diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index 320af4d3953..0aacf0935c2 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -424,6 +424,10 @@ export namespace ProviderTransform {
           result.required = result.required.filter((field: any) => field in result.properties)
         }
 
+        if (result.type === "array" && result.items == null) {
+          result.items = {}
+        }
+
         return result
       }
 
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index 17ae7f55808..78bd296c99c 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -167,6 +167,30 @@ describe("ProviderTransform.maxOutputTokens", () => {
   })
 })
 
+describe("ProviderTransform.schema - gemini array items", () => {
+  test("adds missing items for array properties", () => {
+    const geminiModel = {
+      providerID: "google",
+      api: {
+        id: "gemini-3-pro",
+      },
+    } as any
+
+    const schema = {
+      type: "object",
+      properties: {
+        nodes: { type: "array" },
+        edges: { type: "array", items: { type: "string" } },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.nodes.items).toBeDefined()
+    expect(result.properties.edges.items.type).toBe("string")
+  })
+})
+
 describe("ProviderTransform.message - DeepSeek reasoning content", () => {
   test("DeepSeek with tool calls includes reasoning_content in providerOptions", () => {
     const msgs = [