diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..3c32f93
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,2 @@
+[tool.pytest.ini_options]
+pythonpath = ["."]
diff --git a/tests/compatibility/conftest.py b/tests/compatibility/conftest.py
index 38d00a7..38ac65b 100644
--- a/tests/compatibility/conftest.py
+++ b/tests/compatibility/conftest.py
@@ -13,6 +13,44 @@
 DEVCLOUD_URL = os.environ.get("DEVCLOUD_URL", f"http://localhost:{DEVCLOUD_PORT}")
 
 
+def _build_devcloud_cmd(project_root, bin_path):
+    """Build the devcloud command, conditionally adding -config if devcloud.yaml exists."""
+    config_path = os.path.join(project_root, "devcloud.yaml")
+
+    if bin_path:
+        cmd = [bin_path]
+    else:
+        cmd = ["go", "run", "./cmd/devcloud"]
+
+    if os.path.isfile(config_path):
+        cmd.extend(["-config", "devcloud.yaml"])
+
+    return cmd
+
+
+def _start_server_error(cmd, project_root, env):
+    """Re-run server with PIPE to capture stderr, then raise with diagnostic info."""
+    debug_proc = subprocess.Popen(
+        cmd,
+        cwd=project_root,
+        env=env,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+    )
+    try:
+        _wait_for_server(DEVCLOUD_URL, timeout=5)
+    except RuntimeError:
+        pass
+    debug_proc.kill()
+    debug_proc.wait()
+    stderr = debug_proc.stderr.read().decode(errors="replace")
+    raise RuntimeError(
+        f"devcloud server did not start within 30s.\n"
+        f"command: {' '.join(cmd)}\n"
+        f"stderr:\n{stderr}"
+    ) from None
+
+
 def _wait_for_server(url, timeout=30, interval=0.5):
     """Poll server until it responds or timeout."""
     deadline = time.time() + timeout
@@ -46,10 +84,7 @@ def devcloud_server():
     # collide with stale data.  The directory is cleaned up after the session.
     data_dir = tempfile.mkdtemp(prefix="devcloud-test-")
 
-    if bin_path:
-        cmd = [bin_path, "-config", "devcloud.yaml"]
-    else:
-        cmd = ["go", "run", "./cmd/devcloud", "-config", "devcloud.yaml"]
+    cmd = _build_devcloud_cmd(project_root, bin_path)
 
     env = os.environ.copy()
     env["CGO_ENABLED"] = "1"
@@ -62,7 +97,12 @@ def devcloud_server():
         stdout=subprocess.DEVNULL,
         stderr=subprocess.DEVNULL,
     )
-    _wait_for_server(DEVCLOUD_URL)
+    try:
+        _wait_for_server(DEVCLOUD_URL)
+    except RuntimeError:
+        proc.kill()
+        proc.wait()
+        _start_server_error(cmd, project_root, env)
     yield proc
     proc.send_signal(signal.SIGINT)
     try:
diff --git a/tests/compatibility/test_conftest_helpers.py b/tests/compatibility/test_conftest_helpers.py
new file mode 100644
index 0000000..49840e6
--- /dev/null
+++ b/tests/compatibility/test_conftest_helpers.py
@@ -0,0 +1,80 @@
+import io
+
+from tests.compatibility.conftest import _build_devcloud_cmd, _start_server_error
+
+
+class TestBuildDevcloudCmd:
+    def test_includes_config_when_file_exists(self, monkeypatch, tmp_path):
+        def fake_isfile(path):
+            return path.endswith("devcloud.yaml")
+
+        monkeypatch.setattr("tests.compatibility.conftest.os.path.isfile", fake_isfile)
+
+        cmd = _build_devcloud_cmd(str(tmp_path), None)
+
+        assert cmd[:3] == ["go", "run", "./cmd/devcloud"]
+        assert "-config" in cmd
+        assert "devcloud.yaml" in cmd
+
+    def test_omits_config_when_file_missing(self, monkeypatch, tmp_path):
+        monkeypatch.setattr(
+            "tests.compatibility.conftest.os.path.isfile", lambda p: False
+        )
+
+        cmd = _build_devcloud_cmd(str(tmp_path), None)
+
+        assert cmd == ["go", "run", "./cmd/devcloud"]
+        assert "-config" not in cmd
+        assert "devcloud.yaml" not in cmd
+
+    def test_uses_bin_path_when_provided(self, monkeypatch, tmp_path):
+        monkeypatch.setattr(
+            "tests.compatibility.conftest.os.path.isfile", lambda p: False
+        )
+
+        cmd = _build_devcloud_cmd(str(tmp_path), "/usr/local/bin/devcloud")
+
+        assert cmd == ["/usr/local/bin/devcloud"]
+
+    def test_bin_path_with_config(self, monkeypatch, tmp_path):
+        def fake_isfile(path):
+            return path.endswith("devcloud.yaml")
+
+        monkeypatch.setattr("tests.compatibility.conftest.os.path.isfile", fake_isfile)
+
+        cmd = _build_devcloud_cmd(str(tmp_path), "/usr/local/bin/devcloud")
+
+        assert cmd[0] == "/usr/local/bin/devcloud"
+        assert "-config" in cmd
+
+
+class TestDevcloudServerErrorHandling:
+    def test_raises_runtime_error_with_stderr_on_startup_failure(self, monkeypatch):
+        fake_stderr = io.BytesIO(b"fatal: config file not found\n")
+
+        class FakeProc:
+            def kill(self):
+                pass
+
+            def wait(self):
+                return 1
+
+            stderr = fake_stderr
+
+        monkeypatch.setattr(
+            "tests.compatibility.conftest.subprocess.Popen", lambda *a, **kw: FakeProc()
+        )
+        monkeypatch.setattr(
+            "tests.compatibility.conftest._wait_for_server",
+            lambda *a, **kw: (_ for _ in ()).throw(RuntimeError("timeout")),
+        )
+
+        import pytest
+
+        with pytest.raises(RuntimeError) as exc_info:
+            _start_server_error(["go", "run", "./cmd/devcloud"], "/tmp/project", {})
+
+        msg = str(exc_info.value)
+        assert "fatal: config file not found" in msg
+        assert "command: go run ./cmd/devcloud" in msg
+        assert "stderr:" in msg