fix: handle nested array items for Gemini schema validation (#11952)

2 files changed, 122 insertions, 2 deletions

diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index b4f1aaca4..16f2bd3f2 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -768,8 +768,15 @@ export namespace ProviderTransform {
           result.required = result.required.filter((field: any) => field in result.properties)
         }
 
-        if (result.type === "array" && result.items == null) {
-          result.items = {}
+        if (result.type === "array") {
+          if (result.items == null) {
+            result.items = {}
+          }
+          // Ensure items has at least a type if it's an empty object
+          // This handles nested arrays like { type: "array", items: { type: "array", items: {} } }
+          if (typeof result.items === "object" && !Array.isArray(result.items) && !result.items.type) {
+            result.items.type = "string"
+          }
         }
 
         // Remove properties/required from non-object types (Gemini rejects these)
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index 0743049fe..0e0bb440a 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -293,6 +293,119 @@ describe("ProviderTransform.schema - gemini array items", () => {
   })
 })
 
+describe("ProviderTransform.schema - gemini nested array items", () => {
+  const geminiModel = {
+    providerID: "google",
+    api: {
+      id: "gemini-3-pro",
+    },
+  } as any
+
+  test("adds type to 2D array with empty inner items", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        values: {
+          type: "array",
+          items: {
+            type: "array",
+            items: {}, // Empty items object
+          },
+        },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    // Inner items should have a default type
+    expect(result.properties.values.items.items.type).toBe("string")
+  })
+
+  test("adds items and type to 2D array with missing inner items", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        data: {
+          type: "array",
+          items: { type: "array" }, // No items at all
+        },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.data.items.items).toBeDefined()
+    expect(result.properties.data.items.items.type).toBe("string")
+  })
+
+  test("handles deeply nested arrays (3D)", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        matrix: {
+          type: "array",
+          items: {
+            type: "array",
+            items: {
+              type: "array",
+              // No items
+            },
+          },
+        },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.matrix.items.items.items).toBeDefined()
+    expect(result.properties.matrix.items.items.items.type).toBe("string")
+  })
+
+  test("preserves existing item types in nested arrays", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        numbers: {
+          type: "array",
+          items: {
+            type: "array",
+            items: { type: "number" }, // Has explicit type
+          },
+        },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    // Should preserve the explicit type
+    expect(result.properties.numbers.items.items.type).toBe("number")
+  })
+
+  test("handles mixed nested structures with objects and arrays", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        spreadsheetData: {
+          type: "object",
+          properties: {
+            rows: {
+              type: "array",
+              items: {
+                type: "array",
+                items: {}, // Empty items
+              },
+            },
+          },
+        },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.spreadsheetData.properties.rows.items.items.type).toBe("string")
+  })
+})
+
 describe("ProviderTransform.schema - gemini non-object properties removal", () => {
   const geminiModel = {
     providerID: "google",