diff --git a/open-sse/services/autoCombo/__tests__/chaosEngine.test.ts b/open-sse/services/autoCombo/__tests__/chaosEngine.test.ts
index 758ba41c57..7f61e7ab43 100644
--- a/open-sse/services/autoCombo/__tests__/chaosEngine.test.ts
+++ b/open-sse/services/autoCombo/__tests__/chaosEngine.test.ts
@@ -85,18 +85,29 @@ describe("runChaosPanel", () => {
 });
 
 describe("serializeChaosPart", () => {
-  it("emits a comment + omni-chaos-part event envelope", () => {
+  it("emits a comment + omni-chaos-part event envelope when custom event is requested", () => {
     const part: ChaosPart = { model: "a/gpt", index: 0, ok: true, text: "hi" };
-    const s = serializeChaosPart(part, false);
+    const s = serializeChaosPart(part, false, true);
     expect(s).toContain("event: omni-chaos-part");
     expect(s).toContain('"type":"omni-chaos-part"');
     expect(s).toContain('"model":"a/gpt"');
     expect(s).toContain(": chaos 0 ok a/gpt");
   });
+
+  it("emits ONLY the SSE comment (no event/data) by default for OpenAI-compatible clients", () => {
+    const part: ChaosPart = { model: "a/gpt", index: 0, ok: true, text: "hi" };
+    const s = serializeChaosPart(part, false);
+    // comment line kept (ignored by every SSE parser by spec)
+    expect(s).toContain(": chaos 0 ok a/gpt");
+    // NO custom event/data — those break openai-node / @ai-sdk validators
+    expect(s).not.toContain("event: omni-chaos-part");
+    expect(s).not.toContain('"type":"omni-chaos-part"');
+    expect(s).not.toMatch(/^data:/m);
+  });
 });
 
 describe("handleChaosChat", () => {
-  it("emits broadcast events + final OpenAI chunk", async () => {
+  it("emits ONLY SSE comments (no custom event) by default + final OpenAI chunk", async () => {
     const handle = fakeHandle(async (model) => textResponse(`ans-${model}`));
     const res = await handleChaosChat({
       body: { messages: [] },
@@ -106,13 +117,27 @@ describe("handleChaosChat", () => {
     expect(res.headers.get("X-OmniRoute-Chaos")).toBe("true");
     expect(res.headers.get("X-OmniRoute-Chaos-Panel")).toBe("2");
     const body = await res.text();
-    // each model gets a broadcast event
-    expect(body.match(/event: omni-chaos-part/g)?.length).toBe(2);
+    // NO custom event by default — OpenAI-compatible parsers choke on it
+    expect(body.match(/event: omni-chaos-part/g)?.length ?? 0).toBe(0);
+    expect(body.match(/^: chaos /gm)?.length ?? 0).toBe(2);
     // final canonical chunk carries the primary answer
     expect(body).toContain("ans-b/opus");
     expect(body).toContain("[DONE]");
   });
 
+  it("emits omni-chaos-part events when stream_options.include_chaos_parts is set", async () => {
+    const handle = fakeHandle(async (model) => textResponse(`ans-${model}`));
+    const res = await handleChaosChat({
+      body: { messages: [], stream_options: { include_chaos_parts: true } },
+      models: ["a/gpt", "b/opus"],
+      handleSingleModel: handle,
+    });
+    const body = await res.text();
+    expect(body.match(/event: omni-chaos-part/g)?.length).toBe(2);
+    expect(body).toContain("ans-b/opus");
+    expect(body).toContain("[DONE]");
+  });
+
   it("degrades to a direct call when only one model", async () => {
     const handle = fakeHandle(async () => textResponse("solo"));
     const res = await handleChaosChat({
@@ -138,8 +163,8 @@ describe("handleChaosChat", () => {
     // client learns via the error final chunk rather than a bare 503.
     expect(res.status).toBe(200);
     const body = await res.text();
-    // each model gets a broadcast fail event
-    expect(body.match(/event: omni-chaos-part/g)?.length).toBe(2);
+    // NO custom events by default (comments only), error conveyed via final chunk
+    expect(body.match(/event: omni-chaos-part/g)?.length ?? 0).toBe(0);
     expect(body).toContain("All chaos panel models failed");
     expect(body).toContain("[DONE]");
   });
diff --git a/open-sse/services/autoCombo/chaosEngine.ts b/open-sse/services/autoCombo/chaosEngine.ts
index 6c05e50236..08e581e3fc 100644
--- a/open-sse/services/autoCombo/chaosEngine.ts
+++ b/open-sse/services/autoCombo/chaosEngine.ts
@@ -53,16 +53,28 @@ export type ChaosPart = {
 };
 
