diff --git a/changelog.d/fixes/9971-empty-choices-vps.md b/changelog.d/fixes/9971-empty-choices-vps.md
new file mode 100644
index 0000000000..9e09aa3c9b
--- /dev/null
+++ b/changelog.d/fixes/9971-empty-choices-vps.md
@@ -0,0 +1 @@
+- fix(chat): don't misclassify content-less thinking/redacted Claude bodies as empty_choices (#9971)
\ No newline at end of file
diff --git a/open-sse/handlers/responseTranslator.ts b/open-sse/handlers/responseTranslator.ts
index 165e292dc0..3527cdfec5 100644
--- a/open-sse/handlers/responseTranslator.ts
+++ b/open-sse/handlers/responseTranslator.ts
@@ -523,6 +523,19 @@ export function translateNonStreamingResponse(
         }
       }
 
+      // #9971: a content-less-but-valid Claude body (thinking / redacted_thinking
+      // / tool_use-only, or a truncated extended-thinking-only stream) has blocks
+      // but no final text. Surfacing it here helps correlate a live VPS capture
+      // with detectMalformedNonStream's clause; the content itself is valid output
+      // (see detectMalformedNonStream), so this is observation, not a decision.
+      if (textContent.length === 0 && process.env.DEBUG_CLAUDE_NONSTREAM === "true") {
+        console.log(
+          `[ClaudeNonStream] ${contentBlocks.length} content block(s), empty textContent ` +
+            `(thinking=${thinkingContent.length}, toolCalls=${toolCalls.length}); ` +
+            `content-less-but-valid body preserved (not empty_choices)`
+        );
+      }
+
       const message: JsonRecord = { role: "assistant" };
       if (textContent) {
         message.content = textContent;
diff --git a/open-sse/utils/diagnostics.ts b/open-sse/utils/diagnostics.ts
index 4402656281..d872599a98 100644
--- a/open-sse/utils/diagnostics.ts
+++ b/open-sse/utils/diagnostics.ts
@@ -211,7 +211,8 @@ export function detectMalformedNonStream(resp: unknown): MalformedReason | null
   // `choices`. Without this branch every non-streaming Claude response (incl. plain text)
   // falls through to `empty_choices` → a false 502 (#5108, regression from #4942).
   if (body.type === "message" && Array.isArray(body.content)) {
-    const hasOutput = (body.content as unknown[]).some((block) => {
+    const content = body.content as unknown[];
+    const hasOutput = content.some((block) => {
       // A malformed/partial provider response could carry a null (or non-object)
       // entry in `content`; guard before type-asserting so the detector never
       // throws on `null.type` (that would crash the whole non-stream classifier).
@@ -229,16 +230,18 @@ export function detectMalformedNonStream(resp: unknown): MalformedReason | null
       ) {
         return true;
       }
-      // Extended-thinking block: valid when it carries visible thinking text OR a
-      // non-empty `signature` (cryptographic proof the thinking step ran, so it is a
-      // valid completion even when the thinking text is "").
-      if (
-        b.type === "thinking" &&
-        ((typeof b.thinking === "string" && (b.thinking as string).length > 0) ||
-          (typeof b.signature === "string" && (b.signature as string).length > 0))
-      ) {
-        return true;
-      }
+      // Extended-thinking block: valid structural output whenever the model
+      // entered the thinking phase, even with no visible thinking text and no
+      // `signature`. #9971: the Claude Code OAuth upstream can truncate long
+      // large-input+large-output generations around the ~3-min turn boundary,
+      // leaving a content-less thinking-only body whose final text (and, when
+      // cut mid-think, its signature) never arrived. The block's very presence
+      // is proof the turn produced output upstream, so it is a valid
+      // in-progress completion, NOT a genuinely empty terminal response.
+      // (Previously only a non-empty `thinking` text OR `signature` counted —
+      // #5108 — which misclassified these content-less bodies as empty_choices
+      // → 502.)
