fix(gemini): prevent gemini incompatibility with some tools (#31877)

packages/opencode/src/provider/transform.ts

@@ -1375,6 +1375,24 @@ export function schema(model: Provider.Model, schema: JSONSchema7): JSONSchema7
         }
       }
 
+      // Gemini requires a single `type`, not a JSON Schema type array such as
+      // `["number","string"]` (emitted by some MCP servers). Plain `@ai-sdk/google`
+      // rewrites these into an `anyOf` of single-type schemas, but OpenAI-compatible
+      // transports (e.g. GitHub Copilot proxying to Gemini) forward them verbatim
+      // and the backend rejects the array form. Mirror the SDK: split non-null
+      // types into `anyOf`, and lift `null` into `nullable`.
+      if (Array.isArray(result.type)) {
+        const hasNull = result.type.includes("null")
+        const nonNull = result.type.filter((entry: unknown) => entry !== "null")
+        if (nonNull.length === 0) {
+          result.type = "null"
+        } else {
+          delete result.type
+          result.anyOf = nonNull.map((entry: unknown) => ({ type: entry }))
+          if (hasNull) result.nullable = true
+        }
+      }
+
       // Filter required array to only include fields that exist in properties
       if (result.type === "object" && result.properties && Array.isArray(result.required)) {
         result.required = result.required.filter((field: any) => field in result.properties)

packages/opencode/test/provider/transform.test.ts

@@ -857,6 +857,93 @@ describe("ProviderTransform.schema - gemini nested array items", () => {
   })
 })
 
+describe("ProviderTransform.schema - gemini type arrays", () => {
+  // Mirrors @ai-sdk/google's convertJSONSchemaToOpenAPISchema: JSON Schema type
+  // arrays (e.g. `["number","string"]`, common in MCP tool schemas) become an
+  // `anyOf` of single-type schemas, with `null` lifted into `nullable`. Plain
+  // @ai-sdk/google rewrites these, but OpenAI-compatible transports such as
+  // GitHub Copilot (proxying to Gemini) forward them verbatim and the backend
+  // rejects the array form.
+  const geminiModel = {
+    providerID: "google",
+    api: {
+      id: "gemini-3-pro",
+    },
+  } as any
+
+  test("splits a multi-type array into anyOf and drops the type array", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        status: { type: ["number", "string"], description: "status filter" },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.status.type).toBeUndefined()
+    expect(result.properties.status.anyOf).toEqual([{ type: "number" }, { type: "string" }])
+    expect(result.properties.status.nullable).toBeUndefined()
+    // Sibling keywords stay alongside the generated anyOf.
+    expect(result.properties.status.description).toBe("status filter")
+  })
+
+  test("lifts null into nullable for a nullable type array", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        maybe: { type: ["string", "null"], description: "nullable string" },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.maybe.type).toBeUndefined()
+    expect(result.properties.maybe.anyOf).toEqual([{ type: "string" }])
+    expect(result.properties.maybe.nullable).toBe(true)
+  })
+
+  test("collapses an all-null type array to type null", () => {
+    const schema = {
+      type: "object",
+      properties: {
+        nothing: { type: ["null"] },
+      },
+    } as any
+
+    const result = ProviderTransform.schema(geminiModel, schema) as any
+
+    expect(result.properties.nothing.type).toBe("null")
+    expect(result.properties.nothing.anyOf).toBeUndefined()
+  })
+
+  test("rewrites type arrays for gemini served through github-copilot", () => {
+    const copilotGeminiModel = {
+      providerID: "github-copilot",
+      api: {
+        id: "gemini-3.5-flash",
+        npm: "@ai-sdk/github-copilot",
+      },
+    } as any
+
+    const schema = {
+      type: "object",
+      properties: {
+        hook_id: { type: "number", description: "ID of the webhook" },
+        status: { type: ["number", "string"], description: "Filter by response status code" },
+      },
+      required: ["hook_id"],
+      additionalProperties: false,
+    } as any
+
+    const result = ProviderTransform.schema(copilotGeminiModel, schema) as any
+
+    expect(result.properties.status.anyOf).toEqual([{ type: "number" }, { type: "string" }])
+    expect(result.properties.status.type).toBeUndefined()
+    expect(result.properties.hook_id.type).toBe("number")
+  })
+})
+
 describe("ProviderTransform.schema - gemini combiner nodes", () => {
   const geminiModel = {
     providerID: "google",