diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 7434983c79..c67b2b4dd4 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -2403,7 +2403,12 @@ export async function handleChatCore({
         extractSystemRoleMessages(translatedBody);
       } else {
         // Non-CC path: full normalization including content type conversion.
-        normalizeClaudeUpstreamMessages(translatedBody, { preserveToolResultBlocks: true });
+        // Preserve tool_result blocks only when the upstream target speaks the
+        // Anthropic Messages format — OpenAI-compatible gateways reject them
+        // and return 503. See issue #13971.
+        normalizeClaudeUpstreamMessages(translatedBody, {
+          preserveToolResultBlocks: targetFormat === FORMATS.CLAUDE,
+        });
       }
     } else if (isClaudePassthrough) {
       // Pure passthrough: forward the body as-is without OpenAI round-trip.
@@ -2454,7 +2459,16 @@ export async function handleChatCore({
           ensureCacheControlOnLastUserMessage(translatedBody);
         }
       } else {
-        normalizeClaudeUpstreamMessages(translatedBody, { preserveToolResultBlocks: true });
+        // Same guard as the CC-bridge path: only preserve tool_result blocks
+        // for Anthropic-native targets. See issue #13971. This branch only runs
+        // under isClaudePassthrough (sourceFormat === targetFormat === CLAUDE,
+        // defined above), so targetFormat === FORMATS.CLAUDE always holds here —
+        // the guard is a no-op on this call site, kept for symmetry with the
+        // CC-bridge one above rather than a change to code the issue said not
+        // to touch.
+        normalizeClaudeUpstreamMessages(translatedBody, {
+          preserveToolResultBlocks: targetFormat === FORMATS.CLAUDE,
+        });
       }
 
       log?.debug?.("FORMAT", `claude passthrough (preserveCache=${preserveCacheControl})`);
diff --git a/tests/unit/chatcore-translation-paths.test.ts b/tests/unit/chatcore-translation-paths.test.ts
index 12bab97f0f..fd6221adf2 100644
--- a/tests/unit/chatcore-translation-paths.test.ts
+++ b/tests/unit/chatcore-translation-paths.test.ts
@@ -379,6 +379,7 @@ async function invokeChatCore({
   reasoningTransportFallback = "drop",
   managedLease = null,
   cachedSettings = null,
+  modelTargetFormat = undefined,
 }: any = {}) {
   const calls: any[] = [];
 
@@ -408,7 +409,10 @@ async function invokeChatCore({
     const requestBody = structuredClone(body);
     const result = await handleChatCore({
       body: requestBody,
-      modelInfo: { provider, model, extendedContext: false },
+      modelInfo:
+        modelTargetFormat !== undefined
+          ? { provider, model, extendedContext: false, targetFormat: modelTargetFormat }
+          : { provider, model, extendedContext: false },
       credentials: credentials || {
         apiKey: "sk-test",
         // #13452/#13798: buildUrl() refuses an `*-compatible-*` node with no baseUrl
@@ -1565,6 +1569,65 @@ test("chatCore normalizes native Claude Code messages before CC-compatible relay
   // user msg[2] (was clientMessages[3]): tool_result preserved (preserveToolResultBlocks:true)
   assert.equal(call.body.messages[2].content[0].type, "tool_result");
 });
+
+// Issue #13971: the CC-bridge unconditionally preserved raw tool_result blocks even when the
+// target speaks OpenAI-compatible (503 on those gateways). Fix: gate preserveToolResultBlocks
+// on targetFormat === FORMATS.CLAUDE. userAgent is plain (non-Claude-Code) so both requests hit
+// the CC-bridge's normalizeClaudeUpstreamMessages branch (chatCore.ts:2377-2385), not the
+// Claude-Code semantic-passthrough branch above it, which this fix does not touch.
+function ccBridgeToolResultCall(modelTargetFormat?: string) {
+  return invokeChatCore({
+    provider: "anthropic-compatible-cc-test",
+    model: "claude-sonnet-4-6",
+    endpoint: "/v1/messages",
+    credentials: {
+      apiKey: "sk-test",
+      providerSpecificData: { baseUrl: "https://proxy.example.com/v1/messages" },
+    },
+    body: {
+      model: "claude-sonnet-4-6",
+      max_tokens: 64,
+      messages: [
+        {
+          role: "assistant",
+          content: [{ type: "tool_use", id: "toolu_x", name: "Read", input: {} }],
+        },
+        {
+          role: "user",
+          content: [{ type: "tool_result", tool_use_id: "toolu_x", content: "file contents" }],
+        },
+      ],
+      tools: [{ name: "Read", input_schema: { type: "object", properties: {} } }],
+    },
+    userAgent: "unit-test",
+    responseFormat: "claude",
+    modelTargetFormat,
+  });
+}
+test("chatCore strips tool_result blocks on the CC-bridge path when the target is OpenAI-compatible", async () => {
+  const { call, result } = await ccBridgeToolResultCall("openai");
+  assert.equal(result.success, true);
+  // No block may be raw tool_result/tool_use — that shape 503'd on #13971; the
+  // orphan-tool-use cleanup also drops the now-unmatched assistant turn, a stronger guard.
+  for (const message of call.body.messages) {
+    for (const block of message.content) {
+      assert.notEqual(block.type, "tool_result");
+      assert.notEqual(block.type, "tool_use");
+    }
+  }
+  const flattened = call.body.messages
+    .flatMap((m: { content: Array<{ text?: string }> }) => m.content)
+    .map((b: { text?: string }) => b.text)
+    .join("\n");
+  assert.match(flattened, /file contents/);
+});
+// Same branch, real (Claude-native) target format — tool_result stays preserved raw.
+test("chatCore still preserves tool_result blocks on the CC-bridge path when the target is Claude-native", async () => {
+  const { call, result } = await ccBridgeToolResultCall();
+  assert.equal(result.success, true);
+  assert.equal(call.body.messages[0].content[0].type, "tool_use");
+  assert.equal(call.body.messages[1].content[0].type, "tool_result");
+});
 test("chatCore preserves cache_control automatically for Claude Code single-model requests", async () => {
   await settingsDb.updateSettings({ alwaysPreserveClientCache: "auto" });
   invalidateCacheControlSettingsCache();