diff --git a/README.md b/README.md
index 2d8c0b0..2c527d8 100644
--- a/README.md
+++ b/README.md
@@ -142,6 +142,7 @@ Environment variables:
 | Variable | Effect |
 |---|---|
 | `HYPERWALL_WEB=1` | Enable web remote on port 8585 (off by default) |
+| `HYPERWALL_SERVER_URL` | Per-launch Emby endpoint override; leaves `config.ini` unchanged (use for LAN/public A/B) |
 | `HYPERWALL_STATS=1` | Enable per-cell playback stats |
 | `HYPERWALL_HWDEC` | Override hardware decoder (nvdec, d3d11va, etc.) |
 | `HYPERWALL_VO` | Override video output (gpu-next, gpu) |
diff --git a/hyperwall/app.py b/hyperwall/app.py
index bd9e395..cfd64ad 100644
--- a/hyperwall/app.py
+++ b/hyperwall/app.py
@@ -25,7 +25,7 @@
 )
 
 from . import __version__, runtime_banner
-from .config import HyperwallConfig, ConfigMissingError
+from .config import HyperwallConfig, ConfigMissingError, effective_server_url
 from .constants import (
     CONFIG_FILE,
     LOG_FILE,
@@ -275,8 +275,13 @@ def main() -> None:
         sys.exit(1)
 
     # 7. Emby client
+    server_url = effective_server_url(
+        cfg.server_url, os.environ.get("HYPERWALL_SERVER_URL")
+    )
+    if server_url != cfg.server_url:
+        logger.info("Endpoint override active for this launch: %s", server_url)
     client = EmbyClient(
-        cfg.server_url, cfg.username, cfg.password, verify_ssl=cfg.verify_ssl,
+        server_url, cfg.username, cfg.password, verify_ssl=cfg.verify_ssl,
         backend=resolve_backend(cfg.backend),
     )
     if not cfg.verify_ssl:
@@ -289,7 +294,7 @@ def main() -> None:
     if not client.test_connection():
         _show_error_dialog(
             "Connection Error",
-            f"Cannot reach Emby server at:\n{cfg.server_url}",
+            f"Cannot reach Emby server at:\n{server_url}",
         )
         sys.exit(1)
 
diff --git a/hyperwall/config.py b/hyperwall/config.py
index 35e7444..0ca5976 100644
--- a/hyperwall/config.py
+++ b/hyperwall/config.py
@@ -14,6 +14,16 @@
 from .constants import CONFIG_FILE
 
 
+def effective_server_url(configured: str, override: str | None = None) -> str:
+    """Return a per-launch endpoint override without changing config.ini.
+
+    This supports controlled LAN-vs-public delivery tests while leaving the
+    user's normal configured endpoint and credentials untouched.
+    """
+    candidate = (override or "").strip()
+    return candidate or configured
+
+
 @dataclass(frozen=True)
 class HyperwallConfig:
     """Immutable configuration loaded from config.ini."""
diff --git a/tests/run_all.py b/tests/run_all.py
index d06be44..8b07453 100644
--- a/tests/run_all.py
+++ b/tests/run_all.py
@@ -34,6 +34,7 @@
     "test_platform",
     "test_soak_telemetry",
     "test_macos_playback_performance",
+    "test_endpoint_override",
 ]
 
 
diff --git a/tests/test_endpoint_override.py b/tests/test_endpoint_override.py
new file mode 100644
index 0000000..bc05107
--- /dev/null
+++ b/tests/test_endpoint_override.py
@@ -0,0 +1,42 @@
+"""Regression contract for ephemeral Hyperwall endpoint overrides.
+
+A macOS playback A/B must be able to use Greg's LAN Emby endpoint without
+rewriting a configured public endpoint or exposing credentials in commands.
+"""
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from hyperwall.config import effective_server_url  # noqa: E402
+
+
+def test_endpoint_override_wins_without_mutating_default():
+    configured = "https://mb.perseus.observer"
+    override = "http://10.168.168.29:8096"
+    assert effective_server_url(configured, override) == override
+    assert configured == "https://mb.perseus.observer"
+
+
+def test_blank_override_keeps_configured_endpoint():
+    assert effective_server_url("http://emby:8096", "   ") == "http://emby:8096"
+    assert effective_server_url("http://emby:8096", None) == "http://emby:8096"
+
+
+def run_all() -> int:
+    tests = [v for k, v in sorted(globals().items()) if k.startswith("test_")]
+    passed = failed = 0
+    for test in tests:
+        try:
+            test()
+            passed += 1
+            print(f"  PASS  {test.__name__}")
+        except Exception as e:  # noqa: BLE001
+            failed += 1
+            print(f"  FAIL  {test.__name__}: {e}")
+    print(f"\n{passed} passed, {failed} failed out of {len(tests)} tests.")
+    return failed
+
+
+if __name__ == "__main__":
+    raise SystemExit(1 if run_all() else 0)