diff --git a/packages/ai/src/protocols/utils/gemini-tool-schema.ts b/packages/ai/src/protocols/utils/gemini-tool-schema.ts
index 991dfdfbf96c..a5790f35fdfb 100644
--- a/packages/ai/src/protocols/utils/gemini-tool-schema.ts
+++ b/packages/ai/src/protocols/utils/gemini-tool-schema.ts
@@ -74,7 +74,10 @@ const projectNode = (schema: unknown, nested = false): Record<string, unknown> |
       ["description", schema.description],
       ["required", schema.required],
       ["format", schema.format],
-      ["type", types ? (types.length === 0 ? "null" : undefined) : schema.type],
+      // A single non-null type stays a plain `type` so an optional object keeps carrying its
+      // `properties` and `required`; Gemini reads those only on an OBJECT-typed node and rejects
+      // the whole request when the type is expressed as a combiner instead.
+      ["type", types ? (types.length === 0 ? "null" : types.length === 1 ? types[0] : undefined) : schema.type],
       [
         "nullable",
         (Array.isArray(schema.type) && schema.type.includes("null") && types && types.length > 0) || hasNullAnyOf
@@ -103,7 +106,7 @@ const projectNode = (schema: unknown, nested = false): Record<string, unknown> |
           ? hasNullAnyOf && anyOfTypes.length === 1
             ? undefined
             : anyOfTypes.map((item) => projectNode(item, true))
-          : types && types.length > 0
+          : types && types.length > 1
             ? types.map((type) => ({ type }))
             : undefined,
       ],
diff --git a/packages/ai/test/provider/gemini.test.ts b/packages/ai/test/provider/gemini.test.ts
index 42d7754a5906..f48a38430f09 100644
--- a/packages/ai/test/provider/gemini.test.ts
+++ b/packages/ai/test/provider/gemini.test.ts
@@ -465,8 +465,8 @@ describe("Gemini route", () => {
             anyOf: [{ type: "number" }, { type: "string" }],
           },
           maybe: {
+            type: "string",
             nullable: true,
-            anyOf: [{ type: "string" }],
           },
           nothing: {
             type: "null",
diff --git a/packages/ai/test/tool-schema-projection.test.ts b/packages/ai/test/tool-schema-projection.test.ts
index 4f83942a6608..47233180d181 100644
--- a/packages/ai/test/tool-schema-projection.test.ts
+++ b/packages/ai/test/tool-schema-projection.test.ts
@@ -50,6 +50,36 @@ describe("tool schema projections", () => {
     })
   })
 
+  test("gemini keeps an optional object typed so its properties survive", () => {
+    // Shape an MCP server emits for an optional object parameter.
+    expect(
+      ToolSchemaProjection.gemini({
+        type: "object",
+        properties: {
+          cursor_theme: {
+            type: ["object", "null"],
+            properties: { theme_id: { type: "string" } },
+            required: ["theme_id"],
+          },
+          note: { type: ["string", "null"] },
+          either: { type: ["string", "number"] },
+        },
+      }),
+    ).toEqual({
+      type: "object",
+      properties: {
+        cursor_theme: {
+          type: "object",
+          nullable: true,
+          required: ["theme_id"],
+          properties: { theme_id: { type: "string" } },
+        },
+        note: { type: "string", nullable: true },
+        either: { anyOf: [{ type: "string" }, { type: "number" }] },
+      },
+    })
+  })
+
   test("openai keeps one flat object top-level schema", () => {
     expect(
       ToolSchemaProjection.openAI({