPathComponent("tmux.log").path,
             "CMUX_TMUX_SESSION_STATE": statePath,
+            "CMUX_TMUX_SET_OPTION_STATUS": String(setOptionStatus),
         ])
     }