diff --git a/nextcloud_mcp_server/config.py b/nextcloud_mcp_server/config.py
index c42a6860..993e1497 100644
--- a/nextcloud_mcp_server/config.py
+++ b/nextcloud_mcp_server/config.py
@@ -174,9 +174,10 @@ _DEFAULTS: dict[str, Any] = {
     "fact_event_emitter": "none",  # none | nats | stdout
     "ingest_bus_url": None,  # required when ingest_mode=external
     "embedding_gateway_url": None,  # required when embedding_provider=gateway
-    # Logical model the gateway routes on (e.g. mistral-embed → Mistral for
-    # the MVP). Only consulted when embedding_provider=gateway.
-    "embedding_gateway_model": "mistral-embed",
+    # Provider-namespaced model the gateway serves, "<provider>/<model>"
+    # (the gateway routes on the "/"-prefix; mistral/mistral-embed → Mistral
+    # for the MVP). Only consulted when embedding_provider=gateway.
+    "embedding_gateway_model": "mistral/mistral-embed",
     # Gateway auth: the MCP server is an OIDC *client* in the gateway's own
     # M2M realm (parallel to, and distinct from, the tenant realm it already
     # serves). It obtains a client-credentials token and the gateway maps the
diff --git a/nextcloud_mcp_server/embedding/gateway_client.py b/nextcloud_mcp_server/embedding/gateway_client.py
index 2d729b7c..72aedc1a 100644
--- a/nextcloud_mcp_server/embedding/gateway_client.py
+++ b/nextcloud_mcp_server/embedding/gateway_client.py
@@ -147,6 +147,66 @@ class GatewayProvider(OpenAIProvider):
         if self._token_provider is not None:
             self.client.api_key = await self._token_provider.get_token()
 
+    async def _detect_dimension(self) -> None:
+        """Resolve the embedding dimension from the gateway's ``GET /v1/models``
+        before the first embed.
+
+        Qdrant collection init needs the vector size at startup (it calls
+        ``get_dimension()`` before any ``embed()``); the vector-sync bootstrap
+        invokes this hook first (``vector/qdrant_client.py`` —
+        ``hasattr(provider, "_detect_dimension")``). The gateway is the
+        authority on the dimensions of the models it serves, so we read it from
+        there rather than hardcoding (``mistral-embed`` isn't an OpenAI model,
+        so the OpenAI-wire base class can't know its size statically).
+
+        Best-effort: any failure (old gateway without /v1/models, model absent,
+        network) leaves ``_dimension`` unset so the inherited lazy
+        detect-on-first-embed path still applies. Never raises.
+        """
+        if self._dimension is not None:
+            return  # already known (e.g. an OpenAI model in the static map)
+
+        # ``models`` is a sibling of ``embeddings`` under the gateway's base —
+        # derive it from the same base_url the OpenAI client uses for embeds so
+        # the two stay consistent (str(base_url) has a trailing slash).
+        models_url = str(self.client.base_url).rstrip("/") + "/models"
+        headers: dict[str, str] = {}
+        if self._token_provider is not None:
+            headers["Authorization"] = (
+                f"Bearer {await self._token_provider.get_token()}"
+            )
+        try:
+            async with httpx.AsyncClient(
+                timeout=httpx.Timeout(10.0, connect=5.0)
+            ) as client:
+                resp = await client.get(models_url, headers=headers)
+                resp.raise_for_status()
+                catalog = resp.json().get("data", [])
+            for entry in catalog:
+                if entry.get("id") == self.embedding_model:
+                    dim = entry.get("dimension")
+                    if isinstance(dim, int):
+                        self._dimension = dim
+                        logger.info(
+                            "Resolved embedding dimension %d for model %s via "
+                            "gateway /v1/models",
+                            dim,
+                            self.embedding_model,
+                        )
+                        return
+            logger.warning(
+                "Gateway /v1/models reported no dimension for model %s; "
+                "falling back to lazy detection on first embed",
+                self.embedding_model,
+            )
+        except Exception as exc:  # noqa: BLE001 - best-effort, never fatal
+            logger.warning(
+                "Could not fetch model dimensions from gateway %s: %s; "
+                "falling back to lazy detection on first embed",
+                models_url,
+                exc,
+            )
+
     async def embed(self, text: str) -> list[float]:
         await self._ensure_bearer()
         return await super().embed(text)
diff --git a/tests/unit/providers/test_gateway_provider.py b/tests/unit/providers/test_gateway_provider.py
index 76b53563..923b33f3 100644
--- a/tests/unit/providers/test_gateway_provider.py
+++ b/tests/unit/providers/test_gateway_provider.py
@@ -30,12 +30,12 @@ def test_gateway_selected_unauthenticated(monkeypatch):
     settings = Settings(
         embedding_provider="gateway",
         embedding_gateway_url="https://gateway:8083",
-        embedding_gateway_model="mistral-embed",
+        embedding_gateway_model="mistral/mistral-embed",
     )
     _patch_settings(monkeypatch, settings)
     provider = ProviderRegistry.create_provider()
     assert isinstance(provider, GatewayProvider)
-    assert provider.embedding_model == "mistral-embed"
+    assert provider.embedding_model == "mistral/mistral-embed"
     assert provider.supports_embeddings is True
     assert provider.supports_generation is False
     assert provider._token_provider is None  # unauthenticated
@@ -160,3 +160,132 @@ async def test_token_provider_concurrent_callers_issue_single_request(monkeypatc
