diff --git a/tests/test_litellm_kwargs.py b/tests/test_litellm_kwargs.py
new file mode 100644
index 0000000..19b753b
--- /dev/null
+++ b/tests/test_litellm_kwargs.py
@@ -0,0 +1,112 @@
+"""Regression tests for PR #2026 — litellm_kwargs injection from ProviderSpec."""
+
+from __future__ import annotations
+
+from types import SimpleNamespace
+from typing import Any
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from nanobot.providers.litellm_provider import LiteLLMProvider
+
+
+def _fake_response(content: str = "ok") -> SimpleNamespace:
+    """Build a minimal acompletion-shaped response object."""
+    message = SimpleNamespace(
+        content=content,
+        tool_calls=None,
+        reasoning_content=None,
+        thinking_blocks=None,
+    )
+    choice = SimpleNamespace(message=message, finish_reason="stop")
+    usage = SimpleNamespace(prompt_tokens=10, completion_tokens=5, total_tokens=15)
+    return SimpleNamespace(choices=[choice], usage=usage)
+
+
+@pytest.mark.asyncio
+async def test_openrouter_injects_litellm_kwargs() -> None:
+    """OpenRouter gateway must inject custom_llm_provider into acompletion call."""
+    mock_acompletion = AsyncMock(return_value=_fake_response())
+
+    with patch("nanobot.providers.litellm_provider.acompletion", mock_acompletion):
+        provider = LiteLLMProvider(
+            api_key="sk-or-test-key",
+            api_base="https://openrouter.ai/api/v1",
+            default_model="anthropic/claude-sonnet-4-5",
+            provider_name="openrouter",
+        )
+        await provider.chat(
+            messages=[{"role": "user", "content": "hello"}],
+            model="anthropic/claude-sonnet-4-5",
+        )
+
+    call_kwargs = mock_acompletion.call_args.kwargs
+    assert call_kwargs.get("custom_llm_provider") == "openrouter", (
+        "OpenRouter gateway should pass custom_llm_provider='openrouter' to acompletion"
+    )
+
+
+@pytest.mark.asyncio
+async def test_non_gateway_provider_does_not_inject_litellm_kwargs() -> None:
+    """Standard (non-gateway) providers must NOT inject any litellm_kwargs."""
+    mock_acompletion = AsyncMock(return_value=_fake_response())
+
+    with patch("nanobot.providers.litellm_provider.acompletion", mock_acompletion):
+        provider = LiteLLMProvider(
+            api_key="sk-ant-test-key",
+            default_model="claude-sonnet-4-5",
+        )
+        await provider.chat(
+            messages=[{"role": "user", "content": "hello"}],
+            model="claude-sonnet-4-5",
+        )
+
+    call_kwargs = mock_acompletion.call_args.kwargs
+    assert "custom_llm_provider" not in call_kwargs, (
+        "Standard Anthropic provider should NOT inject custom_llm_provider"
+    )
+
+
+@pytest.mark.asyncio
+async def test_gateway_without_litellm_kwargs_injects_nothing_extra() -> None:
+    """Gateways without litellm_kwargs (e.g. AiHubMix) must not add extra keys."""
+    mock_acompletion = AsyncMock(return_value=_fake_response())
+
+    with patch("nanobot.providers.litellm_provider.acompletion", mock_acompletion):
+        provider = LiteLLMProvider(
+            api_key="sk-aihub-test-key",
+            api_base="https://aihubmix.com/v1",
+            default_model="claude-sonnet-4-5",
+            provider_name="aihubmix",
+        )
+        await provider.chat(
+            messages=[{"role": "user", "content": "hello"}],
+            model="claude-sonnet-4-5",
+        )
+
+    call_kwargs = mock_acompletion.call_args.kwargs
+    assert "custom_llm_provider" not in call_kwargs, (
+        "AiHubMix gateway has no litellm_kwargs, should not add custom_llm_provider"
+    )
+
+
+@pytest.mark.asyncio
+async def test_openrouter_autodetect_by_key_prefix() -> None:
+    """OpenRouter should be auto-detected by sk-or- key prefix even without explicit provider_name."""
+    mock_acompletion = AsyncMock(return_value=_fake_response())
+
+    with patch("nanobot.providers.litellm_provider.acompletion", mock_acompletion):
+        provider = LiteLLMProvider(
+            api_key="sk-or-auto-detect-key",
+            default_model="anthropic/claude-sonnet-4-5",
+        )
+        await provider.chat(
+            messages=[{"role": "user", "content": "hello"}],
+            model="anthropic/claude-sonnet-4-5",
+        )
+
+    call_kwargs = mock_acompletion.call_args.kwargs
+    assert call_kwargs.get("custom_llm_provider") == "openrouter", (
+        "Auto-detected OpenRouter (by sk-or- prefix) should still inject custom_llm_provider"
+    )