diff --git a/canvas/src/lib/__tests__/externalRuntimes.test.ts b/canvas/src/lib/__tests__/externalRuntimes.test.ts
new file mode 100644
index 00000000..0af8520e
--- /dev/null
+++ b/canvas/src/lib/__tests__/externalRuntimes.test.ts
@@ -0,0 +1,60 @@
+/**
+ * Tests for `isExternalLikeRuntime` — mirrors the backend's
+ * isExternalLikeRuntime() in workspace-server/internal/handlers/runtime_registry.go.
+ *
+ * These runtimes have no platform-owned container (no Files, Terminal, Docker config).
+ * Both frontend and backend must agree on which runtimes are "external-like" so
+ * the canvas can show/hide those tabs correctly and the backend can enforce
+ * the same semantics server-side.
+ */
+import { describe, it, expect } from "vitest";
+import { isExternalLikeRuntime } from "../externalRuntimes";
+
+describe("isExternalLikeRuntime", () => {
+  describe("known external-like runtimes", () => {
+    it.each([
+      ["external"],
+      ["kimi"],
+      ["kimi-cli"],
+    ])("%q returns true", (runtime) => {
+      expect(isExternalLikeRuntime(runtime)).toBe(true);
+    });
+  });
+
+  describe("non-external runtimes", () => {
+    it.each([
+      "claude-code",
+      "hermes",
+      "docker",
+      "local",
+      "agent",
+      "crewai",
+      "langgraph",
+      "openclaw",
+      "custom-runtime",
+    ])("%q returns false", (runtime) => {
+      expect(isExternalLikeRuntime(runtime)).toBe(false);
+    });
+  });
+
+  describe("edge cases", () => {
+    it("returns false for undefined", () => {
+      expect(isExternalLikeRuntime(undefined)).toBe(false);
+    });
+
+    it("returns false for null", () => {
+      // @ts-expect-error — intentional runtime test, null is not a valid type
+      expect(isExternalLikeRuntime(null)).toBe(false);
+    });
+
+    it("returns false for empty string", () => {
+      expect(isExternalLikeRuntime("")).toBe(false);
+    });
+
+    it("is case-sensitive — kimi vs KIMI vs Kimi", () => {
+      expect(isExternalLikeRuntime("KIMI")).toBe(false);
+      expect(isExternalLikeRuntime("Kimi")).toBe(false);
+      expect(isExternalLikeRuntime("kimi")).toBe(true);
+    });
+  });
+});