diff --git a/open-sse/services/combo.ts b/open-sse/services/combo.ts
index 431efd2a02..402f033096 100644
--- a/open-sse/services/combo.ts
+++ b/open-sse/services/combo.ts
@@ -4923,7 +4923,12 @@ async function handleRoundRobinCombo({
               }
             })();
           }
-          return result;
+          // validateResponseQuality peeks streaming bodies via getReader(),
+          // which locks `result.body`. It returns a clonedResponse that replays
+          // the buffered prefix and forwards the rest. Returning the original
+          // (now-locked) `result` makes Next.js throw "ReadableStream is locked"
+          // → 500. Mirror the priority strategy and return the replay response.
+          return quality.clonedResponse ?? result;
         }
 
         // Extract error info
diff --git a/tests/unit/combo-round-robin-streaming-lock-3811.test.ts b/tests/unit/combo-round-robin-streaming-lock-3811.test.ts
new file mode 100644
index 0000000000..b9a5baa88a
--- /dev/null
+++ b/tests/unit/combo-round-robin-streaming-lock-3811.test.ts
@@ -0,0 +1,109 @@
+/**
+ * Issue #3811 — a round-robin combo serving a STREAMING response returned a 500
+ * (`TypeError: Invalid state: The ReadableStream is locked`, ERR_INVALID_STATE).
+ *
+ * validateResponseQuality() peeks streaming bodies via `response.body.getReader()`
+ * to detect empty/content-filtered streams. That call LOCKS `result.body` and hands
+ * back an unlocked replay stream in `quality.clonedResponse`. The priority strategy
+ * returns `quality.clonedResponse ?? result`, but the round-robin success path
+ * returned the original (now-locked) `result`, so Next.js could not pipe the body and
+ * surfaced a 500 even though the upstream call succeeded.
+ *
+ * Regression guard: a round-robin combo whose target returns a streaming response with
+ * content must return a response whose body is READABLE (not locked) and reproduces the
+ * streamed content.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-combo-rr-3811-"));
+process.env.DATA_DIR = TEST_DATA_DIR;
+
+const { handleComboChat } = await import("../../open-sse/services/combo.ts");
+
+const encoder = new TextEncoder();
+
+function createLog() {
+  return { info: () => {}, warn: () => {}, error: () => {}, debug: () => {} };
+}
+
+/** Claude 200 streaming response WITH a content block, one chunk per SSE event. */
+function makeClaudeContentStream(): Response {
+  const events = [
+    `event: message_start\ndata: ${JSON.stringify({
+      type: "message_start",
+      message: {
+        id: "msg_rr_3811",
+        type: "message",
+        role: "assistant",
+        model: "claude-sonnet-4-6",
+        content: [],
+        stop_reason: null,
+        usage: { input_tokens: 5, output_tokens: 0 },
+      },
+    })}\n\n`,
+    `event: content_block_start\ndata: ${JSON.stringify({
+      type: "content_block_start",
+      index: 0,
+      content_block: { type: "text", text: "" },
+    })}\n\n`,
+    `event: content_block_delta\ndata: ${JSON.stringify({
+      type: "content_block_delta",
+      index: 0,
+      delta: { type: "text_delta", text: "hello round-robin" },
+    })}\n\n`,
+    `event: content_block_stop\ndata: ${JSON.stringify({ type: "content_block_stop", index: 0 })}\n\n`,
+    `event: message_delta\ndata: ${JSON.stringify({
+      type: "message_delta",
+      delta: { stop_reason: "end_turn" },
+      usage: { output_tokens: 3 },
+    })}\n\n`,
+    `event: message_stop\ndata: ${JSON.stringify({ type: "message_stop" })}\n\n`,
+  ];
+  const chunks = events.map((e) => encoder.encode(e));
+  let idx = 0;
+  const body = new ReadableStream<Uint8Array>({
+    pull(controller) {
+      if (idx < chunks.length) controller.enqueue(chunks[idx++]);
+      else controller.close();
+    },
+  });
+  return new Response(body, {
+    status: 200,
+    headers: { "content-type": "text/event-stream" },
+  });
+}
+
+test("#3811 round-robin streaming response returns an unlocked, replayable body", async () => {
+  const result = await handleComboChat({
+    body: { stream: true },
+    combo: {
+      name: "rr-streaming-lock-3811",
+      strategy: "round-robin",
+      models: ["model-a"],
+      config: { maxRetries: 0, concurrencyPerModel: 1, queueTimeoutMs: 1000 },
+    },
+    handleSingleModel: async () => makeClaudeContentStream(),
+    isModelAvailable: async () => true,
+    log: createLog(),
+    settings: null,
+    allCombos: null,
+  });
+
+  assert.equal(result.ok, true);
+  assert.ok(result.body, "combo response must expose a body stream");
+  // Without the fix the round-robin path returns the original `result` whose body was
+  // locked by validateResponseQuality's getReader() peek.
+  assert.equal(
+    result.body!.locked,
+    false,
+    "round-robin streaming body must not be locked (must return quality.clonedResponse)"
+  );
+
+  // Draining the body must succeed (a locked body throws here) and replay the content.
+  const text = await result.text();
+  assert.match(text, /hello round-robin/, "replayed stream must contain the upstream content");
+});