diff --git a/apps/oxlint/src-js/js_config.ts b/apps/oxlint/src-js/js_config.ts
index 6ddcc43073b63..d91ed5de5eada 100644
--- a/apps/oxlint/src-js/js_config.ts
+++ b/apps/oxlint/src-js/js_config.ts
@@ -12,6 +12,47 @@ type LoadJsConfigsResult =
   | { Failures: { path: string; error: string }[] }
   | { Error: string };
 
+function validateConfigExtends(root: object): void {
+  const visited = new Set<object>();
+  const stack = new Set<object>();
+
+  const visit = (config: object): void => {
+    if (visited.has(config)) return;
+    visited.add(config);
+
+    if (stack.has(config)) {
+      // Defensive: this should never happen because we check before recursing.
+      throw new Error("`extends` contains a circular reference.");
+    }
+    stack.add(config);
+
+    const maybeExtends = (config as Record<string, unknown>).extends;
+    if (maybeExtends !== undefined) {
+      if (!Array.isArray(maybeExtends)) {
+        throw new Error(
+          "`extends` must be an array of config objects (strings/paths are not supported).",
+        );
+      }
+      for (let i = 0; i < maybeExtends.length; i++) {
+        const item = maybeExtends[i];
+        if (typeof item !== "object" || item === null || Array.isArray(item)) {
+          throw new Error(
+            `\`extends[${i}]\` must be a config object (strings/paths are not supported).`,
+          );
+        }
+        if (stack.has(item)) {
+          throw new Error("`extends` contains a circular reference.");
+        }
+        visit(item);
+      }
+    }
+
+    stack.delete(config);
+  };
+
+  visit(root);
+}
+
 /**
  * Load JavaScript config files in parallel.
  *
@@ -43,6 +84,8 @@ export async function loadJsConfigs(paths: string[]): Promise<string> {
           );
         }
 
+        validateConfigExtends(config as object);
+
         return { path, config };
       }),
     );
diff --git a/apps/oxlint/test/fixtures/js_config_extends_cycle/files/test.js b/apps/oxlint/test/fixtures/js_config_extends_cycle/files/test.js
new file mode 100644
index 0000000000000..eab74692130a6
--- /dev/null
+++ b/apps/oxlint/test/fixtures/js_config_extends_cycle/files/test.js
@@ -0,0 +1 @@
+debugger;
diff --git a/apps/oxlint/test/fixtures/js_config_extends_cycle/output.snap.md b/apps/oxlint/test/fixtures/js_config_extends_cycle/output.snap.md
new file mode 100644
index 0000000000000..ae0d0ec92e8a1
--- /dev/null
+++ b/apps/oxlint/test/fixtures/js_config_extends_cycle/output.snap.md
@@ -0,0 +1,15 @@
+# Exit code
+1
+
+# stdout
+```
+Failed to parse oxlint configuration file.
+
+  x Failed to load config: <fixture>/oxlint.config.ts
+  | 
+  | Error: `extends` contains a circular reference.
+```
+
+# stderr
+```
+```
diff --git a/apps/oxlint/test/fixtures/js_config_extends_cycle/oxlint.config.ts b/apps/oxlint/test/fixtures/js_config_extends_cycle/oxlint.config.ts
new file mode 100644
index 0000000000000..469bf3331c54f
--- /dev/null
+++ b/apps/oxlint/test/fixtures/js_config_extends_cycle/oxlint.config.ts
@@ -0,0 +1,9 @@
+import { defineConfig } from "#oxlint";
+
+const a = defineConfig({}) as any;
+const b = defineConfig({ extends: [a] }) as any;
+a.extends = [b];
+
+export default defineConfig({
+  extends: [a],
+});
diff --git a/apps/oxlint/test/fixtures/js_config_extends_invalid_non_array/files/test.js b/apps/oxlint/test/fixtures/js_config_extends_invalid_non_array/files/test.js
new file mode 100644
index 0000000000000..eab74692130a6
--- /dev/null
+++ b/apps/oxlint/test/fixtures/js_config_extends_invalid_non_array/files/test.js
@@ -0,0 +1 @@
+debugger;
diff --git a/apps/oxlint/test/fixtures/js_config_extends_invalid_non_array/output.snap.md b/apps/oxlint/test/fixtures/js_config_extends_invalid_non_array/output.snap.md
new file mode 100644
index 0000000000000..bcbb36a3affbb
--- /dev/null
+++ b/apps/oxlint/test/fixtures/js_config_extends_invalid_non_array/output.snap.md
@@ -0,0 +1,15 @@
+# Exit code
+1
+
+# stdout
+```
+Failed to parse oxlint configuration file.
+
+  x Failed to load config: <fixture>/oxlint.config.ts
+  | 
+  | Error: `extends` must be an array of config objects (strings/paths are not supported).
+```
+
+# stderr
+```
+```
diff --git a/apps/oxlint/test/fixtures/js_config_extends_invalid_non_array/oxlint.config.ts b/apps/oxlint/test/fixtures/js_config_extends_invalid_non_array/oxlint.config.ts
new file mode 100644
index 0000000000000..5e0273d57e801
--- /dev/null
+++ b/apps/oxlint/test/fixtures/js_config_extends_invalid_non_array/oxlint.config.ts
@@ -0,0 +1,6 @@
+import { defineConfig } from "#oxlint";
+
+export default defineConfig({
+  // @ts-expect-error - we are testing invalid config
+  extends: {},
+});
diff --git a/apps/oxlint/test/fixtures/js_config_extends_invalid_string/files/test.js b/apps/oxlint/test/fixtures/js_config_extends_invalid_string/files/test.js
new file mode 100644
index 0000000000000..eab74692130a6
--- /dev/null
+++ b/apps/oxlint/test/fixtures/js_config_extends_invalid_string/files/test.js
@@ -0,0 +1 @@
+debugger;
diff --git a/apps/oxlint/test/fixtures/js_config_extends_invalid_string/output.snap.md b/apps/oxlint/test/fixtures/js_config_extends_invalid_string/output.snap.md
new file mode 100644
index 0000000000000..bacb2f99d5b1d
--- /dev/null
+++ b/apps/oxlint/test/fixtures/js_config_extends_invalid_string/output.snap.md
@@ -0,0 +1,15 @@
+# Exit code
+1
+
+# stdout
+```
+Failed to parse oxlint configuration file.
+
+  x Failed to load config: <fixture>/oxlint.config.ts
+  | 
+  | Error: `extends[0]` must be a config object (strings/paths are not supported).
+```
+
+# stderr
+```
+```
diff --git a/apps/oxlint/test/fixtures/js_config_extends_invalid_string/oxlint.config.ts b/apps/oxlint/test/fixtures/js_config_extends_invalid_string/oxlint.config.ts
new file mode 100644
index 0000000000000..a89e3c5538a89
--- /dev/null
+++ b/apps/oxlint/test/fixtures/js_config_extends_invalid_string/oxlint.config.ts
@@ -0,0 +1,6 @@
+import { defineConfig } from "#oxlint";
+
+export default defineConfig({
+  // @ts-expect-error - we are testing invalid config
+  extends: ["./base.ts"],
+});