🤖 fix: add downgrade compatibility for incompatible runtime configs

When users upgrade to a version with new runtime types (like 'local' without
srcBaseDir for project-dir mode, or new 'worktree' type) and then downgrade,
the old version shows a clear error instead of crashing.

Changes:
- Add IncompatibleRuntimeError for workspaces from newer mux versions
- Add isIncompatibleRuntimeConfig helper to detect future configs
- Add incompatibleRuntime field to FrontendWorkspaceMetadata
- Show error in AIView when workspace is incompatible
- Add 'incompatible_workspace' SendMessageError type as fallback
- Mark incompatible_workspace as non-retryable

This prepares for #824 (Local/Worktree runtime distinction) by ensuring
users can safely downgrade without losing access to their other workspaces.

Author: ammar-agent

src/browser/App.tsx

@@ -574,6 +574,9 @@ function AppInner() {
                   runtimeConfig={
                     workspaceMetadata.get(selectedWorkspace.workspaceId)?.runtimeConfig
                   }
+                  incompatibleRuntime={
+                    workspaceMetadata.get(selectedWorkspace.workspaceId)?.incompatibleRuntime
+                  }
                 />
               </ErrorBoundary>
             ) : creationProjectPath ? (

src/browser/components/AIView.tsx

@@ -49,6 +49,8 @@ interface AIViewProps {
   namedWorkspacePath: string; // User-friendly path for display and terminal
   runtimeConfig?: RuntimeConfig;
   className?: string;
+  /** If set, workspace is incompatible (from newer mux version) and this error should be displayed */
+  incompatibleRuntime?: string;
 }
 
 const AIViewInner: React.FC<AIViewProps> = ({
@@ -607,8 +609,37 @@ const AIViewInner: React.FC<AIViewProps> = ({
   );
 };
 
+/**
+ * Incompatible workspace error display.
+ * Shown when a workspace was created with a newer version of mux.
+ */
+const IncompatibleWorkspaceView: React.FC<{ message: string; className?: string }> = ({
+  message,
+  className,
+}) => (
+  <div className={cn("flex h-full flex-col items-center justify-center p-8", className)}>
+    <div className="max-w-md text-center">
+      <div className="mb-4 text-4xl">⚠️</div>
+      <h2 className="mb-2 text-xl font-semibold text-[var(--color-text-primary)]">
+        Incompatible Workspace
+      </h2>
+      <p className="mb-4 text-[var(--color-text-secondary)]">{message}</p>
+      <p className="text-sm text-[var(--color-text-tertiary)]">
+        You can delete this workspace and create a new one, or upgrade mux to use it.
+      </p>
+    </div>
+  </div>
+);
+
 // Wrapper component that provides the mode and thinking contexts
 export const AIView: React.FC<AIViewProps> = (props) => {
+  // Early return for incompatible workspaces - no hooks called in this path
+  if (props.incompatibleRuntime) {
+    return (
+      <IncompatibleWorkspaceView message={props.incompatibleRuntime} className={props.className} />
+    );
+  }
+
   return (
     <ModeProvider workspaceId={props.workspaceId}>
       <ProviderOptionsProvider>

src/browser/components/ChatInputToasts.test.ts

@@ -69,5 +69,18 @@ describe("ChatInputToasts", () => {
       expect(toast.title).toBe("Message Send Failed");
       expect(toast.message).toContain("unexpected error");
     });
+
+    test("should create toast for incompatible_workspace error", () => {
+      const error: SendMessageError = {
+        type: "incompatible_workspace",
+        message: "This workspace uses a runtime configuration from a newer version of mux.",
+      };
+
+      const toast = createErrorToast(error);
+
+      expect(toast.type).toBe("error");
+      expect(toast.title).toBe("Incompatible Workspace");
+      expect(toast.message).toContain("newer version");
+    });
   });
 });

src/browser/components/ChatInputToasts.tsx

@@ -195,6 +195,21 @@ export const createErrorToast = (error: SendMessageErrorType): Toast => {
       };
     }
 