 /**
- * Build the SSE comment/event wrapper for one chaos panel part.
- * We emit a custom event name `omni-chaos-part` so a protocol-aware IDE can
- * split it out; non-aware clients reading OpenAI-style SSE will simply ignore
- * the unknown event and use the final `data:` chunk below.
+ * Build the SSE wrapper for one chaos panel part.
+ *
+ * By DEFAULT only an SSE comment (`: chaos ...`) is emitted — comments are
+ * ignored by every SSE parser per spec, so OpenAI-compatible clients
+ * (openai-node, @ai-sdk/openai-compatible, …) never see a non-`choices`
+ * `data:` payload and their schema validation cannot fail with an
+ * `invalid_union` error.
+ *
+ * When `emitCustomEvent` is true (opt-in via
+ * `stream_options.include_chaos_parts`), the custom event name
+ * `omni-chaos-part` + metadata `data:` block is also emitted so a
+ * protocol-aware IDE can split panels out.
  *
  * The part's text is NOT included in the metadata event — it arrives in the
  * final `data:` chunk for the primary model. This keeps each broadcast event
  * small (metadata-only) so SSE buffering stays predictable.
  */
-export function serializeChaosPart(part: ChaosPart, isFinal: boolean): string {
+export function serializeChaosPart(
+  part: ChaosPart,
+  isFinal: boolean,
+  emitCustomEvent = false
+): string {
   const meta = {
     type: "omni-chaos-part",
     model: part.model,
@@ -71,11 +83,11 @@ export function serializeChaosPart(part: ChaosPart, isFinal: boolean): string {
     final: isFinal,
     ...(part.error ? { error: part.error } : {}),
   };
-  return (
-    `: chaos ${part.index} ${part.ok ? "ok" : "fail"} ${part.model}\n` +
-    `event: omni-chaos-part\n` +
-    `data: ${JSON.stringify(meta)}\n\n`
-  );
+  const comment = `: chaos ${part.index} ${part.ok ? "ok" : "fail"} ${part.model}\n`;
+  if (!emitCustomEvent) {
+    return comment + "\n";
+  }
+  return comment + `event: omni-chaos-part\n` + `data: ${JSON.stringify(meta)}\n\n`;
 }
 
 /**
@@ -330,9 +342,11 @@ function concatSseText(sse: string): string {
  * `config.chaos.enabled` flag is set (the `auto/chaos` virtual combo).
  *
  * Returns a single Response whose body is an SSE stream:
- *   - one `omni-chaos-part` event per panel model, enqueued PROGRESSIVELY as
- *     each model lands (so the client starts receiving answers immediately,
- *     without waiting for the whole panel to finish)
+ *   - one SSE comment (`: chaos N ...`) per panel model, enqueued
+ *     PROGRESSIVELY as each model lands (comments are ignored by every SSE
+ *     parser, so OpenAI-compatible clients see only the final chunk)
+ *   - when `stream_options.include_chaos_parts: true` is set, the per-panel
+ *     `omni-chaos-part` custom event is emitted instead of the bare comment
  *   - a final `data:` OpenAI-style chunk carrying the primary model's answer
  *     (so non-aware clients / IDEs still get a usable completion)
  *   - a terminating `data: [DONE]`
@@ -354,6 +368,14 @@ export async function handleChaosChat(opts: {
   const panel = Array.isArray(models) ? models.filter(Boolean) : [];
   const hardTimeout = tuning?.panelHardTimeoutMs ?? CHAOS_DEFAULTS.panelHardTimeoutMs;
   const minPanel = tuning?.minPanel ?? CHAOS_DEFAULTS.minPanel;
+  // Opt-in gate: only protocol-aware clients request the custom event. OpenAI
+  // SDK validators choke on any `data:` payload without `choices`/`error`, so
+  // the default MUST be comment-only output.
+  const streamOptions = (body as Record<string, unknown> | null | undefined)?.stream_options;
+  const emitCustomEvent =
+    typeof streamOptions === "object" &&
+    streamOptions !== null &&
+    (streamOptions as Record<string, unknown>).include_chaos_parts === true;
   if (panel.length === 0) {
     return errorResponse(400, "Chaos combo has no models");
   }
@@ -396,7 +418,7 @@ export async function handleChaosChat(opts: {
           hardTimeout,
           log,
           onResult: async (part) => {
-            await safeEnqueue(serializeChaosPart(part, false));
+            await safeEnqueue(serializeChaosPart(part, false, emitCustomEvent));
           },
         });
       });