diff --git a/.changeset/fix-strict-schema-optional-composites.md b/.changeset/fix-strict-schema-optional-composites.md
new file mode 100644
index 000000000..9cc97c2b9
--- /dev/null
+++ b/.changeset/fix-strict-schema-optional-composites.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/ai': patch
+---
+
+fix(ai): make optional nested objects and arrays nullable under `forStructuredOutput`. Previously `makeStructuredOutputCompatible` recursed into optional composites and skipped the `'null'`-wrap, but still added them to `required[]`, producing a schema that OpenAI-style strict `json_schema` providers reject. Any schema with an optional `z.object({...}).optional()` or `z.array(...).optional()` field now serializes as `type: ['object','null']` / `['array','null']` and passes strict validation.
diff --git a/packages/typescript/ai-openrouter/tests/openrouter-adapter.test.ts b/packages/typescript/ai-openrouter/tests/openrouter-adapter.test.ts
index 25dae0daf..1c7ac4ed9 100644
--- a/packages/typescript/ai-openrouter/tests/openrouter-adapter.test.ts
+++ b/packages/typescript/ai-openrouter/tests/openrouter-adapter.test.ts
@@ -964,8 +964,11 @@ describe('OpenRouter structured output', () => {
     // Root object: all props required, additionalProperties: false
     expect(sentSchema.additionalProperties).toBe(false)
     expect(sentSchema.required).toEqual(['title', 'description', 'tags'])
-    // Optional field is made nullable
+    // Optional primitive is made nullable
     expect(sentSchema.properties.description.type).toEqual(['string', 'null'])
+    // Optional array must also be made nullable (strict mode requires every
+    // required property to be nullable if it was originally optional)
+    expect(sentSchema.properties.tags.type).toEqual(['array', 'null'])
     // Nested array items: same transformation applied recursively
     expect(sentSchema.properties.tags.items.additionalProperties).toBe(false)
     expect(sentSchema.properties.tags.items.required).toEqual([
@@ -978,6 +981,46 @@ describe('OpenRouter structured output', () => {
     ])
   })
 
+  it('makes optional nested objects nullable under strict mode', async () => {
+    const nonStreamResponse = {
+      choices: [{ message: { content: '{"id":"x","meta":null}' } }],
+    }
+    setupMockSdkClient([], nonStreamResponse)
+    const adapter = createAdapter()
+
+    const outputSchema = {
+      type: 'object',
+      properties: {
+        id: { type: 'string' },
+        meta: {
+          type: 'object',
+          properties: {
+            createdAt: { type: 'string' },
+          },
+          required: ['createdAt'],
+        },
+      },
+      required: ['id'],
+    }
+
+    await adapter.structuredOutput({
+      chatOptions: {
+        model: 'openai/gpt-4o-mini',
+        messages: [{ role: 'user', content: 'Generate' }],
+      },
+      outputSchema,
+    })
+
+    const [rawParams] = mockSend.mock.calls[0]!
+    const sentSchema = rawParams.chatRequest.responseFormat.jsonSchema.schema
+
+    expect(sentSchema.required).toEqual(['id', 'meta'])
+    expect(sentSchema.properties.meta.type).toEqual(['object', 'null'])
+    // Inner object still strict-compatible
+    expect(sentSchema.properties.meta.additionalProperties).toBe(false)
+    expect(sentSchema.properties.meta.required).toEqual(['createdAt'])
+  })
+
   it('flows through core chat() entrypoint with strict transformation', async () => {
     // End-to-end via chat(): schema converted by the core, then made
     // strict-compatible by the adapter before the SDK call.
diff --git a/packages/typescript/ai/src/activities/chat/tools/schema-converter.ts b/packages/typescript/ai/src/activities/chat/tools/schema-converter.ts
index 735ef79ab..8f9d481eb 100644
--- a/packages/typescript/ai/src/activities/chat/tools/schema-converter.ts
+++ b/packages/typescript/ai/src/activities/chat/tools/schema-converter.ts
@@ -75,18 +75,24 @@ function makeStructuredOutputCompatible(
 
       // Recursively transform nested objects/arrays
       if (prop.type === 'object' && prop.properties) {
-        properties[propName] = makeStructuredOutputCompatible(
+        const transformed = makeStructuredOutputCompatible(
           prop,
           prop.required || [],
         )
+        properties[propName] = wasOptional
+          ? { ...transformed, type: ['object', 'null'] }
+          : transformed
       } else if (prop.type === 'array' && prop.items) {
-        properties[propName] = {
+        const transformed = {
           ...prop,
           items: makeStructuredOutputCompatible(
             prop.items,
             prop.items.required || [],
           ),
         }
+        properties[propName] = wasOptional
+          ? { ...transformed, type: ['array', 'null'] }
+          : transformed
       } else if (wasOptional) {
         // Make optional fields nullable by adding null to the type
         if (prop.type && !Array.isArray(prop.type)) {