diff --git a/src/agents/schema/clean-for-gemini.test.ts b/src/agents/schema/clean-for-gemini.test.ts
new file mode 100644
index 00000000000..22fa7a4d162
--- /dev/null
+++ b/src/agents/schema/clean-for-gemini.test.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from "vitest";
+import { cleanSchemaForGemini } from "./clean-for-gemini.js";
+
+describe("cleanSchemaForGemini", () => {
+  it("coerces null properties to an empty object", () => {
+    const cleaned = cleanSchemaForGemini({
+      type: "object",
+      properties: null,
+    }) as { type?: unknown; properties?: unknown };
+
+    expect(cleaned.type).toBe("object");
+    expect(cleaned.properties).toEqual({});
+  });
+
+  it("coerces non-object properties to an empty object", () => {
+    const cleaned = cleanSchemaForGemini({
+      type: "object",
+      properties: "invalid",
+    }) as { properties?: unknown };
+
+    expect(cleaned.properties).toEqual({});
+  });
+
+  it("coerces nested null properties while preserving valid siblings", () => {
+    const cleaned = cleanSchemaForGemini({
+      type: "object",
+      properties: {
+        bad: {
+          type: "object",
+          properties: null,
+        },
+        good: {
+          type: "string",
+        },
+      },
+    }) as {
+      properties?: {
+        bad?: { properties?: unknown };
+        good?: { type?: unknown };
+      };
+    };
+
+    expect(cleaned.properties?.bad?.properties).toEqual({});
+    expect(cleaned.properties?.good?.type).toBe("string");
+  });
+});
diff --git a/src/agents/schema/clean-for-gemini.ts b/src/agents/schema/clean-for-gemini.ts
index b416c32168e..669d8b9ac03 100644
--- a/src/agents/schema/clean-for-gemini.ts
+++ b/src/agents/schema/clean-for-gemini.ts
@@ -304,14 +304,20 @@ function cleanSchemaForGeminiWithDefs(
       continue;
     }
 
-    if (key === "properties" && value && typeof value === "object") {
-      const props = value as Record<string, unknown>;
-      cleaned[key] = Object.fromEntries(
-        Object.entries(props).map(([k, v]) => [
-          k,
-          cleanSchemaForGeminiWithDefs(v, nextDefs, refStack),
-        ]),
-      );
+    if (key === "properties") {
+      if (value && typeof value === "object" && !Array.isArray(value)) {
+        const props = value as Record<string, unknown>;
+        cleaned[key] = Object.fromEntries(
+          Object.entries(props).map(([k, v]) => [
+            k,
+            cleanSchemaForGeminiWithDefs(v, nextDefs, refStack),
+          ]),
+        );
+      } else {
+        // Guard malformed schemas (e.g. properties: null) that can trigger
+        // downstream Object.* crashes in strict provider validators.
+        cleaned[key] = {};
+      }
     } else if (key === "items" && value) {
       if (Array.isArray(value)) {
         cleaned[key] = value.map((entry) =>