diff --git a/tests/test_ensure_model_loaded.py b/tests/test_ensure_model_loaded.py
new file mode 100644
index 0000000..5fa8db4
--- /dev/null
+++ b/tests/test_ensure_model_loaded.py
@@ -0,0 +1,195 @@
+# SPDX-License-Identifier: Apache-2.0
+"""Tests for `_is_model_loaded` and `ensure_model_loaded` in service.helpers.
+
+These helpers unify the previously inconsistent "is the request model name
+something we can serve?" check across /v1/chat/completions, /v1/completions,
+and /v1/messages routes.
+
+Future on-demand auto-loading (see follow-up PR) will hook into
+`ensure_model_loaded`; today it strictly raises 404 for unloaded models.
+"""
+
+from unittest.mock import MagicMock
+
+import pytest
+from fastapi import HTTPException
+
+from vllm_mlx.config import get_config
+from vllm_mlx.runtime.model_registry import ModelRegistry
+from vllm_mlx.service.helpers import _is_model_loaded, ensure_model_loaded
+
+
+@pytest.fixture
+def reset_config():
+    """Snapshot mutated config fields and restore after the test."""
+    cfg = get_config()
+    fields = ("model_name", "model_alias", "model_path", "model_registry")
+    snap = {f: getattr(cfg, f) for f in fields}
+    yield cfg
+    for f, v in snap.items():
+        setattr(cfg, f, v)
+
+
+def _make_registry(*names: str) -> MagicMock:
+    """Spec'd registry mock — catches interface drift if ModelRegistry changes."""
+    reg = MagicMock(spec=ModelRegistry)
+    reg.__contains__ = lambda self, x: x in names
+    reg.list_model_names.return_value = list(names)
+    return reg
+
+
+# ---------------------------------------------------------------------------
+# _is_model_loaded — single-model mode
+# ---------------------------------------------------------------------------
+
+
+class TestIsModelLoadedSingleMode:
+    def test_matches_model_name(self, reset_config):
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        assert _is_model_loaded("qwen3.5-4b") is True
+
+    def test_matches_model_alias(self, reset_config):
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = "qwen"
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        assert _is_model_loaded("qwen") is True
+
+    def test_matches_model_path(self, reset_config):
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = None
+        reset_config.model_path = "mlx-community/Qwen2.5-4B-Instruct-4bit"
+        reset_config.model_registry = None
+        assert _is_model_loaded("mlx-community/Qwen2.5-4B-Instruct-4bit") is True
+
+    def test_rejects_unknown_model(self, reset_config):
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        assert _is_model_loaded("kimi-48b") is False
+
+    def test_default_accepted_in_single_mode(self, reset_config):
+        """P2-1 fix: 'default' is loaded in BOTH single-model and registry mode.
+
+        Pre-fix, `_validate_model_name` accepted 'default' only when a registry
+        was configured — single-model servers 404'd on `model: "default"` even
+        though the request unambiguously targets the one model that's loaded.
+        """
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        assert _is_model_loaded("default") is True
+
+    def test_returns_false_when_no_model_configured(self, reset_config):
+        reset_config.model_name = None
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        assert _is_model_loaded("anything") is False
+
+
+# ---------------------------------------------------------------------------
+# _is_model_loaded — registry (multi-model) mode
+# ---------------------------------------------------------------------------
+
+
+class TestIsModelLoadedRegistryMode:
+    def test_matches_registry_entry(self, reset_config):
+        reset_config.model_registry = _make_registry("qwen3.5-4b", "phi4-14b")
+        reset_config.model_name = "qwen3.5-4b"
+        assert _is_model_loaded("phi4-14b") is True
+
+    def test_rejects_non_registry_entry(self, reset_config):
+        reset_config.model_registry = _make_registry("qwen3.5-4b", "phi4-14b")
