diff --git a/changelog.d/fixes/16646-reasoning-alias-collision.md b/changelog.d/fixes/16646-reasoning-alias-collision.md
new file mode 100644
index 000000000000..28b54b7f0c4a
--- /dev/null
+++ b/changelog.d/fixes/16646-reasoning-alias-collision.md
@@ -0,0 +1 @@
+- sse: drop the `reasoning` alias once `reasoning_content` carries the replay, fixing GLM/Kimi 400 "duplicate field reasoning_content" (#16646)
diff --git a/open-sse/translator/index.ts b/open-sse/translator/index.ts
index 2055d4123f65..cbfca95b9508 100644
--- a/open-sse/translator/index.ts
+++ b/open-sse/translator/index.ts
@@ -246,6 +246,13 @@ function replayOpenAIReasoningMessage(
   ) {
     message.reasoning_content = message.reasoning;
   }
+  // Serde-based upstreams (z-ai GLM, Moonshot Kimi) alias `reasoning` to
+  // `reasoning_content`, so a turn carrying both is rejected with 400
+  // "duplicate field `reasoning_content`" (9router#4841). Once
+  // `reasoning_content` holds the text it is the canonical carrier.
+  if (typeof message.reasoning === "string" && hasNonEmptyReasoningContent(message)) {
+    delete message.reasoning;
+  }
 
   const toolCalls = Array.isArray(message.tool_calls) ? message.tool_calls : [];
   const hasToolCalls = toolCalls.length > 0;
diff --git a/tests/unit/translator-reasoning-alias-collision-4841.test.ts b/tests/unit/translator-reasoning-alias-collision-4841.test.ts
new file mode 100644
index 000000000000..0b1d8793e355
--- /dev/null
+++ b/tests/unit/translator-reasoning-alias-collision-4841.test.ts
@@ -0,0 +1,64 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { translateRequest } = await import("../../open-sse/translator/index.ts");
+const { FORMATS } = await import("../../open-sse/translator/formats.ts");
+
+// Regression for upstream issue 9router#4841: serde-based OpenAI-compatible
+// upstreams (z-ai GLM, Moonshot Kimi) declare `reasoning` as an alias of
+// `reasoning_content`. A replayed assistant turn carrying BOTH keys is rejected
+// with 400 "duplicate field `reasoning_content`". On reasoning-replay targets
+// `reasoning_content` is the canonical carrier, so `reasoning` must be dropped.
+const historyWith = (assistant: Record<string, unknown>) => ({
+  messages: [
+    { role: "user", content: "hi" },
+    {
+      role: "assistant",
+      content: "ok",
+      tool_calls: [{ id: "c1", type: "function", function: { name: "f", arguments: "{}" } }],
+      ...assistant,
+    },
+    { role: "tool", tool_call_id: "c1", content: "r" },
+    { role: "user", content: "say ok" },
+  ],
+});
+
+const TARGETS: Array<[string, string]> = [
+  ["glm", "glm-4.6-thinking"],
+  ["moonshot", "kimi-k2.5"],
+  ["moonshot", "kimi-k3"],
+];
+
+for (const [provider, model] of TARGETS) {
+  test(`${provider}/${model}: assistant turn with both reasoning and reasoning_content forwards only reasoning_content (9router#4841)`, () => {
+    const result = translateRequest(
+      FORMATS.OPENAI,
+      FORMATS.OPENAI,
+      model,
+      { model, ...historyWith({ reasoning: "dup test", reasoning_content: "dup test" }) },
+      true,
+      null,
+      provider
+    );
+    const assistant = result.messages.find((m) => m.role === "assistant");
+    assert.ok(assistant, "assistant message present");
+    assert.equal(assistant.reasoning_content, "dup test");
+    assert.equal("reasoning" in assistant, false, "alias `reasoning` must not be forwarded");
+  });
+
+  test(`${provider}/${model}: reasoning-only assistant turn is still promoted to reasoning_content (9router#4841)`, () => {
+    const result = translateRequest(
+      FORMATS.OPENAI,
+      FORMATS.OPENAI,
+      model,
+      { model, ...historyWith({ reasoning: "only reasoning" }) },
+      true,
+      null,
+      provider
+    );
+    const assistant = result.messages.find((m) => m.role === "assistant");
+    assert.ok(assistant, "assistant message present");
+    assert.equal(assistant.reasoning_content, "only reasoning");
+    assert.equal("reasoning" in assistant, false, "alias `reasoning` must not be forwarded");
+  });
+}