diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index e564c54a1e8..05d2d6c2d3b 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -754,6 +754,39 @@ export namespace ProviderTransform {
 
     // Convert integer enums to string enums for Google/Gemini
     if (model.providerID === "google" || model.api.id.includes("gemini")) {
+      // Expand $ref references inline - Gemini rejects $ref alongside other fields
+      const expand = (obj: any, defs: Record<string, any>, seen = new Set<string>()): any => {
+        if (obj === null || typeof obj !== "object") return obj
+        if (Array.isArray(obj)) return obj.map((item) => expand(item, defs, seen))
+
+        if (obj.$ref && typeof obj.$ref === "string") {
+          const match = obj.$ref.match(/^#\/(\$defs|definitions)\/(.+)$/)
+          if (match) {
+            const name = match[2]
+            if (seen.has(name)) return { type: "object" }
+            if (defs[name]) {
+              seen.add(name)
+              const rest = Object.fromEntries(Object.entries(obj).filter(([k]) => k !== "$ref"))
+              const result = expand({ ...defs[name], ...rest }, defs, new Set(seen))
+              seen.delete(name)
+              return result
+            }
+          }
+        }
+
+        const result: any = {}
+        for (const [key, value] of Object.entries(obj)) {
+          if (key === "$defs" || key === "definitions") continue
+          result[key] = expand(value, defs, seen)
+        }
+        return result
+      }
+
+      const defs = (schema as any).$defs || (schema as any).definitions || {}
+      if (Object.keys(defs).length > 0) {
+        schema = expand(schema, defs)
+      }
+
       const sanitizeGemini = (obj: any): any => {
         if (obj === null || typeof obj !== "object") {
           return obj
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index 0e0bb440aa8..222f8e92ed0 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -516,6 +516,167 @@ describe("ProviderTransform.schema - gemini non-object properties removal", () =
   })
 })
 
+describe("ProviderTransform.schema - gemini $ref expansion", () => {
+  const geminiModel = {
+    providerID: "google",
+    api: { id: "gemini-3-pro" },
+  } as any
+
+  test("expands simple $ref with $defs", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        option: { $ref: "#/$defs/QuestionOption" },
+      },
+      $defs: {
+        QuestionOption: {
+          type: "object",
+          properties: {
+            label: { type: "string" },
+            description: { type: "string" },
+          },
+          required: ["label", "description"],
+        },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.$defs).toBeUndefined()
+    expect(result.properties.option.$ref).toBeUndefined()
+    expect(result.properties.option.type).toBe("object")
+    expect(result.properties.option.properties.label.type).toBe("string")
+  })
+
+  test("expands $ref in array items", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        options: {
+          type: "array",
+          items: { $ref: "#/$defs/Option" },
+        },
+      },
+      $defs: {
+        Option: { type: "object", properties: { value: { type: "string" } } },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.options.items.$ref).toBeUndefined()
+    expect(result.properties.options.items.type).toBe("object")
+    expect(result.properties.options.items.properties.value.type).toBe("string")
+  })
+
+  test("expands nested $ref references", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        question: { $ref: "#/$defs/Question" },
+      },
+      $defs: {
+        Question: {
+          type: "object",
+          properties: {
+            options: { type: "array", items: { $ref: "#/$defs/Option" } },
+          },
+        },
+        Option: { type: "object", properties: { label: { type: "string" } } },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.question.properties.options.items.type).toBe("object")
+    expect(result.properties.question.properties.options.items.properties.label.type).toBe("string")
+  })
+
+  test("handles circular $ref by returning simple object", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        node: { $ref: "#/$defs/TreeNode" },
+      },
+      $defs: {
+        TreeNode: {
+          type: "object",
+          properties: {
+            value: { type: "string" },
+            children: { type: "array", items: { $ref: "#/$defs/TreeNode" } },
+          },
+        },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.node.type).toBe("object")
+    expect(result.properties.node.properties.value.type).toBe("string")
+    expect(result.properties.node.properties.children.items.type).toBe("object")
+  })
+
+  test("expands definitions (legacy format)", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        item: { $ref: "#/definitions/Item" },
+      },
+      definitions: {
+        Item: { type: "object", properties: { name: { type: "string" } } },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.definitions).toBeUndefined()
+    expect(result.properties.item.$ref).toBeUndefined()
+    expect(result.properties.item.type).toBe("object")
+  })
+
+  test("preserves description when expanding $ref", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        option: {
+          $ref: "#/$defs/Option",
+          description: "Custom description",
+        },
+      },
+      $defs: {
+        Option: { type: "string" },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.option.type).toBe("string")
+    expect(result.properties.option.description).toBe("Custom description")
+  })
+
+  test("does not expand $ref for non-gemini providers", () => {
+    const openaiModel = {
+      providerID: "openai",
+      api: { id: "gpt-4" },
+    } as any
+
+    const schema = {
+      type: "object",
+      properties: {
+        option: { $ref: "#/$defs/Option" },
+      },
+      $defs: {
+        Option: { type: "string" },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(openaiModel, schema) as any
+
+    expect(result.$defs).toBeDefined()
+    expect(result.properties.option.$ref).toBe("#/$defs/Option")
+  })
+})
+
 describe("ProviderTransform.message - DeepSeek reasoning content", () => {
   test("DeepSeek with tool calls includes reasoning_content in providerOptions", () => {
     const msgs = [