+        reset_config.model_name = "qwen3.5-4b"
+        assert _is_model_loaded("kimi-48b") is False
+
+    def test_default_accepted_in_registry_mode(self, reset_config):
+        reset_config.model_registry = _make_registry("qwen3.5-4b")
+        reset_config.model_name = "qwen3.5-4b"
+        assert _is_model_loaded("default") is True
+
+
+# ---------------------------------------------------------------------------
+# ensure_model_loaded — strict-404 semantics (PR #1 baseline)
+# ---------------------------------------------------------------------------
+
+
+class TestEnsureModelLoaded:
+    @pytest.mark.asyncio
+    async def test_noop_when_model_is_loaded(self, reset_config):
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        # Should not raise.
+        await ensure_model_loaded("qwen3.5-4b")
+
+    @pytest.mark.asyncio
+    async def test_noop_for_empty_or_default(self, reset_config):
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        await ensure_model_loaded(None)
+        await ensure_model_loaded("")
+        await ensure_model_loaded("default")
+
+    @pytest.mark.asyncio
+    async def test_raises_404_when_model_not_loaded(self, reset_config):
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        with pytest.raises(HTTPException) as exc:
+            await ensure_model_loaded("kimi-48b")
+        assert exc.value.status_code == 404
+        assert "kimi-48b" in exc.value.detail
+
+    @pytest.mark.asyncio
+    async def test_404_detail_lists_available_single_mode(self, reset_config):
+        """Locks the 404 contract: detail must include the `Available:` hint.
+
+        Mirrors `_validate_model_name`'s message shape so the two helpers are
+        interchangeable on the strict-404 path — and so #319's swap logic can
+        replace this `raise` without changing what clients see on a miss.
+        """
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        with pytest.raises(HTTPException) as exc:
+            await ensure_model_loaded("kimi-48b")
+        assert exc.value.status_code == 404
+        assert "Available:" in exc.value.detail
+        assert "qwen3.5-4b" in exc.value.detail
+
+    @pytest.mark.asyncio
+    async def test_404_detail_lists_available_registry_mode(self, reset_config):
+        reset_config.model_registry = _make_registry("qwen3.5-4b", "phi4-14b")
+        reset_config.model_name = "qwen3.5-4b"
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        with pytest.raises(HTTPException) as exc:
+            await ensure_model_loaded("kimi-48b")
+        assert exc.value.status_code == 404
+        assert "Available:" in exc.value.detail
+        assert "phi4-14b" in exc.value.detail
+        assert "qwen3.5-4b" in exc.value.detail
+
+    @pytest.mark.asyncio
+    async def test_noop_when_server_unconfigured(self, reset_config):
+        """Theoretical unconfigured-server case — preserves parity with
+        `_validate_model_name`, which silently returns when neither
+        `model_name` nor `model_registry` is set."""
+        reset_config.model_name = None
+        reset_config.model_alias = None
+        reset_config.model_path = None
+        reset_config.model_registry = None
+        # Should not raise even for a clearly-unknown name.
+        await ensure_model_loaded("anything")
diff --git a/vllm_mlx/routes/chat.py b/vllm_mlx/routes/chat.py
index 621c5b7..5f4d798 100644
--- a/vllm_mlx/routes/chat.py
+++ b/vllm_mlx/routes/chat.py
@@ -54,10 +54,10 @@
     _resolve_model_name,
     _resolve_temperature,
     _resolve_top_p,
-    _validate_model_name,
     _validate_tool_call_params,
     _wait_with_disconnect,
     build_extended_sampling_kwargs,
+    ensure_model_loaded,
     get_engine,
     get_usage,
 )
@@ -203,7 +203,7 @@ async def create_chat_completion(request: ChatCompletionRequest, raw_request: Re
     }
     ```
     """
-    _validate_model_name(request.model)
+    await ensure_model_loaded(request.model)
     engine = get_engine(request.model)
 
