diff --git a/src/agents/model-compat.test.ts b/src/agents/model-compat.test.ts
index 56b9c16203c..733d9a2f47f 100644
--- a/src/agents/model-compat.test.ts
+++ b/src/agents/model-compat.test.ts
@@ -28,6 +28,10 @@ function supportsUsageInStreaming(model: Model<Api>): boolean | undefined {
     ?.supportsUsageInStreaming;
 }
 
+function supportsStrictMode(model: Model<Api>): boolean | undefined {
+  return (model.compat as { supportsStrictMode?: boolean } | undefined)?.supportsStrictMode;
+}
+
 function createTemplateModel(provider: string, id: string): Model<Api> {
   return {
     id,
@@ -94,6 +98,13 @@ function expectSupportsUsageInStreamingForcedOff(overrides?: Partial<Model<Api>>
   expect(supportsUsageInStreaming(normalized)).toBe(false);
 }
 
+function expectSupportsStrictModeForcedOff(overrides?: Partial<Model<Api>>): void {
+  const model = { ...baseModel(), ...overrides };
+  delete (model as { compat?: unknown }).compat;
+  const normalized = normalizeModelCompat(model as Model<Api>);
+  expect(supportsStrictMode(normalized)).toBe(false);
+}
+
 function expectResolvedForwardCompat(
   model: Model<Api> | undefined,
   expected: { provider: string; id: string },
@@ -226,6 +237,17 @@ describe("normalizeModelCompat", () => {
     });
   });
 
+  it("forces supportsStrictMode off for z.ai models", () => {
+    expectSupportsStrictModeForcedOff();
+  });
+
+  it("forces supportsStrictMode off for custom openai-completions provider", () => {
+    expectSupportsStrictModeForcedOff({
+      provider: "custom-cpa",
+      baseUrl: "https://cpa.example.com/v1",
+    });
+  });
+
   it("forces supportsDeveloperRole off for Qwen proxy via openai-completions", () => {
     expectSupportsDeveloperRoleForcedOff({
       provider: "qwen-proxy",
@@ -283,6 +305,18 @@ describe("normalizeModelCompat", () => {
     const normalized = normalizeModelCompat(model);
     expect(supportsDeveloperRole(normalized)).toBe(false);
     expect(supportsUsageInStreaming(normalized)).toBe(false);
+    expect(supportsStrictMode(normalized)).toBe(false);
+  });
+
+  it("respects explicit supportsStrictMode true on non-native endpoints", () => {
+    const model = {
+      ...baseModel(),
+      provider: "custom-cpa",
+      baseUrl: "https://proxy.example.com/v1",
+      compat: { supportsStrictMode: true },
+    };
+    const normalized = normalizeModelCompat(model);
+    expect(supportsStrictMode(normalized)).toBe(true);
   });
 
   it("does not mutate caller model when forcing supportsDeveloperRole off", () => {
@@ -296,16 +330,23 @@ describe("normalizeModelCompat", () => {
     expect(normalized).not.toBe(model);
     expect(supportsDeveloperRole(model)).toBeUndefined();
     expect(supportsUsageInStreaming(model)).toBeUndefined();
+    expect(supportsStrictMode(model)).toBeUndefined();
     expect(supportsDeveloperRole(normalized)).toBe(false);
     expect(supportsUsageInStreaming(normalized)).toBe(false);
+    expect(supportsStrictMode(normalized)).toBe(false);
   });
 
   it("does not override explicit compat false", () => {
     const model = baseModel();
-    model.compat = { supportsDeveloperRole: false, supportsUsageInStreaming: false };
+    model.compat = {
+      supportsDeveloperRole: false,
+      supportsUsageInStreaming: false,
+      supportsStrictMode: false,
+    };
     const normalized = normalizeModelCompat(model);
     expect(supportsDeveloperRole(normalized)).toBe(false);
     expect(supportsUsageInStreaming(normalized)).toBe(false);
+    expect(supportsStrictMode(normalized)).toBe(false);
   });
 });
 
diff --git a/src/agents/model-compat.ts b/src/agents/model-compat.ts
index 72deb0c655f..46e37733aec 100644
--- a/src/agents/model-compat.ts
+++ b/src/agents/model-compat.ts
@@ -54,9 +54,10 @@ export function normalizeModelCompat(model: Model<Api>): Model<Api> {
 
   // The `developer` role and stream usage chunks are OpenAI-native behaviors.
   // Many OpenAI-compatible backends reject `developer` and/or emit usage-only
-  // chunks that break strict parsers expecting choices[0]. For non-native
-  // openai-completions endpoints, force both compat flags off — unless the
-  // user has explicitly opted in via their model config.
+  // chunks that break strict parsers expecting choices[0]. Additionally, the
+  // `strict` boolean inside tools validation is rejected by several providers
+  // causing tool calls to be ignored. For non-native openai-completions endpoints,
+  // default these compat flags off unless explicitly opted in.
   const compat = model.compat ?? undefined;
   // When baseUrl is empty the pi-ai library defaults to api.openai.com, so
   // leave compat unchanged and let default native behavior apply.
@@ -64,13 +65,14 @@ export function normalizeModelCompat(model: Model<Api>): Model<Api> {
   if (!needsForce) {
     return model;
   }
-
-  // Respect explicit user overrides: if the user has set a compat flag to
-  // true in their model definition, they know their endpoint supports it.
   const forcedDeveloperRole = compat?.supportsDeveloperRole === true;
   const forcedUsageStreaming = compat?.supportsUsageInStreaming === true;
-
-  if (forcedDeveloperRole && forcedUsageStreaming) {
+  const targetStrictMode = compat?.supportsStrictMode ?? false;
+  if (
+    compat?.supportsDeveloperRole !== undefined &&
+    compat?.supportsUsageInStreaming !== undefined &&
+    compat?.supportsStrictMode !== undefined
+  ) {
     return model;
   }
 
@@ -82,7 +84,12 @@ export function normalizeModelCompat(model: Model<Api>): Model<Api> {
           ...compat,
           supportsDeveloperRole: forcedDeveloperRole || false,
           supportsUsageInStreaming: forcedUsageStreaming || false,
+          supportsStrictMode: targetStrictMode,
         }
-      : { supportsDeveloperRole: false, supportsUsageInStreaming: false },
+      : {
+          supportsDeveloperRole: false,
+          supportsUsageInStreaming: false,
+          supportsStrictMode: false,
+        },
   } as typeof model;
 }