diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index e459380faa..0063bb753c 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -1537,6 +1537,19 @@ export function schema(model: Provider.Model, schema: JSONSchema7): JSONSchema7
     schema = sanitizeGemini(schema)
   }
 
+  // DeepSeek rejects function schemas whose root type is implicit. Effect
+  // emits object-only discriminated unions as a root `anyOf`; retaining the
+  // union while declaring its shared object type preserves every branch.
+  if (
+    model.api.id.toLowerCase().includes("deepseek") &&
+    schema.type === undefined &&
+    Array.isArray(schema.anyOf) &&
+    schema.anyOf.length > 0 &&
+    schema.anyOf.every((branch) => isPlainObject(branch) && branch.type === "object")
+  ) {
+    schema = { ...schema, type: "object" }
+  }
+
   return schema
 }
 
diff --git a/packages/opencode/test/tool/workflow-provider-schema.test.ts b/packages/opencode/test/tool/workflow-provider-schema.test.ts
index b7be8f53dc..665fd6159c 100644
--- a/packages/opencode/test/tool/workflow-provider-schema.test.ts
+++ b/packages/opencode/test/tool/workflow-provider-schema.test.ts
@@ -11,6 +11,10 @@ import { ProviderTransform } from "../../src/provider/transform"
 const openaiModel = { providerID: "openai", api: { id: "gpt-4.1", npm: "@ai-sdk/openai" } } as never
 const azureModel = { providerID: "azure", api: { id: "gpt-4.1", npm: "@ai-sdk/azure" } } as never
 const geminiModel = { providerID: "google", api: { id: "gemini-3-pro", npm: "@ai-sdk/google" } } as never
+const deepseekModel = {
+  providerID: "deepseek",
+  api: { id: "deepseek-v4-pro", npm: "@ai-sdk/openai-compatible" },
+} as never
 
 type JsonSchemaNode = {
   anyOf?: JsonSchemaNode[]
@@ -117,6 +121,16 @@ describe("workflow provider-facing schema", () => {
     expect(Object.keys(record(resultBranch))).toEqual(expect.arrayContaining(["cursor", "limit"]))
   })
 
+  test("DeepSeek transformation presents the workflow union as an object-root function schema", () => {
+    const transformed = ProviderTransform.schema(
+      deepseekModel,
+      ToolJsonSchema.fromSchema(Parameters as never),
+    ) as JsonSchemaNode
+    expect(transformed.type).toBe("object")
+    expect(branches(transformed)).toHaveLength(10)
+    expect(branchByAction(transformed, "start", "spec_path")).toHaveLength(1)
+  })
+
   test("post-change byte sizes stay at the recorded evidence", async () => {
     const evidence = (await Bun.file(
       new URL("./fixtures/workflow-parameters-post-change.json", import.meta.url),