     # and both callers observe the same cached token.
     assert calls["n"] == 1
     assert results == ["tok1", "tok1"]
+
+
+# --- Dimension discovery via gateway GET /v1/models -------------------------
+
+
+def _mock_async_client(monkeypatch, handler):
+    """Route every httpx.AsyncClient through a MockTransport (mirrors the
+    token-provider tests above)."""
+    transport = httpx.MockTransport(handler)
+    orig = httpx.AsyncClient
+
+    def _client(*args, **kwargs):
+        kwargs["transport"] = transport
+        return orig(*args, **kwargs)
+
+    monkeypatch.setattr(httpx, "AsyncClient", _client)
+
+
+async def test_detect_dimension_from_models_endpoint(monkeypatch):
+    """_detect_dimension() resolves the dimension from /v1/models with no embed
+    call — the regression that crashed external-mode startup."""
+    seen = {}
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        seen["url"] = str(request.url)
+        seen["auth"] = request.headers.get("Authorization")
+        return httpx.Response(
+            200,
+            json={
+                "object": "list",
+                "data": [
+                    {
+                        "id": "mistral/mistral-embed",
+                        "object": "model",
+                        "dimension": 1024,
+                    },
+                    {
+                        "id": "text-embedding-3-large",
+                        "object": "model",
+                        "dimension": 3072,
+                    },
+                ],
+            },
+        )
+
+    _mock_async_client(monkeypatch, handler)
+    provider = GatewayProvider(
+        base_url="http://gw:8083/v1", embedding_model="mistral/mistral-embed"
+    )
+    await provider._detect_dimension()
+    assert provider.get_dimension() == 1024
+    assert seen["url"].endswith("/v1/models")
+    assert seen["auth"] is None  # unauthenticated gateway
+
+
+async def test_detect_dimension_sends_bearer(monkeypatch):
+    """When a token provider is configured, discovery presents the M2M bearer."""
+    captured = {}
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        if request.url.path.endswith("/token"):
+            return httpx.Response(200, json={"access_token": "tok", "expires_in": 3600})
+        captured["auth"] = request.headers.get("Authorization")
+        return httpx.Response(
+            200, json={"data": [{"id": "mistral/mistral-embed", "dimension": 1024}]}
+        )
+
+    _mock_async_client(monkeypatch, handler)
+    tp = GatewayTokenProvider(
+        token_url="http://idp.example/token", client_id="c", client_secret="s"
+    )
+    provider = GatewayProvider(
+        base_url="http://gw:8083/v1",
+        embedding_model="mistral/mistral-embed",
+        token_provider=tp,
+    )
+    await provider._detect_dimension()
+    assert provider.get_dimension() == 1024
+    assert captured["auth"] == "Bearer tok"
+
+
+async def test_detect_dimension_non_fatal_on_http_error(monkeypatch):
+    """An old gateway without /v1/models (404) must not crash startup —
+    dimension stays unknown so lazy detect-on-first-embed still applies."""
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(404, json={"detail": "not found"})
+
+    _mock_async_client(monkeypatch, handler)
+    provider = GatewayProvider(
+        base_url="http://gw:8083/v1", embedding_model="mistral/mistral-embed"
+    )
+    await provider._detect_dimension()  # must not raise
+    with pytest.raises(RuntimeError):
+        provider.get_dimension()  # still unknown
+
+
+async def test_detect_dimension_model_absent(monkeypatch):
+    """Gateway reachable but doesn't list our model → no dimension set,
+    no raise."""
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        return httpx.Response(200, json={"data": [{"id": "other", "dimension": 99}]})
+
+    _mock_async_client(monkeypatch, handler)
+    provider = GatewayProvider(
+        base_url="http://gw:8083/v1", embedding_model="mistral/mistral-embed"
+    )
+    await provider._detect_dimension()
+    with pytest.raises(RuntimeError):
+        provider.get_dimension()
+
+
+async def test_detect_dimension_skips_when_already_known(monkeypatch):
+    """If the dimension is already known, discovery makes no HTTP call."""
+    called = {"n": 0}
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        called["n"] += 1
+        return httpx.Response(200, json={"data": []})
+
+    _mock_async_client(monkeypatch, handler)
+    provider = GatewayProvider(
+        base_url="http://gw:8083/v1", embedding_model="mistral/mistral-embed"
+    )
+    provider._dimension = 1024  # pre-set (e.g. explicit override / OpenAI model)
+    await provider._detect_dimension()
+    assert called["n"] == 0
+    assert provider.get_dimension() == 1024