diff --git a/pm/environment.py b/pm/environment.py
index 69663053a7..a2f7d03c3c 100644
--- a/pm/environment.py
+++ b/pm/environment.py
@@ -272,6 +272,16 @@ class PythonEnvironment:
         # Explicit index credentials survive, but cannot redirect the project,
         # interpreter or cache selected by the operation.
         env = _base_environment(self.env)
+        if self.no_config:
+            # --no-config only removes config files; the pip-mirror bridge
+            # (pm.index_config) reaches uv as env vars. The pinned runtime
+            # builder asked to be isolated from ambient index configuration:
+            # re-resolving its official-index lockfile against a mirror trips
+            # `--locked` (#124418), and a fully-pinned graph never needs a
+            # mirror to resolve.
+            from pm.index_config import is_forwarded
+
+            env = {key: value for key, value in env.items() if not is_forwarded(key)}
         env.update(UV_PYTHON=str(self.python), UV_PROJECT_ENVIRONMENT=str(self.destination),
                    UV_CACHE_DIR=str(self.cache), UV_PYTHON_DOWNLOADS="never")
         with tempfile.TemporaryDirectory(prefix="pm-uv-config-") as config:
diff --git a/tests/pm/test_environment_build.py b/tests/pm/test_environment_build.py
index 500b1292b6..f0b027d3cf 100644
--- a/tests/pm/test_environment_build.py
+++ b/tests/pm/test_environment_build.py
@@ -787,3 +787,32 @@ def test_live_apply_keeps_selection_on_failed_union(locked_project, tmp_path, mo
     assert set(generations.iterdir()) == prior_generations
     assert (source / "uv.lock").read_bytes() == source_lock
     assert env == original_env
+
+
+def test_no_config_drops_bridged_index_settings_from_child_env(tmp_path):
+    """no_config must isolate the pinned runtime builder from ALL ambient uv
+    configuration: --no-config covers config files, but PM's pip-mirror bridge
+    injects UV_INDEX_URL as an env var, and re-resolving the official-index
+    runtime lockfile against that mirror trips `uv sync --locked` (#124418).
+    The default path keeps the bridge — mirrored networks need it to resolve
+    their application dependencies at all."""
+    import io
+    from pm.environment import PythonEnvironment
+
+    ambient = dict(os.environ, UV_INDEX_URL="https://fixture-mirror.invalid/simple",
+                   UV_HTTP_TIMEOUT="99")
+    child = ("import os, sys\n"
+             "present = sorted(k for k in ('UV_INDEX_URL', 'UV_HTTP_TIMEOUT') if k in os.environ)\n"
+             "sys.exit(', '.join(present) if present else None)\n")
+
+    def run(**kwargs):
+        environment = PythonEnvironment(
+            uv=Path(sys.executable), python=Path(sys.executable), destination=tmp_path / "venv",
+            cache=tmp_path / "cache", env=ambient, output=io.StringIO(), **kwargs)
+        return environment._run(["-c", child], cwd=tmp_path, timeout=60)
+
+    isolated = run(no_config=True)
+    assert isolated.returncode == 0, f"bridged settings leaked into no_config child: {isolated.stderr}"
+    bridged = run()
+    assert bridged.returncode == 1, "default path must keep the bridge"
+    assert "UV_INDEX_URL" in bridged.stderr and "UV_HTTP_TIMEOUT" in bridged.stderr