diff --git a/src/tools/diagnostics.test.ts b/src/tools/diagnostics.test.ts
new file mode 100644
index 0000000..4b6fa95
--- /dev/null
+++ b/src/tools/diagnostics.test.ts
@@ -0,0 +1,53 @@
+import { expect, test, describe, beforeEach } from "bun:test";
+import { mkdtempSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { runDiagnostics, configureDiagnostics } from "./diagnostics";
+
+let tmp: string;
+
+beforeEach(() => {
+  tmp = mkdtempSync(join(tmpdir(), "klaatai-diag-"));
+  configureDiagnostics({ enabled: true, timeoutMs: 8_000 });
+});
+
+describe("Ruby diagnostics (.rb)", () => {
+  test("returns null for .rb when rubocop is not on PATH (silently skipped)", () => {
+    const absPath = join(tmp, "foo.rb");
+    writeFileSync(absPath, 'puts "hello"\n');
+    const result = runDiagnostics(absPath, tmp);
+    expect(result === null || typeof result === "string").toBe(true);
+  });
+
+  test("returns null for .rb when diagnostics are disabled", () => {
+    configureDiagnostics({ enabled: false });
+    const absPath = join(tmp, "foo.rb");
+    writeFileSync(absPath, 'puts "hello"\n');
+    expect(runDiagnostics(absPath, tmp)).toBeNull();
+  });
+
+  test("returns null for .rb when rubocop is absent (no override)", () => {
+    // No config override, rubocop not on PATH in CI — must return null, never throw.
+    const absPath = join(tmp, "bar.rb");
+    writeFileSync(absPath, "def foo; end\n");
+    expect(() => runDiagnostics(absPath, tmp)).not.toThrow();
+    const result = runDiagnostics(absPath, tmp);
+    // null = rubocop absent (expected in CI); string = rubocop present and found issues
+    expect(result === null || typeof result === "string").toBe(true);
+  });
+
+  test("non-.rb files are unaffected by the Ruby branch", () => {
+    const absPath = join(tmp, "foo.py");
+    writeFileSync(absPath, "x = 1\n");
+    expect(() => runDiagnostics(absPath, tmp)).not.toThrow();
+  });
+
+  test("returns null for .rb when config explicitly disables the extension via empty commands", () => {
+    configureDiagnostics({ enabled: true, timeoutMs: 8_000, commands: {} });
+    const absPath = join(tmp, "baz.rb");
+    writeFileSync(absPath, 'puts "test"\n');
+    // No override for .rb, rubocop not on PATH -> null
+    const result = runDiagnostics(absPath, tmp);
+    expect(result === null || typeof result === "string").toBe(true);
+  });
+});
diff --git a/src/tools/diagnostics.ts b/src/tools/diagnostics.ts
index baa86ef..46ee499 100644
--- a/src/tools/diagnostics.ts
+++ b/src/tools/diagnostics.ts
@@ -69,6 +69,10 @@ function commandFor(absPath: string, projectRoot: string): string[] | null {
     if (onPath("gofmt")) return ["gofmt", "-e", "-l", absPath]; // -e reports syntax errors
     return null;
   }
+  if (ext === ".rb") {
+    if (onPath("rubocop")) return ["rubocop", "--format", "emacs", "--no-color", absPath];
+    return null;
+  }
   if (ext === ".rs") {
     // cargo check is whole-crate/slow — only via explicit config override.
     return null;