diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..4b6d747
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,89 @@
+"""Shared pytest fixtures + import shims for the adapter test suite.
+
+`adapter.py` imports at module load:
+  - molecule_runtime.adapters.base (BaseAdapter, AdapterConfig, RuntimeCapabilities)
+  - molecule_runtime.plugins (lazy in setup(), but stubbed proactively)
+  - a2a.server.agent_execution (AgentExecutor)
+  - claude_sdk_executor (lazy in create_executor(), stubbed proactively)
+
+In production those arrive transitively via molecule-ai-workspace-runtime.
+The CI runner only installs `pytest pytest-asyncio pyyaml`, so the import
+chain would fail with ModuleNotFoundError before any test collects —
+exactly the failure that broke CI on the #180 fix branch (PR #4) and
+caused the merge wall to block on a green local but red Gitea CI.
+
+Putting the stub installer here (collected before any test module is
+imported, per pytest semantics) means every test file can do
+`from adapter import ...` at module top without a per-file boilerplate
+copy. It also forces a single shape for the stubs so two files can't
+silently disagree on whether `BaseAdapter` has
+`install_plugins_via_registry` (see test_adapter_prevalidate's
+async-setup tests, which need the method to exist on the parent class).
+"""
+
+import os
+import sys
+import types
+from dataclasses import dataclass
+from unittest.mock import MagicMock
+
+
+@dataclass
+class _StubRuntimeCapabilities:
+    provides_native_session: bool = False
+
+
+@dataclass
+class _StubAdapterConfig:
+    runtime_config: object = None
+    config_path: str = "/tmp/configs"
+    system_prompt: str = ""
+    heartbeat: object = None
+
+
+class _StubBaseAdapter:
+    async def install_plugins_via_registry(self, *_args, **_kwargs):
+        pass
+
+
+def _install_stubs() -> None:
+    """Install the smallest set of import shims that adapter.py needs."""
+    if "molecule_runtime" not in sys.modules:
+        mr = types.ModuleType("molecule_runtime")
+        mr.adapters = types.ModuleType("molecule_runtime.adapters")
+        mr.adapters.base = types.ModuleType("molecule_runtime.adapters.base")
+        mr.adapters.base.BaseAdapter = _StubBaseAdapter
+        mr.adapters.base.AdapterConfig = _StubAdapterConfig
+        mr.adapters.base.RuntimeCapabilities = _StubRuntimeCapabilities
+        mr.plugins = types.ModuleType("molecule_runtime.plugins")
+        mr.plugins.load_plugins = lambda **_kwargs: []
+        sys.modules["molecule_runtime"] = mr
+        sys.modules["molecule_runtime.adapters"] = mr.adapters
+        sys.modules["molecule_runtime.adapters.base"] = mr.adapters.base
+        sys.modules["molecule_runtime.plugins"] = mr.plugins
+    if "a2a" not in sys.modules:
+        a2a = types.ModuleType("a2a")
+        a2a.server = types.ModuleType("a2a.server")
+        a2a.server.agent_execution = types.ModuleType("a2a.server.agent_execution")
+        a2a.server.agent_execution.AgentExecutor = type("AgentExecutor", (), {})
+        sys.modules["a2a"] = a2a
+        sys.modules["a2a.server"] = a2a.server
+        sys.modules["a2a.server.agent_execution"] = a2a.server.agent_execution
+    if "claude_sdk_executor" not in sys.modules:
+        mod = types.ModuleType("claude_sdk_executor")
+        mod.ClaudeSDKExecutor = MagicMock(name="ClaudeSDKExecutor")
+        sys.modules["claude_sdk_executor"] = mod
+
+
+# Run at conftest import time — pytest collects conftest.py before any
+# test module, so the stubs are in sys.modules before `from adapter
+# import ...` ever executes.
+_install_stubs()
+
+# adapter.py lives in the parent dir of tests/ (template root). pytest's
+# `--import-mode=importlib` + tests/pytest.ini anchoring rootdir at
+# tests/ means the parent isn't on sys.path automatically. Add it here
+# once so every test file can do `from adapter import ...` cleanly.
+_PARENT_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+if _PARENT_DIR not in sys.path:
+    sys.path.insert(0, _PARENT_DIR)
diff --git a/tests/test_provider_resolution.py b/tests/test_provider_resolution.py
index 030ea7c..cb51343 100644
--- a/tests/test_provider_resolution.py
+++ b/tests/test_provider_resolution.py
@@ -11,6 +11,11 @@ These tests mirror the production failure mode reported by Hongming
 adapter ignored it entirely, the SDK kept calling the Anthropic API with
 CLAUDE_CODE_OAUTH_TOKEN, hit the OAuth quota, and the canvas surfaced
 "Agent error (Exception)" with no clue why.
+
+Import-shim setup (sys.path + molecule_runtime / a2a / claude_sdk_executor
+stubs) lives in tests/conftest.py — shared with test_adapter_prevalidate
+so the two stub installers can't disagree on shape (e.g. BaseAdapter
+having install_plugins_via_registry).
 """
 
 import pytest