     # Validate messages is non-empty
diff --git a/vllm_mlx/routes/completions.py b/vllm_mlx/routes/completions.py
index da82c0a..8a67f8d 100644
--- a/vllm_mlx/routes/completions.py
+++ b/vllm_mlx/routes/completions.py
@@ -24,9 +24,9 @@
     _resolve_model_name,
     _resolve_temperature,
     _resolve_top_p,
-    _validate_model_name,
     _wait_with_disconnect,
     build_extended_sampling_kwargs,
+    ensure_model_loaded,
     get_engine,
     get_usage,
 )
@@ -42,7 +42,7 @@
 )
 async def create_completion(request: CompletionRequest, raw_request: Request):
     """Create a text completion."""
-    _validate_model_name(request.model)
+    await ensure_model_loaded(request.model)
     engine = get_engine(request.model)
 
     # Handle single prompt or list of prompts
diff --git a/vllm_mlx/service/helpers.py b/vllm_mlx/service/helpers.py
index 4b0cbd6..76833f4 100644
--- a/vllm_mlx/service/helpers.py
+++ b/vllm_mlx/service/helpers.py
@@ -367,35 +367,79 @@ def get_engine(model_name: str | None = None) -> BaseEngine:
     return cfg.engine
 
 
+def _is_model_loaded(model_name: str | None) -> bool:
+    """Return True when model_name refers to a currently served model.
+
+    Treats `None`, empty string, and the literal "default" as "loaded": the
+    request does not target a specific model, so the currently configured
+    default serves it. This unifies single-model and registry modes — the
+    pre-fix behavior accepted "default" only in registry mode, which surfaced
+    as a confusing 404 for clients sending `model: "default"` against a
+    single-model server.
+    """
+    if not model_name or model_name == "default":
+        return True
+    cfg = get_config()
+    if cfg.model_registry is not None and model_name in cfg.model_registry:
+        return True
+    if not cfg.model_name:
+        return False
+    accepted = {cfg.model_name}
+    if cfg.model_alias:
+        accepted.add(cfg.model_alias)
+    if cfg.model_path:
+        accepted.add(cfg.model_path)
+    return model_name in accepted
+
+
 def _validate_model_name(request_model: str) -> None:
     """Validate that the request model name matches a served model."""
-    if not request_model:
+    if _is_model_loaded(request_model):
         return
 
     cfg = get_config()
-    if cfg.model_registry and request_model in cfg.model_registry:
-        return
-    if cfg.model_registry and request_model == "default":
+    if not cfg.model_name and cfg.model_registry is None:
         return
 
-    if not cfg.model_name:
+    available = (
+        ", ".join(cfg.model_registry.list_model_names())
+        if cfg.model_registry is not None
+        else cfg.model_name
+    )
+    raise HTTPException(
+        status_code=404,
+        detail=f"The model `{request_model}` does not exist. Available: {available}",
+    )
+
+
+async def ensure_model_loaded(model_name: str | None) -> None:
+    """Validate that `model_name` is currently loaded; otherwise raise 404.
+
+    Canonical strict-404 check for /v1/chat/completions and /v1/completions —
+    mirrors `_validate_model_name`'s message shape so the two are
+    interchangeable on the strict-404 path. The sync counterpart is kept for
+    /v1/messages (anthropic adapter is model-name-agnostic by design).
+
+    Extension point for on-demand auto-loading: a follow-up PR will let an
+    operator-set flag (`enable_on_demand_loading`) trigger a hot swap to the
+    requested model here instead of raising 404. The flag plumbing, swap
+    machinery, and security/concurrency model are intentionally out of scope
+    for this PR; today the function is strict-404.
+    """
+    if _is_model_loaded(model_name):
         return
-    accepted = {cfg.model_name}
-    if cfg.model_alias:
-        accepted.add(cfg.model_alias)
-    if cfg.model_path:
-        accepted.add(cfg.model_path)
-    if request_model not in accepted:
-        available = (
-            ", ".join(cfg.model_registry.list_model_names())
-            if cfg.model_registry
-            else cfg.model_name
-        )
-        raise HTTPException(
-            status_code=404,
-            detail=f"The model `{request_model}` does not exist. "
-            f"Available: {available}",
-        )
+    cfg = get_config()
+    if not cfg.model_name and cfg.model_registry is None:
+        return
+    available = (
+        ", ".join(cfg.model_registry.list_model_names())
+        if cfg.model_registry is not None
+        else cfg.model_name
+    )
+    raise HTTPException(
+        status_code=404,
+        detail=f"The model `{model_name}` does not exist. Available: {available}",
+    )
 
 
 # ── Tool call parsing ──────────────────────────────────────────────