diff --git a/api/config.py b/api/config.py
index bb79483..642589a 100644
--- a/api/config.py
+++ b/api/config.py
@@ -585,9 +585,26 @@ def get_available_models() -> dict:
                     'models': [{'id': m['id'], 'label': m['label']} for m in _FALLBACK_MODELS],
                 })
             elif pid in _PROVIDER_MODELS:
+                # For non-default providers, prefix model IDs with provider name
+                # so resolve_model_provider() can route them correctly (e.g.
+                # \"minimax/MiniMax-M2.7\" instead of bare \"MiniMax-M2.7\").
+                # The default provider's models keep bare names for direct API routing.
+                # Guard: only prefix when we have a confirmed active_provider, and
+                # normalise case before comparing (config.yaml may use 'Anthropic').
+                raw_models = _PROVIDER_MODELS[pid]
+                _active = (active_provider or '').lower()
+                if _active and pid != _active:
+                    # Shallow copy — don't mutate the shared _PROVIDER_MODELS list.
+                    # Bare IDs get prefixed; already-prefixed IDs pass through as-is.
+                    models = []
+                    for m in raw_models:
+                        mid = m['id']
+                        models.append({'id': mid if '/' in mid else f'{pid}/{mid}', 'label': m['label']})
+                else:
+                    models = list(raw_models)  # shallow copy to protect against insert() mutations
                 groups.append({
                     'provider': provider_name,
-                    'models': _PROVIDER_MODELS[pid],
+                    'models': models,
                 })
             else:
                 # Unknown provider -- use auto-detected models if available,
diff --git a/tests/test_model_resolver.py b/tests/test_model_resolver.py
index a80ff62..fe502bf 100644
--- a/tests/test_model_resolver.py
+++ b/tests/test_model_resolver.py
@@ -98,3 +98,83 @@ def test_empty_model_returns_config_defaults():
     )
     assert model == ''
     assert provider == 'anthropic'
+
+
+# ── Non-default provider prefix routing (Issue #138) ────────────────────
+
+def test_prefixed_non_default_provider_routes_through_openrouter():
+    """minimax/MiniMax-M2.7 with anthropic as default should route via openrouter."""
+    model, provider, base_url = _resolve_with_config(
+        'minimax/MiniMax-M2.7', provider='anthropic',
+    )
+    assert model == 'minimax/MiniMax-M2.7'
+    assert provider == 'openrouter'
+
+
+def test_prefixed_non_default_provider_zai():
+    """zai/GLM-5 with openai as default should route via openrouter."""
+    model, provider, base_url = _resolve_with_config(
+        'zai/GLM-5', provider='openai',
+    )
+    assert model == 'zai/GLM-5'
+    assert provider == 'openrouter'
+
+
+# ── get_available_models() prefix behaviour ───────────────────────────────
+
+def _available_models_with_provider(provider):
+    """Helper: temporarily set active_provider in auth store simulation via config.cfg."""
+    old_cfg = dict(config.cfg)
+    config.cfg['model'] = {'provider': provider}
+    try:
+        return config.get_available_models()
+    finally:
+        config.cfg.clear()
+        config.cfg.update(old_cfg)
+
+
+def test_non_default_provider_models_are_prefixed():
+    """With anthropic as default, minimax model IDs should be prefixed 'minimax/...'."""
+    result = _available_models_with_provider('anthropic')
+    groups = {g['provider']: g['models'] for g in result['groups']}
+    if 'MiniMax' in groups:
+        for m in groups['MiniMax']:
+            assert m['id'].startswith('minimax/'), (
+                f"Expected minimax/ prefix, got: {m['id']!r}"
+            )
+
+
+def test_default_provider_models_not_prefixed():
+    """The active provider's _PROVIDER_MODELS entries remain bare (no prefix added)."""
+    import api.config as _cfg
+    # The bare IDs as stored in _PROVIDER_MODELS (e.g. 'claude-sonnet-4.6')
+    raw_anthropic_ids = {m['id'] for m in _cfg._PROVIDER_MODELS.get('anthropic', [])}
+    result = _available_models_with_provider('anthropic')
+    groups = {g['provider']: g['models'] for g in result['groups']}
+    if 'Anthropic' in groups:
+        returned_ids = {m['id'] for m in groups['Anthropic']}
+        # Every bare _PROVIDER_MODELS ID must still appear bare (not turned into 'anthropic/...')
+        for bare_id in raw_anthropic_ids:
+            assert bare_id in returned_ids, (
+                f"_PROVIDER_MODELS entry '{bare_id}' is missing from the Anthropic group "
+                f"(returned: {sorted(returned_ids)})"
+            )
+
+
+def test_no_active_provider_models_not_prefixed():
+    """With no confirmed active_provider, models should not be prefixed."""
+    old_cfg = dict(config.cfg)
+    config.cfg['model'] = {}  # no provider set
+    try:
+        result = config.get_available_models()
+        for g in result['groups']:
+            for m in g['models']:
+                # No model should have a double-prefix like 'minimax/minimax/...'
+                parts = m['id'].split('/')
+                if len(parts) >= 2:
+                    assert parts[0] != parts[1], (
+                        f"Double-prefix detected: {m['id']!r}"
+                    )
+    finally:
+        config.cfg.clear()
+        config.cfg.update(old_cfg)