diff --git a/changelog.d/fixes/11861-nous-tags-user-injection.md b/changelog.d/fixes/11861-nous-tags-user-injection.md
new file mode 100644
index 0000000000..bfe9bc8464
--- /dev/null
+++ b/changelog.d/fixes/11861-nous-tags-user-injection.md
@@ -0,0 +1 @@
+- **fix(provider/nous):** inject required user= tag into Nous Research inference requests to resolve upstream 400 "missing tags" error ([#11861](https://github.com/diegosouzapw/OmniRoute/issues/11861)) — thanks @Karan825
diff --git a/open-sse/executors/default.ts b/open-sse/executors/default.ts
index 0d2c9182bd..c8395e9d4a 100644
--- a/open-sse/executors/default.ts
+++ b/open-sse/executors/default.ts
@@ -639,8 +639,7 @@ export class DefaultExecutor extends BaseExecutor {
 
     const record = body as Record<string, unknown>;
     const rf = record.response_format as
-      | { type?: string; json_schema?: { schema?: unknown } }
-      | undefined;
+      { type?: string; json_schema?: { schema?: unknown } } | undefined;
     if (!rf) return body;
 
     // openai-compatible-* providers accept json_object natively — only the
@@ -748,6 +747,38 @@ export class DefaultExecutor extends BaseExecutor {
       delete withoutClientMetadata.client_metadata;
       withDefaults = withoutClientMetadata;
     }
+    // Nous Research inference gateway (portal.nousresearch.com) requires a top-level
+    // `tags` array containing at least a `user=` item on raw API-key requests (#11861).
+    // Without `tags`, upstream returns 400 "missing tags".
+    // Without `user=...`, upstream returns 400 "missing user tag".
+    if (
+      this.provider === "nous-research" &&
+      withDefaults &&
+      typeof withDefaults === "object" &&
+      !Array.isArray(withDefaults)
+    ) {
+      const record = withDefaults as Record<string, unknown>;
+      const extraBody = record.extra_body as Record<string, unknown> | undefined;
+
+      const rawTags = Array.isArray(record.tags)
+        ? (record.tags as unknown[])
+        : Array.isArray(extraBody?.tags)
+          ? (extraBody.tags as unknown[])
+          : [];
+
+      const stringTags = rawTags.filter(
+        (t): t is string => typeof t === "string" && t.trim().length > 0
+      );
+
+      const hasUserTag = stringTags.some((t) => t.startsWith("user="));
+      if (!hasUserTag) {
+        const username =
+          typeof record.user === "string" && record.user.trim() ? record.user.trim() : "omniroute";
+        record.tags = [...stringTags, `user=${username}`];
+      } else {
+        record.tags = stringTags;
+      }
+    }
 
     // 9router#1649: Mistral's API returns 422 (extra_forbidden) when an
     // assistant message carries a `reasoning_content` field (replayed thinking
diff --git a/src/lib/providers/validation/audioMiscProviders.ts b/src/lib/providers/validation/audioMiscProviders.ts
index c0976de088..eeb0f3c5eb 100644
--- a/src/lib/providers/validation/audioMiscProviders.ts
+++ b/src/lib/providers/validation/audioMiscProviders.ts
@@ -631,6 +631,7 @@ export async function validateNousResearchProvider({ apiKey, providerSpecificDat
         model: modelId,
         messages: [{ role: "user", content: "test" }],
         max_tokens: 1,
+        tags: ["user=omniroute"],
       }),
     });
 
diff --git a/tests/unit/executor-nous-research.test.ts b/tests/unit/executor-nous-research.test.ts
index a20e9735c0..8694a055f7 100644
--- a/tests/unit/executor-nous-research.test.ts
+++ b/tests/unit/executor-nous-research.test.ts
@@ -29,8 +29,62 @@ test("nous-research DefaultExecutor.buildUrl() targets the correct inference end
 
   const url = executor.buildUrl("Hermes-4-70B", false, 0, null);
 
-  assert.equal(
-    url,
-    "https://inference-api.nousresearch.com/v1/chat/completions"
-  );
+  assert.equal(url, "https://inference-api.nousresearch.com/v1/chat/completions");
+});
+
+test("nous-research DefaultExecutor.transformRequest injects user=omniroute tag when tags is absent (#11861)", () => {
+  const executor = new DefaultExecutor("nous-research");
+  const transformed = executor.transformRequest(
+    "Hermes-4-70B",
+    { model: "Hermes-4-70B", messages: [{ role: "user", content: "hello" }] },
+    false,
+    null
+  ) as Record<string, unknown>;
+
+  assert.ok(Array.isArray(transformed.tags), "Expected tags array on nous-research body");
+  assert.deepEqual(transformed.tags, ["user=omniroute"]);
+});
+
+test("nous-research DefaultExecutor.transformRequest respects client-sent user in tags (#11861)", () => {
+  const executor = new DefaultExecutor("nous-research");
+  const transformed = executor.transformRequest(
+    "Hermes-4-70B",
+    { model: "Hermes-4-70B", user: "dev-user", messages: [{ role: "user", content: "hello" }] },
+    false,
+    null
+  ) as Record<string, unknown>;
+
+  assert.deepEqual(transformed.tags, ["user=dev-user"]);
+});
+
+test("nous-research DefaultExecutor.transformRequest preserves existing tags and appends user tag (#11861)", () => {
+  const executor = new DefaultExecutor("nous-research");
+  const transformed = executor.transformRequest(
+    "Hermes-4-70B",
+    {
+      model: "Hermes-4-70B",
+      tags: ["client=agent"],
+      messages: [{ role: "user", content: "hello" }],
+    },
+    false,
+    null
+  ) as Record<string, unknown>;
+
+  assert.deepEqual(transformed.tags, ["client=agent", "user=omniroute"]);
+});
+
+test("nous-research DefaultExecutor.transformRequest does not duplicate user tag if already present (#11861)", () => {
+  const executor = new DefaultExecutor("nous-research");
+  const transformed = executor.transformRequest(
+    "Hermes-4-70B",
+    {
+      model: "Hermes-4-70B",
+      tags: ["user=custom-tag"],
+      messages: [{ role: "user", content: "hello" }],
+    },
+    false,
+    null
+  ) as Record<string, unknown>;
+
+  assert.deepEqual(transformed.tags, ["user=custom-tag"]);
 });