diff --git a/open-sse/translator/helpers/openaiHelper.ts b/open-sse/translator/helpers/openaiHelper.ts
index 375364c725..67186eaa69 100644
--- a/open-sse/translator/helpers/openaiHelper.ts
+++ b/open-sse/translator/helpers/openaiHelper.ts
@@ -178,7 +178,9 @@ export function filterToOpenAIFormat(body) {
             type: "function",
             function: {
               name: tool.name,
-              description: tool.description || "",
+              // Coerce: strict upstream validators (NVIDIA NIM, Codex) reject
+              // non-string descriptions. Ports decolua/9router#397.
+              description: String(tool.description ?? ""),
               parameters: tool.input_schema || { type: "object", properties: {} },
             },
           };
@@ -190,7 +192,7 @@ export function filterToOpenAIFormat(body) {
             type: "function",
             function: {
               name: fn.name,
-              description: fn.description || "",
+              description: String(fn.description ?? ""),
               parameters: fn.parameters || { type: "object", properties: {} },
             },
           }));
diff --git a/tests/unit/translator-helper-tool-description-coercion.test.ts b/tests/unit/translator-helper-tool-description-coercion.test.ts
new file mode 100644
index 0000000000..12e47f12ad
--- /dev/null
+++ b/tests/unit/translator-helper-tool-description-coercion.test.ts
@@ -0,0 +1,58 @@
+// Regression: tool descriptions must always be strings for strict upstream
+// validation (e.g., NVIDIA NIM, Codex). Ports upstream PR decolua/9router#397
+// (Ibrahim Ryan), restricted to the gap that still exists in OmniRoute —
+// the two non-string-tolerant paths in openaiHelper.filterToOpenAIFormat
+// (Claude-style and Gemini-style tool normalization). The other locations
+// upstream-patched (claude-to-openai.ts, openai-responses.ts) already coerce
+// via String/toString in OmniRoute.
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const openaiHelper = await import(
+  "../../open-sse/translator/helpers/openaiHelper.ts"
+);
+
+test("filterToOpenAIFormat coerces non-string Claude-tool description to string", () => {
+  const result = openaiHelper.filterToOpenAIFormat({
+    messages: [{ role: "user", content: "hi" }],
+    tools: [
+      {
+        name: "claude-tool",
+        // Non-string truthy value — must NOT leak through to upstream.
+        description: { invalid: "object" } as unknown as string,
+        input_schema: { type: "object" },
+      },
+    ],
+  });
+  assert.equal(typeof result.tools[0].function.description, "string");
+});
+
+test("filterToOpenAIFormat coerces null/undefined Claude-tool description to empty string", () => {
+  const result = openaiHelper.filterToOpenAIFormat({
+    messages: [{ role: "user", content: "hi" }],
+    tools: [
+      { name: "t1", description: null as unknown as string, input_schema: {} },
+      { name: "t2", description: undefined, input_schema: {} },
+    ],
+  });
+  assert.equal(result.tools[0].function.description, "");
+  assert.equal(result.tools[1].function.description, "");
+});
+
+test("filterToOpenAIFormat coerces non-string Gemini functionDeclarations description", () => {
+  const result = openaiHelper.filterToOpenAIFormat({
+    messages: [{ role: "user", content: "hi" }],
+    tools: [
+      {
+        functionDeclarations: [
+          {
+            name: "gemini-tool",
+            description: 12345 as unknown as string,
+            parameters: { type: "object" },
+          },
+        ],
+      },
+    ],
+  });
+  assert.equal(typeof result.tools[0].function.description, "string");
+});