+      if (b.type === "thinking") return true;
       // Redacted thinking and tool_use are valid structural output.
       if (b.type === "redacted_thinking") return true;
       if (b.type === "tool_use" && typeof b.id === "string" && (b.id as string).length > 0) {
@@ -246,7 +249,27 @@ export function detectMalformedNonStream(resp: unknown): MalformedReason | null
       }
       return false;
     });
-    return hasOutput ? null : "empty_choices";
+    if (hasOutput) return null;
+
+    // No per-block output. Two distinct situations remain:
+    //  1) A block IS present but invalid (e.g. text:"", a lone "(empty response)"
+    //     sentinel, or only null entries) — the model genuinely produced no
+    //     usable output. That is a MALFORMED-200 empty_choices regardless of
+    //     stop_reason (parity with the OpenAI content:"" path).
+    //  2) `content: []` — no block at all. Only a genuinely *terminal* response
+    //     (a final stop_reason with no output) is empty_choices. #9971: a
+    //     truncated / non-terminal body — the Claude Code OAuth upstream cutting
+    //     a long generation mid-turn, or a content-less thinking-only stream
+    //     that never emitted a terminal event — carries content:[] with no
+    //     reachable end, so flagging it would turn an upstream truncation into a
+    //     false 502. Require a terminal stop_reason before calling a block-less
+    //     response genuinely empty.
+    if (content.length === 0) {
+      const stopReason = typeof body.stop_reason === "string" ? body.stop_reason : "";
+      const isTerminal = stopReason.length > 0;
+      return isTerminal ? "empty_choices" : null;
+    }
+    return "empty_choices";
   }
 
   // ── Chat Completions shape ──
diff --git a/tests/unit/diagnostics-claude-thinking-5108.test.ts b/tests/unit/diagnostics-claude-thinking-5108.test.ts
index 0cfae4fced..4ef9caf3f6 100644
--- a/tests/unit/diagnostics-claude-thinking-5108.test.ts
+++ b/tests/unit/diagnostics-claude-thinking-5108.test.ts
@@ -12,8 +12,11 @@
  * the thinking step actually ran, so this is a valid completion, not an empty one.
  *
  * The detector must understand the Claude shape: text blocks with text, thinking blocks
- * with a signature, and tool_use blocks count as output; a genuinely empty `content:[]`
- * (or thinking with neither text nor signature) is still flagged.
+ * (with or without a signature), redacted_thinking, and tool_use blocks count as output;
+ * a genuinely empty terminal `content:[]` (or a terminal `(empty response)` text sentinel)
+ * is still flagged. #9971 refined #5108's rule: an empty thinking block is also valid
+ * structural output (the upstream can truncate a thinking-only generation before any
+ * text/signature lands), so it is no longer flagged — only content-less *terminal* bodies are.
  */
 import test from "node:test";
 import assert from "node:assert/strict";
@@ -48,9 +51,12 @@ test("#5108 genuinely empty Claude content:[] is still flagged malformed", () =>
   assert.equal(detectMalformedNonStream(claudeMsg([])), "empty_choices");
 });
 
-test("#5108 Claude thinking block with neither text nor signature is still flagged", () => {
+test("#5108/#9971 Claude thinking block with neither text nor signature is valid output (was empty_choices)", () => {
   const body = claudeMsg([{ type: "thinking", thinking: "", signature: "" }]);
-  assert.equal(detectMalformedNonStream(body), "empty_choices");
+  // #9971: an empty thinking block is valid structural output — the upstream can
+  // truncate a thinking-only generation before any text/signature lands, and a
+  // content-less thinking-only body must not turn into an empty_choices 502.
+  assert.equal(detectMalformedNonStream(body), null);
 });
 
 // Existing OpenAI / Responses behavior must be unchanged.
diff --git a/tests/unit/issue-9971-empty-choices-contentless-claude.test.ts b/tests/unit/issue-9971-empty-choices-contentless-claude.test.ts
new file mode 100644
index 0000000000..47f7bcd05e
--- /dev/null
+++ b/tests/unit/issue-9971-empty-choices-contentless-claude.test.ts
@@ -0,0 +1,84 @@
+/**
+ * #9971 — Non-stream MALFORMED-200/empty_choices false positive on `cc/` routes.
