diff --git a/src/app/api/v1/models/[...model]/route.ts b/src/app/api/v1/models/[...model]/route.ts
new file mode 100644
index 0000000000..cb3d22e8b0
--- /dev/null
+++ b/src/app/api/v1/models/[...model]/route.ts
@@ -0,0 +1,26 @@
+import { handleCorsOptions } from "@/shared/utils/cors";
+import { getUnifiedModelsResponse } from "../catalog";
+import { handleGetModelById } from "../modelById";
+
+/**
+ * Handle CORS preflight
+ */
+export async function OPTIONS() {
+  return handleCorsOptions();
+}
+
+/**
+ * GET /v1/models/{model} — OpenAI-compatible single-model retrieval (#4674).
+ *
+ * Catch-all (`[...model]`) so provider-prefixed ids that contain a slash
+ * (e.g. `cgpt-web/gpt-5.5`, `claude/claude-sonnet-4-6`) are captured intact.
+ */
+export async function GET(
+  request: Request,
+  { params }: { params: Promise<{ model: string[] }> }
+) {
+  const { model } = await params;
+  const segments = Array.isArray(model) ? model : [model];
+  const requestedId = decodeURIComponent(segments.join("/"));
+  return handleGetModelById(request, requestedId, getUnifiedModelsResponse);
+}
diff --git a/src/app/api/v1/models/modelById.ts b/src/app/api/v1/models/modelById.ts
new file mode 100644
index 0000000000..5a8b2eadb8
--- /dev/null
+++ b/src/app/api/v1/models/modelById.ts
@@ -0,0 +1,62 @@
+import { CORS_HEADERS } from "@/shared/utils/cors";
+
+/**
+ * #4674 — Shared logic for `GET /v1/models/{model}`.
+ *
+ * Before this route existed, a request for a single model fell through to the
+ * Next.js catch-all and returned the HTML dashboard, which broke Claude Code's
+ * model-validation probe (it expects the OpenAI `{ id, object: "model", ... }`
+ * shape). Kept in a sibling module (not the route file) so the lookup stays
+ * unit-testable without the DB-backed catalog.
+ */
+
+type CatalogModel = { id?: unknown } & Record<string, unknown>;
+
+/** Find a model entry in the unified catalog `data` array by its exact id. */
+export function findModelById(
+  data: CatalogModel[] | null | undefined,
+  requestedId: string
+): CatalogModel | null {
+  if (!Array.isArray(data)) return null;
+  return data.find((m) => typeof m?.id === "string" && m.id === requestedId) ?? null;
+}
+
+/**
+ * Resolve a single model from the unified catalog response.
+ *
+ * @param getModels  Returns the `{ object: "list", data: [...] }` Response — in
+ *   the route this is `getUnifiedModelsResponse`; tests inject a fake.
+ */
+export async function handleGetModelById(
+  request: Request,
+  requestedId: string,
+  getModels: (request: Request, corsHeaders?: Record<string, string>) => Promise<Response>
+): Promise<Response> {
+  const listResp = await getModels(request, CORS_HEADERS);
+  // Propagate auth rejections / 5xx (already JSON) unchanged.
+  if (!listResp.ok) return listResp;
+
+  let data: CatalogModel[] | undefined;
+  try {
+    const body = (await listResp.json()) as { data?: CatalogModel[] };
+    data = body?.data;
+  } catch {
+    data = undefined;
+  }
+
+  const found = findModelById(data, requestedId);
+  if (found) {
+    return Response.json(found, { headers: CORS_HEADERS });
+  }
+
+  return Response.json(
+    {
+      error: {
+        message: `The model '${requestedId}' does not exist`,
+        type: "invalid_request_error",
+        code: "model_not_found",
+      },
+    },
+    { status: 404, headers: CORS_HEADERS }
+  );
+}
diff --git a/tests/unit/v1-models-by-id-4674.test.ts b/tests/unit/v1-models-by-id-4674.test.ts
new file mode 100644
index 0000000000..4af0bb7193
--- /dev/null
+++ b/tests/unit/v1-models-by-id-4674.test.ts
@@ -0,0 +1,77 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  findModelById,
+  handleGetModelById,
+} from "@/app/api/v1/models/modelById";
+
+// #4674 — GET /v1/models/{model} previously had no route handler, so the request
+// fell through to the Next.js catch-all and returned the HTML dashboard instead of
+// JSON, breaking Claude Code's model-validation probe. These tests pin the JSON
+// contract for both the found and not-found paths.
+
+const CATALOG = [
+  { id: "claude/claude-sonnet-4-6", object: "model", owned_by: "claude" },
+  { id: "cgpt-web/gpt-5.5", object: "model", owned_by: "chatgpt-web" },
+  { id: "gpt-5", object: "model", owned_by: "openai" },
+];
+
+function listResponse() {
+  return Response.json({ object: "list", data: CATALOG });
+}
+
+test("findModelById returns the exact-id match", () => {
+  const found = findModelById(CATALOG, "claude/claude-sonnet-4-6");
+  assert.ok(found);
+  assert.equal(found.id, "claude/claude-sonnet-4-6");
+  assert.equal(found.object, "model");
+});
+
+test("findModelById handles provider-prefixed ids containing a slash", () => {
+  const found = findModelById(CATALOG, "cgpt-web/gpt-5.5");
+  assert.ok(found);
+  assert.equal(found.id, "cgpt-web/gpt-5.5");
+});
+
+test("findModelById returns null for an unknown model", () => {
+  assert.equal(findModelById(CATALOG, "does-not-exist"), null);
+});
+
+test("findModelById tolerates a non-array catalog", () => {
+  assert.equal(findModelById(undefined, "gpt-5"), null);
+  assert.equal(findModelById(null, "gpt-5"), null);
+});
+
+test("handleGetModelById returns 200 JSON for an existing model", async () => {
+  const req = new Request("http://localhost:20128/v1/models/gpt-5");
+  const res = await handleGetModelById(req, "gpt-5", listResponse);
+  assert.equal(res.status, 200);
+  assert.match(res.headers.get("content-type") || "", /application\/json/);
+  const body = await res.json();
+  assert.equal(body.id, "gpt-5");
+  assert.equal(body.object, "model");
+});
+
+test("handleGetModelById returns 404 JSON (not HTML) for an unknown model", async () => {
+  const req = new Request("http://localhost:20128/v1/models/ghost");
+  const res = await handleGetModelById(req, "ghost", listResponse);
+  assert.equal(res.status, 404);
+  // The whole point of #4674: never serve the HTML dashboard here.
+  const ct = res.headers.get("content-type") || "";
+  assert.match(ct, /application\/json/);
+  assert.doesNotMatch(ct, /text\/html/);
+  const body = await res.json();
+  assert.equal(body.error.code, "model_not_found");
+  assert.match(body.error.message, /ghost/);
+});
+
+test("handleGetModelById propagates an upstream auth/error response unchanged", async () => {
+  const req = new Request("http://localhost:20128/v1/models/gpt-5");
+  const rejection = async () =>
+    Response.json({ error: { message: "unauthorized" } }, { status: 401 });
+  const res = await handleGetModelById(req, "gpt-5", rejection);
+  assert.equal(res.status, 401);
+  const body = await res.json();
+  assert.equal(body.error.message, "unauthorized");
+});