diff --git a/src/agentic_cli/config.py b/src/agentic_cli/config.py
index 9287a8d..1d5da9f 100644
--- a/src/agentic_cli/config.py
+++ b/src/agentic_cli/config.py
@@ -10,11 +10,17 @@
         set_settings(my_settings)
         settings = get_settings()
 
-    2. Context-based (isolated contexts, multi-tenant):
+    2. Context-based (isolated settings lookup, e.g. tests or per-request
+       overrides):
         with SettingsContext(my_settings):
             # Code here sees my_settings via get_settings()
             settings = get_settings()  # Returns my_settings
 
+       Note: isolation covers settings *lookup* only. API credentials are
+       exported to process-global env vars at manager initialization
+       (provider SDKs read them from the environment), so a SettingsContext
+       does not isolate credentials between contexts in one process.
+
 Settings Loading Priority (highest to lowest):
     1. Environment variables (AGENTIC_* prefix)
     2. Project config (./.{app_name}/settings.json)
diff --git a/src/agentic_cli/workflow/settings.py b/src/agentic_cli/workflow/settings.py
index a154a86..852970d 100644
--- a/src/agentic_cli/workflow/settings.py
+++ b/src/agentic_cli/workflow/settings.py
@@ -652,11 +652,25 @@ def set_thinking_effort(self, effort: str) -> None:
         object.__setattr__(self, "thinking_effort", effort)
 
     def export_api_keys_to_env(self) -> None:
-        """Export API keys to environment variables."""
+        """Export configured API keys to provider environment variables.
+
+        Provider SDKs used by the orchestrators (ADK's AnthropicLlm/Gemini,
+        LangChain clients) read credentials from process env vars. The export
+        OVERWRITES the env from this settings instance: the key fields bind
+        only via their env alias (real env vars are the highest-priority
+        source), so settings and env diverge only when the process env
+        changed after this instance loaded — e.g. an earlier manager's
+        export, or a key loaded from a class-specific env_file — and then
+        this instance's configured value must win. (The previous
+        set-if-absent guard let the first exporting manager pin credentials
+        for every later one.) A key unset in settings leaves the environment
+        untouched. Credentials are process-global; SettingsContext does not
+        isolate them.
+        """
         import os
 
-        if self.google_api_key and not os.environ.get("GOOGLE_API_KEY"):
+        if self.google_api_key:
             os.environ["GOOGLE_API_KEY"] = self.google_api_key
 
-        if self.anthropic_api_key and not os.environ.get("ANTHROPIC_API_KEY"):
+        if self.anthropic_api_key:
             os.environ["ANTHROPIC_API_KEY"] = self.anthropic_api_key
diff --git a/tests/test_config.py b/tests/test_config.py
index 054733e..a1338ef 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -248,6 +248,41 @@ def test_export_api_keys_to_env(self, temp_workspace: Path):
             else:
                 os.environ.pop("GOOGLE_API_KEY", None)
 
+    def test_export_overwrites_stale_env_key(self, temp_workspace: Path, monkeypatch):
+        """Export re-asserts this instance's key over a mutated env var.
+
+        The key fields bind only via their env alias, so settings and env
+        diverge when the process env changed after this instance loaded —
+        exactly what an earlier manager's export does in a two-settings
+        process. The previous set-if-absent guard then silently kept the
+        first manager's credentials for every later one.
+        """
+        monkeypatch.setenv("ANTHROPIC_API_KEY", "current-key")
+        settings = BaseSettings(workspace_dir=temp_workspace)
+        assert settings.anthropic_api_key == "current-key"
+
+        # Another settings instance exported its key in the meantime
+        monkeypatch.setenv("ANTHROPIC_API_KEY", "stale-key")
+
+        settings.export_api_keys_to_env()
+
+        assert os.environ["ANTHROPIC_API_KEY"] == "current-key"
+
+    def test_export_leaves_env_when_setting_unset(
+        self, temp_workspace: Path, monkeypatch, tmp_path: Path
+    ):
+        """A key unset in settings never clobbers an externally-set env var."""
+        monkeypatch.setenv("HOME", str(tmp_path))  # no user config leakage
+        monkeypatch.delenv("GOOGLE_API_KEY", raising=False)
+        settings = BaseSettings(workspace_dir=temp_workspace)
+        assert settings.google_api_key is None
+
+        monkeypatch.setenv("GOOGLE_API_KEY", "external-key")
+
+        settings.export_api_keys_to_env()
+
+        assert os.environ["GOOGLE_API_KEY"] == "external-key"
+
 
 class TestSettingsContext:
     """Tests for context-based settings management."""