+ *
+ * A content-less-but-valid Claude body — thinking-only (with no visible text AND
+ * no signature), redacted_thinking-only, or a truncated extended-thinking-only
+ * stream cut before any text/signature landed — must be treated as VALID output,
+ * not flagged as `empty_choices` (which became a false 502 / BAD_GATEWAY).
+ *
+ * Root cause (plan-file): the Claude Code OAuth subscription upstream can truncate
+ * long large-input+large-output generations around the ~3-min turn boundary; the
+ * non-stream path's `detectMalformedNonStream` then misclassified the resulting
+ * content-less/thinking-only Claude body as `empty_choices`. The guard may only
+ * fire for a genuinely malformed upstream response — a non-200 or a truly empty
+ * *terminal* completion (terminal stop_reason with no usable output).
+ *
+ * Live note: the exact large-recvBytes+empty signature (33–65KB) needs a live VPS
+ * capture to confirm the upstream truncation; this test encodes the
+ * offline-reproducible mechanism (content-less thinking/redacted bodies), which
+ * failed to `empty_choices` on the unfixed code and must pass after the fix.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import { detectMalformedNonStream } from "../../open-sse/utils/diagnostics.ts";
+
+const claudeMsg = (content: unknown[], stopReason = "end_turn") => ({
+  type: "message",
+  role: "assistant",
+  id: "msg_x",
+  model: "claude-sonnet-4-5",
+  content,
+  stop_reason: stopReason,
+  usage: { input_tokens: 30000, output_tokens: 120 },
+});
+
+// ── Content-less-but-valid Claude bodies must NOT be empty_choices ───────────
+
+test("#9971 content-less thinking-only body (no text, no signature) is valid output", () => {
+  // Truncated extended-thinking stream: a thinking block arrived but the model
+  // never emitted its final text (and was cut before producing a signature).
+  const body = claudeMsg([{ type: "thinking", thinking: "", signature: "" }], "");
+  assert.equal(detectMalformedNonStream(body), null);
+});
+
+test("#9971 redacted_thinking-only body is valid output", () => {
+  // OAuth-style redacted footprint: the control plane suppresses the raw thinking
+  // text, leaving only a redacted_thinking marker — still a valid completion.
+  const body = claudeMsg([{ type: "redacted_thinking", data: "" }]);
+  assert.equal(detectMalformedNonStream(body), null);
+});
+
+test("#9971 thinking-only body with visible thinking text is valid output", () => {
+  const body = claudeMsg([
+    { type: "thinking", thinking: "working through the request", signature: "" },
+  ]);
+  assert.equal(detectMalformedNonStream(body), null);
+});
+
+test("#9971 functional/structural tool_use body is valid output", () => {
+  const body = claudeMsg([
+    { type: "tool_use", id: "toolu_1", name: "bash", input: { command: "ls" } },
+  ]);
+  assert.equal(detectMalformedNonStream(body), null);
+});
+
+// ── Genuinely malformed / truly-empty terminal bodies must STILL be flagged ──
+
+test("#9971 genuinely empty terminal content:[] is still flagged", () => {
+  // Terminal stop_reason + no blocks at all = a truly empty completion.
+  assert.equal(detectMalformedNonStream(claudeMsg([], "end_turn")), "empty_choices");
+});
+
+test("#9971 terminal '(empty response)' text sentinel is still flagged", () => {
+  // The OpenAI->Claude converter's sentinel for an upstream that produced no
+  // content: a terminal body carrying only that sentinel is genuinely empty.
+  const body = claudeMsg([{ type: "text", text: "(empty response)" }], "end_turn");
+  assert.equal(detectMalformedNonStream(body), "empty_choices");
+});
+
+test("#9971 truncated non-terminal empty body is valid (no false 502)", () => {
+  // Upstream cut mid-turn before a stop_reason landed: not a terminal completion,
+  // so the guard must not fire even though there is no output block yet.
+  const body = claudeMsg([], "");
+  assert.equal(detectMalformedNonStream(body), null);
+});