+    case "incompatible_workspace": {
+      return {
+        id: Date.now().toString(),
+        type: "error",
+        title: "Incompatible Workspace",
+        message: error.message,
+        solution: (
+          <>
+            <SolutionLabel>Solution:</SolutionLabel>
+            Upgrade mux to use this workspace, or delete it and create a new one.
+          </>
+        ),
+      };
+    }
+
     case "unknown":
     default: {
       const formatted = formatSendMessageError(error);

src/browser/utils/messages/retryEligibility.test.ts

@@ -594,4 +594,12 @@ describe("isNonRetryableSendError", () => {
     };
     expect(isNonRetryableSendError(error)).toBe(false);
   });
+
+  it("returns true for incompatible_workspace error", () => {
+    const error: SendMessageError = {
+      type: "incompatible_workspace",
+      message: "This workspace uses a runtime configuration from a newer version of mux.",
+    };
+    expect(isNonRetryableSendError(error)).toBe(true);
+  });
 });

src/browser/utils/messages/retryEligibility.ts

@@ -52,6 +52,7 @@ export function isNonRetryableSendError(error: SendMessageError): boolean {
     case "api_key_not_found": // Missing API key - user must configure
     case "provider_not_supported": // Unsupported provider - user must switch
     case "invalid_model_string": // Bad model format - user must fix
+    case "incompatible_workspace": // Workspace from newer mux version - user must upgrade
       return true;
     case "unknown":
       return false; // Unknown errors might be transient

src/common/types/errors.ts

@@ -12,6 +12,7 @@ export type SendMessageError =
   | { type: "api_key_not_found"; provider: string }
   | { type: "provider_not_supported"; provider: string }
   | { type: "invalid_model_string"; message: string }
+  | { type: "incompatible_workspace"; message: string }
   | { type: "unknown"; raw: string };
 
 /**

src/common/types/workspace.ts

@@ -74,6 +74,13 @@ export interface GitStatus {
 export interface FrontendWorkspaceMetadata extends WorkspaceMetadata {
   /** Worktree path (uses workspace name as directory) */
   namedWorkspacePath: string;
+
+  /**
+   * If set, this workspace has an incompatible runtime configuration
+   * (e.g., from a newer version of mux). The workspace should be displayed
+   * but interactions should show this error message.
+   */
+  incompatibleRuntime?: string;
 }
 
 export interface WorkspaceActivitySnapshot {

src/common/utils/errors/formatSendError.ts

@@ -32,6 +32,11 @@ export function formatSendMessageError(error: SendMessageError): FormattedError
         message: error.message,
       };
 
+    case "incompatible_workspace":
+      return {
+        message: error.message,
+      };
+
     case "unknown":
       return {
         message: error.raw || "An unexpected error occurred",

src/common/utils/runtimeCompatibility.ts

@@ -0,0 +1,31 @@
+/**
+ * Runtime configuration compatibility checks.
+ *
+ * This module is intentionally in common/ to avoid circular dependencies
+ * with runtime implementations (LocalRuntime, SSHRuntime, etc.).
+ */
+
+import type { RuntimeConfig } from "@/common/types/runtime";
+
+/**
+ * Check if a runtime config is from a newer version and incompatible.
+ *
+ * This handles downgrade compatibility: if a user upgrades to a version
+ * with new runtime types (e.g., "local" without srcBaseDir for project-dir mode),
+ * then downgrades, those workspaces should show a clear error rather than crashing.
+ */
+export function isIncompatibleRuntimeConfig(config: RuntimeConfig | undefined): boolean {
+  if (!config) {
+    return false;
+  }
+  // Future versions may add "local" without srcBaseDir (project-dir mode)
+  // or new types like "worktree". Detect these as incompatible.
+  if (config.type === "local" && !("srcBaseDir" in config && config.srcBaseDir)) {
+    return true;
+  }
+  // Unknown types from future versions
+  if (config.type !== "local" && config.type !== "ssh") {
+    return true;
+  }
+  return false;
+}