diff --git a/open-sse/executors/opencode.js b/open-sse/executors/opencode.js
index c4c31720..531c5df5 100644
--- a/open-sse/executors/opencode.js
+++ b/open-sse/executors/opencode.js
@@ -411,6 +411,11 @@ export class OpenCodeExecutor extends BaseExecutor {
   transformRequest(model, body, stream, credentials) {
     if (body && typeof body === "object" && model && !body.model) body.model = model;
     if (isResponsesModel(model || body?.model) && body && typeof body === "object") {
+      // ponytail: chỉ model đã xác nhận auto-only; mở allowlist khi có bằng chứng.
+      if ("tool_choice" in body && body.tool_choice !== "auto"
+        && this.config.quirks?.forceAutoToolChoiceModels?.includes(baseModelId(model))) {
+        body.tool_choice = "auto";
+      }
       const normalized = normalizeResponsesInput(body.input);
       if (normalized) body.input = normalized;
       if (!Array.isArray(body.input) || body.input.length === 0) {
diff --git a/open-sse/providers/registry/opencode.js b/open-sse/providers/registry/opencode.js
index fc2ab8c6..0914f431 100644
--- a/open-sse/providers/registry/opencode.js
+++ b/open-sse/providers/registry/opencode.js
@@ -18,6 +18,9 @@ export default {
       "x-opencode-client": "desktop",
     },
     noAuth: true,
+    quirks: {
+      forceAutoToolChoiceModels: ["muse-spark-1.3-contributor-free"],
+    },
   },
   models: [
     // Endpoint formats differ per model, so declare non-chat models explicitly.
diff --git a/tests/unit/opencode-free-tool-choice.test.js b/tests/unit/opencode-free-tool-choice.test.js
new file mode 100644
index 00000000..4d3aca91
--- /dev/null
+++ b/tests/unit/opencode-free-tool-choice.test.js
@@ -0,0 +1,90 @@
+import { describe, expect, it, vi } from "vitest";
+import { PROVIDERS } from "../../open-sse/config/providers.js";
+import { OpenCodeExecutor } from "../../open-sse/executors/opencode.js";
+import { proxyAwareFetch } from "../../open-sse/utils/proxyFetch.js";
+
+vi.mock("../../open-sse/utils/proxyFetch.js", () => ({
+  proxyAwareFetch: vi.fn(async () => ({ ok: true, status: 200, headers: { get: () => "" } })),
+}));
+
+// Break caught: opencode/muse-spark-1.3-contributor-free 400 vì upstream
+// chỉ nhận tool_choice "auto"; named/required/none phải demote sang "auto".
+const FREE_13 = "muse-spark-1.3-contributor-free";
+const CREDS = { connectionId: "opencode-free-tool-choice-test" };
+const INPUT = [{ type: "message", role: "user", content: [{ type: "input_text", text: "hi" }] }];
+const TOOLS = [{ type: "function", name: "get_weather", description: "w", parameters: { type: "object", properties: {} } }];
+
+function responsesBody(model, tool_choice) {
+  const body = { model, input: structuredClone(INPUT), tools: structuredClone(TOOLS) };
+  if (tool_choice !== undefined) body.tool_choice = tool_choice;
+  return body;
+}
+
+describe("opencode Free 1.3 tool_choice auto-only", () => {
+  it("khai quirk đúng model 1.3-Free trong registry", () => {
+    expect(PROVIDERS.opencode.quirks?.forceAutoToolChoiceModels).toEqual([FREE_13]);
+  });
+
+  it.each([
+    ["Responses named", { type: "function", name: "get_weather" }],
+    ["Chat function named", { type: "function", function: { name: "get_weather" } }],
+    ["Claude tool named", { type: "tool", name: "get_weather" }],
+    ["required", "required"],
+    ["none", "none"],
+  ])("demote %s sang auto (plain và max)", (_label, choice) => {
+    for (const model of [FREE_13, `${FREE_13}(max)`]) {
+      const body = responsesBody(model, structuredClone(choice));
+      const out = new OpenCodeExecutor().transformRequest(model, body, true, CREDS);
+      expect(out.tool_choice).toBe("auto");
+      expect(out.tools).toEqual(TOOLS);
+      expect(out.input).toEqual(INPUT);
+    }
+  });
+
+  it("giữ auto và absent; tools/input nguyên vẹn", () => {
+    const autoOut = new OpenCodeExecutor().transformRequest(
+      FREE_13, responsesBody(FREE_13, "auto"), true, CREDS,
+    );
+    expect(autoOut.tool_choice).toBe("auto");
+    expect(autoOut.tools).toEqual(TOOLS);
+    expect(autoOut.input).toEqual(INPUT);
+
+    const absentOut = new OpenCodeExecutor().transformRequest(
+      FREE_13, responsesBody(FREE_13, undefined), true, CREDS,
+    );
+    expect("tool_choice" in absentOut).toBe(false);
+    expect(absentOut.tools).toEqual(TOOLS);
+    expect(absentOut.input).toEqual(INPUT);
+  });
+
+  it.each([
+    ["1.2-Free", "muse-spark-1.2-contributor-free"],
+    ["future 1.4-Free", "muse-spark-1.4-contributor-free"],
+    ["Go id", "muse-spark-1.3-contributor"],
+    ["non-Muse", "big-pickle"],
+  ])("không đổi tool_choice của %s", (_label, model) => {
+    const choice = { type: "function", name: "get_weather" };
+    const body = responsesBody(model, structuredClone(choice));
+    const out = new OpenCodeExecutor().transformRequest(model, body, true, CREDS);
+    expect(out.tool_choice).toEqual(choice);
+  });
+
+  it("wire: execute gửi choice auto tới /zen/v1/responses", async () => {
+    proxyAwareFetch.mockClear();
+    const ex = new OpenCodeExecutor();
+    const body = responsesBody(FREE_13, { type: "function", name: "get_weather" });
+    const { url, transformedBody } = await ex.execute({
+      model: FREE_13, body, stream: true, credentials: CREDS,
+    });
+    expect(url).toBe("https://opencode.ai/zen/v1/responses");
+    expect(transformedBody.tool_choice).toBe("auto");
+    expect(proxyAwareFetch).toHaveBeenCalledTimes(1);
+    const [actualUrl, actualInit] = proxyAwareFetch.mock.calls[0];
+    expect(actualUrl).toBe("https://opencode.ai/zen/v1/responses");
+    const sent = JSON.parse(actualInit.body);
+    expect(sent.tool_choice).toBe("auto");
+    expect(sent.model).toBe(FREE_13);
+    expect(sent.tools).toEqual(TOOLS);
+    expect(sent.input).toEqual(INPUT);
+  });
+});