diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8e3a61628e..305cd7d24d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,6 +22,8 @@
 
 ### 🔧 Bug Fixes
 
+- **translator (Responses → Chat Completions):** strip the Responses-API-only `truncation` field before forwarding a `/v1/responses` request to a non-OpenAI Chat Completions upstream. Strict upstreams (e.g. NVIDIA NIM) rejected it with HTTP 400 `Unsupported parameter(s): truncation`, breaking Codex-style clients routed to those providers. `client_metadata`, `background`, and `safety_identifier` were already stripped — `truncation` was the remaining gap. Regression guard: `tests/unit/responses-strip-truncation-2311.test.ts`. (thanks @TuanNguyen0708)
+
 - **combo (prefer known context capacity over unknown):** when a combo filters out at least one target for exceeding a *known* context limit, the router now prefers the remaining known-compatible targets over targets whose context metadata is simply unknown, instead of letting unknown-metadata targets be the only survivors. If no known-compatible context target remains, context-only candidates fall back to the normal strategy order. Regression guard: `tests/unit/combo-context-window-filter.test.ts`. ([#6088](https://github.com/diegosouzapw/OmniRoute/pull/6088) — thanks @Thinkscape)
 
 - **models (GLM-5.2 context normalization):** stop treating every hosted GLM-5.2 provider alias as the native 1M-context model. Native/bare GLM-5.2 and verified OpenCode / ZenMux routes keep their 1,000,000-token context, while hosted-provider aliases now respect the caps declared in their provider metadata instead of inheriting the native max. Regression guards: `tests/unit/model-capabilities-registry.test.ts`, `tests/unit/models-catalog-route.test.ts`. ([#6091](https://github.com/diegosouzapw/OmniRoute/pull/6091) — thanks @Thinkscape)
diff --git a/open-sse/translator/request/openai-responses.ts b/open-sse/translator/request/openai-responses.ts
index 23f32f53d4..83b1aedb20 100644
--- a/open-sse/translator/request/openai-responses.ts
+++ b/open-sse/translator/request/openai-responses.ts
@@ -520,6 +520,10 @@ export function openaiResponsesToOpenAIRequest(
   // client_metadata is sent by Codex CLI and has no Chat Completions equivalent.
   // Strict upstreams (e.g. Mistral) reject it with HTTP 422 extra_forbidden.
   delete result.client_metadata;
+  // truncation ("auto"/"disabled") is a Responses-API-only field with no Chat
+  // Completions equivalent. Strict non-OpenAI upstreams (e.g. NVIDIA NIM) reject
+  // it with HTTP 400 "Unsupported parameter(s): truncation" (#2311).
+  delete result.truncation;
 
   return result;
 }
diff --git a/tests/unit/responses-strip-truncation-2311.test.ts b/tests/unit/responses-strip-truncation-2311.test.ts
new file mode 100644
index 0000000000..a8bedb873d
--- /dev/null
+++ b/tests/unit/responses-strip-truncation-2311.test.ts
@@ -0,0 +1,44 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+// Regression for port of decolua/9router#2311 (PR #2318): OpenAI Responses-API-only
+// fields must be stripped before forwarding to a Chat Completions upstream, otherwise
+// strict non-OpenAI upstreams (e.g. NVIDIA NIM) reject with HTTP 400
+// "Unsupported parameter(s): ...". OmniRoute already strips `client_metadata`,
+// `background`, and `safety_identifier`; `truncation` was the remaining gap.
+const { openaiResponsesToOpenAIRequest } = await import(
+  "../../open-sse/translator/request/openai-responses.ts"
+);
+
+test("Responses -> OpenAI: truncation is stripped (never forwarded to Chat Completions)", () => {
+  const result = openaiResponsesToOpenAIRequest(
+    "z-ai/glm-5.2",
+    {
+      input: [{ type: "message", role: "user", content: [{ type: "input_text", text: "hi" }] }],
+      truncation: "auto",
+    },
+    false,
+    {}
+  ) as Record<string, unknown>;
+
+  assert.equal("truncation" in result, false);
+});
+
+test("Responses -> OpenAI: full Responses-only field set is stripped together", () => {
+  const result = openaiResponsesToOpenAIRequest(
+    "z-ai/glm-5.2",
+    {
+      input: [{ type: "message", role: "user", content: [{ type: "input_text", text: "hi" }] }],
+      truncation: "disabled",
+      client_metadata: { user_id: "abc" },
+      background: true,
+      safety_identifier: "lobehub-user",
+    },
+    false,
+    {}
+  ) as Record<string, unknown>;
+
+  for (const field of ["truncation", "client_metadata", "background", "safety_identifier"]) {
+    assert.equal(field in result, false, `${field} should be stripped`);
+  }
+});