diff --git a/open-sse/config/providers/registry/opencode/zen/index.ts b/open-sse/config/providers/registry/opencode/zen/index.ts
index 1bb2382aad1c..bb62ad20f2c4 100644
--- a/open-sse/config/providers/registry/opencode/zen/index.ts
+++ b/open-sse/config/providers/registry/opencode/zen/index.ts
@@ -30,9 +30,12 @@ export const opencode_zenProvider: RegistryEntry = {
     ...OPENCODE_ZEN_GO_SHARED_MODELS,
     // models[0] (big-pickle) is the dashboard default; SHARED spread kept after it.
 
-    { id: "gpt-5.6-sol", name: "GPT 5.6 Sol" },
-    { id: "gpt-5.6-terra", name: "GPT 5.6 Terra" },
-    { id: "gpt-5.6-luna", name: "GPT 5.6 Luna" },
+    // #14229: OpenCode Zen serves the GPT-5.6 family through /responses,
+    // not /chat/completions. Without this declaration the executor builds the
+    // wrong upstream URL and returns HTTP 503 "Endpoint is unavailable".
+    { id: "gpt-5.6-sol", name: "GPT 5.6 Sol", targetFormat: "openai-responses" },
+    { id: "gpt-5.6-terra", name: "GPT 5.6 Terra", targetFormat: "openai-responses" },
+    { id: "gpt-5.6-luna", name: "GPT 5.6 Luna", targetFormat: "openai-responses" },
     { id: "gpt-5.4", name: "GPT 5.4" },
     { id: "gpt-5.4-mini", name: "GPT 5.4 Mini" },
     { id: "gpt-5.4-nano", name: "GPT 5.4 Nano" },
diff --git a/tests/unit/opencode-zen-gpt56-responses-14229.test.ts b/tests/unit/opencode-zen-gpt56-responses-14229.test.ts
new file mode 100644
index 000000000000..510d56a0451c
--- /dev/null
+++ b/tests/unit/opencode-zen-gpt56-responses-14229.test.ts
@@ -0,0 +1,24 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { opencode_zenProvider } from "../../open-sse/config/providers/registry/opencode/zen/index.ts";
+import { resolveOpencodeTargetFormat } from "../../open-sse/executors/opencode.ts";
+
+const GPT56_MODELS = ["gpt-5.6-sol", "gpt-5.6-terra", "gpt-5.6-luna"];
+
+test("opencode-zen GPT-5.6 models use the Responses API", () => {
+  const models = new Map((opencode_zenProvider.models ?? []).map((model) => [model.id, model]));
+
+  for (const modelId of GPT56_MODELS) {
+    assert.equal(
+      models.get(modelId)?.targetFormat,
+      "openai-responses",
+      `${modelId} must declare the Responses API wire format`
+    );
+    assert.equal(
+      resolveOpencodeTargetFormat("opencode-zen", modelId),
+      "openai-responses",
+      `${modelId} must resolve to the Responses API through the public provider alias`
+    );
+  }
+});