diff --git a/src/app/api/v1/messages/route.ts b/src/app/api/v1/messages/route.ts
index 9a67080aa8..3e2172b1eb 100644
--- a/src/app/api/v1/messages/route.ts
+++ b/src/app/api/v1/messages/route.ts
@@ -1,6 +1,7 @@
 import { handleChat } from "@/sse/handlers/chat";
 import { initTranslators } from "@omniroute/open-sse/translator/index.ts";
 import { withInjectionGuard } from "@/middleware/promptInjectionGuard";
+import { requireJsonContentType } from "@/shared/middleware/requireJsonContentType";
 import {
   withEarlyStreamKeepalive,
   ANTHROPIC_PING_FRAME,
@@ -39,6 +40,11 @@ export async function OPTIONS() {
  * parsed at most once per request.
  */
 async function postHandler(request: any, context: any, preParsedBody: any = null) {
+  // Reject non-JSON Content-Type with 415 before touching the body — mirrors OpenAI's
+  // reference API and matches /v1/chat/completions (#6414).
+  const ctRejection = requireJsonContentType(request);
+  if (ctRejection) return ctRejection;
+
   await ensureInitialized();
   // Streaming Anthropic clients (Claude Code, the Anthropic SDK) drop the connection
   // when no bytes arrive while a large prompt is processed before the first token — a
diff --git a/src/shared/middleware/requireJsonContentType.ts b/src/shared/middleware/requireJsonContentType.ts
new file mode 100644
index 0000000000..cf3f937f3e
--- /dev/null
+++ b/src/shared/middleware/requireJsonContentType.ts
@@ -0,0 +1,47 @@
+/**
+ * Content-Type guard for chat/message routes (#6414).
+ *
+ * OpenAI's reference API returns HTTP 415 `unsupported_media_type` when a POST to
+ * /v1/chat/completions arrives with a non-JSON Content-Type (or none). OmniRoute
+ * previously admitted such requests, silently parsed the body as JSON (via
+ * `request.clone().json().catch(() => null)`), and let them fall through to the
+ * provider-lookup layer — where they emerged as a misleading `model_not_found` /
+ * generic error rather than the RFC 7231 §6.5.13-mandated 415. This guard closes
+ * that gap at the route boundary, mirroring the pre-parse pattern already used by
+ * `chatBodyAdmission.ts`.
+ *
+ * Returns a ready 415 Response when the request must be rejected, or `null` to
+ * proceed. Only inspects the header — no body read, no I/O.
+ *
+ * @module shared/middleware/requireJsonContentType
+ */
+
+import { CORS_HEADERS } from "../utils/cors";
+
+/**
+ * Route-level guard. Rejects POST/PUT/PATCH requests whose `Content-Type` is not
+ * `application/json` (a `; charset=…` suffix is permitted). A missing header is
+ * treated as unsupported, matching the OpenAI reference behavior cited in #6414.
+ */
+export function requireJsonContentType(request: Request): Response | null {
+  const method = request.method.toUpperCase();
+  if (method !== "POST" && method !== "PUT" && method !== "PATCH") return null;
+
+  const raw = request.headers.get("content-type");
+  const ct = (raw ?? "").trim().toLowerCase();
+  if (ct.startsWith("application/json")) return null;
+
+  return new Response(
+    JSON.stringify({
+      error: {
+        message: "Content-Type must be application/json",
+        type: "invalid_request_error",
+        code: "unsupported_media_type",
+      },
+    }),
+    {
+      status: 415,
+      headers: { ...CORS_HEADERS, "Content-Type": "application/json" },
+    }
+  );
+}
diff --git a/tests/unit/chat-content-type-6414.test.ts b/tests/unit/chat-content-type-6414.test.ts
new file mode 100644
index 0000000000..aa49986024
--- /dev/null
+++ b/tests/unit/chat-content-type-6414.test.ts
@@ -0,0 +1,89 @@
+// #6414: POST /v1/chat/completions (and /v1/messages) must return HTTP 415
+// `unsupported_media_type` when the Content-Type header is not application/json,
+// matching OpenAI's reference API and RFC 7231 §6.5.13. Previously OmniRoute
+// silently parsed such requests as JSON via `.clone().json().catch(() => null)`
+// and let them reach the provider-lookup layer, where they surfaced as misleading
+// `model_not_found` / generic errors instead of a boundary 415.
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { requireJsonContentType } = await import(
+  "../../src/shared/middleware/requireJsonContentType.ts"
+);
+
+function makeRequest(method: string, contentType: string | null): Request {
+  const headers = new Headers();
+  if (contentType !== null) headers.set("content-type", contentType);
+  return new Request("http://localhost/v1/chat/completions", {
+    method,
+    headers,
+    body: method === "GET" || method === "HEAD" ? undefined : "{}",
+  });
+}
+
+test("returns 415 for text/plain on POST", async () => {
+  const rejection = requireJsonContentType(makeRequest("POST", "text/plain"));
+  assert.ok(rejection, "expected a rejection Response");
+  assert.equal(rejection.status, 415);
+  const body = await rejection.json();
+  assert.equal(body.error.code, "unsupported_media_type");
+  assert.equal(body.error.type, "invalid_request_error");
+});
+
+test("returns 415 when Content-Type header is missing on POST", async () => {
+  const rejection = requireJsonContentType(makeRequest("POST", null));
+  assert.ok(rejection, "missing content-type must be rejected");
+  assert.equal(rejection.status, 415);
+});
+
+test("returns 415 for application/x-www-form-urlencoded on POST", async () => {
+  const rejection = requireJsonContentType(
+    makeRequest("POST", "application/x-www-form-urlencoded")
+  );
+  assert.ok(rejection);
+  assert.equal(rejection.status, 415);
+});
+
+test("admits application/json", () => {
+  assert.equal(requireJsonContentType(makeRequest("POST", "application/json")), null);
+});
+
+test("admits application/json with charset suffix", () => {
+  assert.equal(
+    requireJsonContentType(makeRequest("POST", "application/json; charset=utf-8")),
+    null
+  );
+});
+
+test("admits Application/JSON (case-insensitive)", () => {
+  assert.equal(requireJsonContentType(makeRequest("POST", "Application/JSON")), null);
+});
+
+test("admits application/json with leading whitespace", () => {
+  assert.equal(requireJsonContentType(makeRequest("POST", "  application/json")), null);
+});
+
+test("does not touch GET requests (no body → nothing to reject)", () => {
+  assert.equal(requireJsonContentType(makeRequest("GET", "text/plain")), null);
+});
+
+test("does not touch OPTIONS preflight", () => {
+  assert.equal(requireJsonContentType(makeRequest("OPTIONS", null)), null);
+});
+
+test("guards PUT and PATCH the same as POST", () => {
+  const putRejection = requireJsonContentType(makeRequest("PUT", "text/plain"));
+  assert.ok(putRejection);
+  assert.equal(putRejection.status, 415);
+
+  const patchRejection = requireJsonContentType(makeRequest("PATCH", "text/plain"));
+  assert.ok(patchRejection);
+  assert.equal(patchRejection.status, 415);
+});
+
+test("rejection carries CORS + JSON headers", () => {
+  const rejection = requireJsonContentType(makeRequest("POST", "text/plain"));
+  assert.ok(rejection);
+  assert.equal(rejection.headers.get("Content-Type"), "application/json");
+  assert.ok(rejection.headers.get("Access-Control-Allow-Methods"));
+});