diff --git a/hermes_cli/main.py b/hermes_cli/main.py
index 5744d8a9e39..4911de19a7c 100644
--- a/hermes_cli/main.py
+++ b/hermes_cli/main.py
@@ -4609,13 +4609,18 @@ def _run_npm_install_deterministic(
     the working tree dirty and causes the next ``hermes update`` to stash the
     lockfile — repeatedly.
     """
+    # unicode-animations' postinstall animates to /dev/tty (bypasses
+    # --silent/capture_output). It no-ops when CI is set — same as the TUI
+    # install path and nix/lib.nix npm ci hooks.
+    run_env = {**os.environ, **(env or {}), "CI": "1"}
+
     lockfile = cwd / "package-lock.json"
     if lockfile.exists():
         ci_cmd = [npm, "ci", *extra_args]
         ci_result = subprocess.run(
             ci_cmd,
             cwd=cwd,
-            env=env,
+            env=run_env,
             capture_output=capture_output,
             text=True,
             encoding="utf-8",
@@ -4630,7 +4635,7 @@ def _run_npm_install_deterministic(
     return subprocess.run(
         install_cmd,
         cwd=cwd,
-        env=env,
+        env=run_env,
         capture_output=capture_output,
         text=True,
         encoding="utf-8",
diff --git a/tests/hermes_cli/test_web_ui_build.py b/tests/hermes_cli/test_web_ui_build.py
index cc20fa61cf3..8f94458a6ac 100644
--- a/tests/hermes_cli/test_web_ui_build.py
+++ b/tests/hermes_cli/test_web_ui_build.py
@@ -140,6 +140,22 @@ class TestBuildWebUISkipsWhenFresh:
         assert kwargs["encoding"] == "utf-8"
         assert kwargs["errors"] == "replace"
 
+    def test_npm_install_sets_ci_to_suppress_postinstall_tty_output(self, tmp_path):
+        web_dir, _ = _make_web_dir(tmp_path)
+        (web_dir / "package-lock.json").write_text("{}", encoding="utf-8")
+
+        mock_cp = __import__("subprocess").CompletedProcess([], 0, stdout="", stderr="")
+        with patch("hermes_cli.main.subprocess.run", return_value=mock_cp) as mock_run:
+            _run_npm_install_deterministic(
+                "/usr/bin/npm",
+                web_dir,
+                env={"PYTHON": "/nix/store/python"},
+            )
+
+        _, kwargs = mock_run.call_args
+        assert kwargs["env"]["CI"] == "1"
+        assert kwargs["env"]["PYTHON"] == "/nix/store/python"
+
     def test_npm_install_uses_workspace_web_scope(self, tmp_path):
         web_dir, _ = _make_web_dir(tmp_path)
         # Real workspace checkout: the single lockfile lives at the root, so