diff --git a/tests/inner/conftest.py b/tests/inner/conftest.py
index 5cd8999f36..9ed5694712 100644
--- a/tests/inner/conftest.py
+++ b/tests/inner/conftest.py
@@ -12,8 +12,11 @@
 import logging
 import os
 import pathlib
+import shutil
 import sys
+import tempfile
 import time
+from collections.abc import Iterator
 from types import SimpleNamespace
 
 import pytest
@@ -22,6 +25,24 @@
 from tests import _model_pools
 
 
+@pytest.fixture
+def short_tmp_parent() -> Iterator[pathlib.Path]:
+    """A short-pathed temp dir under ``/tmp`` for Unix-socket-binding tests.
+
+    macOS caps an ``AF_UNIX`` path at ~103 bytes and its ``$TMPDIR`` is already
+    ~48 bytes, so pytest's ``tmp_path`` (which embeds the test name) overflows
+    before a socket filename is appended (issue #4279). Tests that bind a real
+    Unix socket — the private tmux server, the egress proxy — must place it
+    under a short parent. Production sockets already live under a short
+    ``$TMPDIR/omnigent-...`` path, so this is a test-path artifact only.
+    """
+    parent = pathlib.Path(tempfile.mkdtemp(prefix="omni-inner-", dir="/tmp"))
+    try:
+        yield parent
+    finally:
+        shutil.rmtree(parent, ignore_errors=True)
+
+
 @pytest.fixture(autouse=True)
 def _stub_executor_catalog_defaults(monkeypatch: pytest.MonkeyPatch) -> None:
     """Keep executor unit tests deterministic without catalog network access."""
diff --git a/tests/inner/egress/test_proxy.py b/tests/inner/egress/test_proxy.py
index 047977579a..1cf4ec9927 100644
--- a/tests/inner/egress/test_proxy.py
+++ b/tests/inner/egress/test_proxy.py
@@ -51,13 +51,14 @@ async def test_proxy_start_stop_tcp(
 
 
 @pytest.mark.asyncio
-async def test_proxy_start_unix(ca_paths: tuple[Path, Path, Path], tmp_path: Path) -> None:
+async def test_proxy_start_unix(ca_paths: tuple[Path, Path, Path], short_tmp_parent: Path) -> None:
     """Proxy can listen on a Unix socket."""
     cert_path, key_path, _ = ca_paths
     rules = parse_rules(["GET example.com/**"])
     proxy = EgressProxy(rules, cert_path, key_path)
 
-    sock_path = tmp_path / "test.sock"
+    # short_tmp_parent (not tmp_path): the AF_UNIX path cap overflows on macOS.
+    sock_path = short_tmp_parent / "test.sock"
     await proxy.start_unix(sock_path)
 
     # Socket file is created
diff --git a/tests/inner/test_terminal.py b/tests/inner/test_terminal.py
index 95b6c89551..caea5d5325 100644
--- a/tests/inner/test_terminal.py
+++ b/tests/inner/test_terminal.py
@@ -368,7 +368,9 @@ async def fake_create_subprocess_exec(
 
 @pytest.mark.skipif(shutil.which("tmux") is None, reason="requires a real tmux binary")
 @pytest.mark.asyncio
-async def test_server_survives_inner_process_exit_real_tmux(tmp_path: Path) -> None:
+async def test_server_survives_inner_process_exit_real_tmux(
+    tmp_path: Path, short_tmp_parent: Path
+) -> None:
     """
     The private tmux server outlives an inner-process exit (issue #540).
 
