diff --git a/src/plugin-sdk/provider-tools.test.ts b/src/plugin-sdk/provider-tools.test.ts
index 153713f87ce..a4e5aeea530 100644
--- a/src/plugin-sdk/provider-tools.test.ts
+++ b/src/plugin-sdk/provider-tools.test.ts
@@ -72,6 +72,35 @@ describe("buildProviderToolCompatFamilyHooks", () => {
     ).toEqual([]);
   });
 
+  it("preserves explicit empty properties maps when normalizing strict openai schemas", () => {
+    const hooks = buildProviderToolCompatFamilyHooks("openai");
+    const parameters = {
+      type: "object",
+      properties: {},
+    };
+    const tools = [{ name: "ping", description: "", parameters }] as never;
+
+    const normalized = hooks.normalizeToolSchemas({
+      provider: "openai",
+      modelId: "gpt-5.4",
+      modelApi: "openai-responses",
+      model: {
+        provider: "openai",
+        api: "openai-responses",
+        baseUrl: "https://api.openai.com/v1",
+        id: "gpt-5.4",
+      } as never,
+      tools,
+    });
+
+    expect(normalized[0]?.parameters).toEqual({
+      type: "object",
+      properties: {},
+      required: [],
+      additionalProperties: false,
+    });
+  });
+
   it("does not tighten permissive object schemas just to satisfy strict mode", () => {
     const hooks = buildProviderToolCompatFamilyHooks("openai");
     const permissiveParameters = {
diff --git a/src/plugin-sdk/provider-tools.ts b/src/plugin-sdk/provider-tools.ts
index fec86fca99f..4c8a62a0efd 100644
--- a/src/plugin-sdk/provider-tools.ts
+++ b/src/plugin-sdk/provider-tools.ts
@@ -229,7 +229,17 @@ function normalizeOpenAIStrictCompatSchemaRecursive(schema: unknown): unknown {
   let changed = false;
   const normalized: Record<string, unknown> = {};
   for (const [key, value] of Object.entries(record)) {
-    const next = normalizeOpenAIStrictCompatSchemaRecursive(value);
+    const next =
+      key === "properties" && value && typeof value === "object" && !Array.isArray(value)
+        ? Object.fromEntries(
+            Object.entries(value as Record<string, unknown>).map(
+              ([propertyName, propertyValue]) => [
+                propertyName,
+                normalizeOpenAIStrictCompatSchemaRecursive(propertyValue),
+              ],
+            ),
+          )
+        : normalizeOpenAIStrictCompatSchemaRecursive(value);
     normalized[key] = next;
     changed ||= next !== value;
   }