diff --git a/api/config.py b/api/config.py
index 6742652..ba6b5de 100644
--- a/api/config.py
+++ b/api/config.py
@@ -391,6 +391,10 @@ def resolve_model_provider(model_id: str) -> tuple:
 
     if '/' in model_id:
         prefix, bare = model_id.split('/', 1)
+        # OpenRouter always needs the full provider/model path (e.g. openrouter/free,
+        # anthropic/claude-sonnet-4.6). Never strip the prefix for OpenRouter.
+        if config_provider == 'openrouter':
+            return model_id, 'openrouter', config_base_url
         # If prefix matches config provider exactly, strip it and use that provider directly.
         # e.g. config=anthropic, model=anthropic/claude-... → bare name to anthropic API
         if config_provider and prefix == config_provider:
diff --git a/tests/test_model_resolver.py b/tests/test_model_resolver.py
new file mode 100644
index 0000000..a80ff62
--- /dev/null
+++ b/tests/test_model_resolver.py
@@ -0,0 +1,100 @@
+"""
+Tests for resolve_model_provider() model routing logic.
+Verifies that model IDs are correctly resolved to (model, provider, base_url)
+tuples for different provider configurations.
+"""
+import api.config as config
+
+
+def _resolve_with_config(model_id, provider=None, base_url=None, default=None):
+    """Helper: temporarily set config.cfg model section, call resolve, restore."""
+    old_cfg = dict(config.cfg)
+    model_cfg = {}
+    if provider:
+        model_cfg['provider'] = provider
+    if base_url:
+        model_cfg['base_url'] = base_url
+    if default:
+        model_cfg['default'] = default
+    config.cfg['model'] = model_cfg if model_cfg else {}
+    try:
+        return config.resolve_model_provider(model_id)
+    finally:
+        config.cfg.clear()
+        config.cfg.update(old_cfg)
+
+
+# ── OpenRouter prefix handling ────────────────────────────────────────────
+
+def test_openrouter_free_keeps_full_path():
+    """openrouter/free must NOT be stripped to 'free' when provider is openrouter."""
+    model, provider, base_url = _resolve_with_config(
+        'openrouter/free', provider='openrouter',
+        base_url='https://openrouter.ai/api/v1',
+    )
+    assert model == 'openrouter/free', f"Expected 'openrouter/free', got '{model}'"
+    assert provider == 'openrouter'
+
+
+def test_openrouter_model_with_provider_prefix():
+    """anthropic/claude-sonnet-4.6 via openrouter keeps full path."""
+    model, provider, base_url = _resolve_with_config(
+        'anthropic/claude-sonnet-4.6', provider='openrouter',
+        base_url='https://openrouter.ai/api/v1',
+    )
+    assert model == 'anthropic/claude-sonnet-4.6'
+    assert provider == 'openrouter'
+
+
+# ── Direct provider prefix stripping ─────────────────────────────────────
+
+def test_anthropic_prefix_stripped_for_direct_api():
+    """anthropic/claude-sonnet-4.6 strips prefix when provider is anthropic."""
+    model, provider, base_url = _resolve_with_config(
+        'anthropic/claude-sonnet-4.6', provider='anthropic',
+    )
+    assert model == 'claude-sonnet-4.6'
+    assert provider == 'anthropic'
+
+
+def test_openai_prefix_stripped_for_direct_api():
+    """openai/gpt-5.4-mini strips prefix when provider is openai."""
+    model, provider, base_url = _resolve_with_config(
+        'openai/gpt-5.4-mini', provider='openai',
+    )
+    assert model == 'gpt-5.4-mini'
+    assert provider == 'openai'
+
+
+# ── Cross-provider routing ───────────────────────────────────────────────
+
+def test_cross_provider_routes_through_openrouter():
+    """Picking openai model when config is anthropic routes via openrouter."""
+    model, provider, base_url = _resolve_with_config(
+        'openai/gpt-5.4-mini', provider='anthropic',
+    )
+    assert model == 'openai/gpt-5.4-mini'
+    assert provider == 'openrouter'
+    assert base_url is None  # openrouter uses its own endpoint
+
+
+# ── Bare model names ─────────────────────────────────────────────────────
+
+def test_bare_model_uses_config_provider():
+    """A model name without / uses the config provider and base_url."""
+    model, provider, base_url = _resolve_with_config(
+        'gemma-4-26B', provider='custom',
+        base_url='http://192.168.1.160:4000',
+    )
+    assert model == 'gemma-4-26B'
+    assert provider == 'custom'
+    assert base_url == 'http://192.168.1.160:4000'
+
+
+def test_empty_model_returns_config_defaults():
+    """Empty model string returns config provider and base_url."""
+    model, provider, base_url = _resolve_with_config(
+        '', provider='anthropic',
+    )
+    assert model == ''
+    assert provider == 'anthropic'