@@ -384,7 +386,9 @@ async def test_server_survives_inner_process_exit_real_tmux(tmp_path: Path) -> N
     instance = TerminalInstance(
         name="bash",
         session_key="s1",
-        socket_path=tmp_path / "tmux.sock",
+        # short_tmp_parent (not tmp_path): tmux's AF_UNIX socket path overflows
+        # the macOS 103-byte cap when it embeds pytest's long tmp_path (#4279).
+        socket_path=short_tmp_parent / "tmux.sock",
         private_dir=tmp_path,
         command="sh",
         args=["-c", "exit 0"],
diff --git a/tests/runner/test_app_sessions_native_terminals_runtime.py b/tests/runner/test_app_sessions_native_terminals_runtime.py
index 91e310e78e..bd3f7a1311 100644
--- a/tests/runner/test_app_sessions_native_terminals_runtime.py
+++ b/tests/runner/test_app_sessions_native_terminals_runtime.py
@@ -158,6 +158,11 @@ async def test_create_session_threads_workspace_to_pi_cwd(
 ) -> None:
     """Pi pre-spawn receives the session workspace, not the bundle dir."""
     monkeypatch.setenv("OMNIGENT_CONFIG_HOME", str(tmp_path / "config-home"))
+    # Isolate $HOME too: ambient provider detection reads ~/.databrickscfg,
+    # which on a multi-profile Databricks dev box otherwise 400s the create
+    # with "match <host>; use --profile" (issue #4279).
+    monkeypatch.setenv("HOME", str(tmp_path / "config-home"))
+    monkeypatch.setenv("USERPROFILE", str(tmp_path / "config-home"))
     session_id = "18f39ab73f49285e4dab0c80ff7b8455"
     runner_workspace = tmp_path / "runner-workspace"
     runner_workspace.mkdir()
diff --git a/tests/runtime/test_openai_agents_sdk_spawn_env.py b/tests/runtime/test_openai_agents_sdk_spawn_env.py
index 1f01f39003..1b515a1a7b 100644
--- a/tests/runtime/test_openai_agents_sdk_spawn_env.py
+++ b/tests/runtime/test_openai_agents_sdk_spawn_env.py
@@ -37,11 +37,19 @@ def _isolate_global_config(monkeypatch: pytest.MonkeyPatch, tmp_path: Path) -> N
     this file so tests that don't explicitly set up a global config are
     not affected by the developer's real ``~/.omnigent/config.yaml``.
 
+    Also redirect ``$HOME`` (and ``$USERPROFILE`` on Windows) to that temp
+    dir: ambient provider detection reads ``~/.codex/config.toml`` and
+    ``~/.databrickscfg``, which live under HOME, not OMNIGENT_CONFIG_HOME. On
+    a Databricks developer machine those otherwise pin a provider and break
+    these tests (issue #4279).
+
     Tests that need a specific global config write their own config.yaml
     into a separate temp dir and set OMNIGENT_CONFIG_HOME themselves —
     that setenv call wins because monkeypatch applies in call order.
     """
     monkeypatch.setenv("OMNIGENT_CONFIG_HOME", str(tmp_path))
+    monkeypatch.setenv("HOME", str(tmp_path))
+    monkeypatch.setenv("USERPROFILE", str(tmp_path))
 
 
 def _make_spec(
diff --git a/tests/runtime/test_provider_spawn_env.py b/tests/runtime/test_provider_spawn_env.py
index 30ade9eac7..dd20b843af 100644
--- a/tests/runtime/test_provider_spawn_env.py
+++ b/tests/runtime/test_provider_spawn_env.py
@@ -80,6 +80,25 @@ def _clear_ambient_keys(monkeypatch: pytest.MonkeyPatch) -> None:
     )
 
 
+@pytest.fixture(autouse=True)
+def _isolate_ambient_provider_state(monkeypatch: pytest.MonkeyPatch, tmp_path: Path) -> None:
+    """Isolate host state ambient provider detection reads (issue #4279).
+
+    Two ambient sources leak past ``$OMNIGENT_CONFIG_HOME``:
+
+    - ``~/.codex/config.toml`` and ``~/.databrickscfg`` live under ``$HOME``
+      (``$USERPROFILE`` on Windows), so redirect it to an empty temp dir.
+    - On macOS ``_claude_login_detected()`` falls back to ``claude auth status``,
+      which reads the **Keychain** — no ``$HOME`` override can hide it. A
+      signed-in Mac would inject a ``subscription`` provider that outranks the
+      test's own configured entry, so stub it to "not logged in". Tests that
+      exercise a detected login can still override this in call order.
+    """
+    monkeypatch.setenv("HOME", str(tmp_path))
+    monkeypatch.setenv("USERPROFILE", str(tmp_path))
+    monkeypatch.setattr("omnigent.onboarding.ambient._claude_login_detected", lambda: False)
+
+
 @pytest.fixture
 def config_home(monkeypatch: pytest.MonkeyPatch, tmp_path: Path) -> Path:
     """