diff --git a/apps/backend/core/auth.py b/apps/backend/core/auth.py
index 53f2f7b0..cbd46efa 100644
--- a/apps/backend/core/auth.py
+++ b/apps/backend/core/auth.py
@@ -10,6 +10,17 @@ import json
 import os
 import platform
 import subprocess
+from typing import TYPE_CHECKING
+
+# Optional import for Linux secret-service support
+# secretstorage provides access to the Freedesktop.org Secret Service API via DBus
+if TYPE_CHECKING:
+    import secretstorage
+else:
+    try:
+        import secretstorage  # type: ignore[import-untyped]
+    except ImportError:
+        secretstorage = None  # type: ignore[assignment]
 
 # Priority order for auth token resolution
 # NOTE: We intentionally do NOT fall back to ANTHROPIC_API_KEY.
@@ -48,7 +59,7 @@ def get_token_from_keychain() -> str | None:
     Reads Claude Code credentials from:
     - macOS: Keychain
     - Windows: Credential Manager
-    - Linux: Not yet supported (use env var)
+    - Linux: Secret Service API (via dbus/secretstorage)
 
     Returns:
         Token string if found, None otherwise
@@ -60,8 +71,8 @@ def get_token_from_keychain() -> str | None:
     elif system == "Windows":
         return _get_token_from_windows_credential_files()
     else:
-        # Linux: secret-service not yet implemented
-        return None
+        # Linux: use secret-service API via DBus
+        return _get_token_from_linux_secret_service()
 
 
 def _get_token_from_macos_keychain() -> str | None:
@@ -131,6 +142,82 @@ def _get_token_from_windows_credential_files() -> str | None:
         return None
 
 
+def _get_token_from_linux_secret_service() -> str | None:
+    """Get token from Linux Secret Service API via DBus.
+
+    Claude Code on Linux stores credentials in the Secret Service API
+    using the 'org.freedesktop.secrets' collection. This implementation
+    uses the secretstorage library which communicates via DBus.
+
+    The credential is stored with:
+    - Label: "Claude Code-credentials"
+    - Attributes: {application: "claude-code"}
+
+    Returns:
+        Token string if found, None otherwise
+    """
+    if secretstorage is None:
+        # secretstorage not installed, fall back to env var
+        return None
+
+    try:
+        # Get the default collection (typically "login" keyring)
+        # secretstorage handles DBus communication internally
+        try:
+            collection = secretstorage.get_default_collection(None)
+        except (
+            AttributeError,
+            secretstorage.exceptions.SecretServiceNotAvailableException,
+        ):
+            # DBus not available or secret-service not running
+            return None
+
+        if collection.is_locked():
+            # Try to unlock the collection (may prompt user for password)
+            try:
+                collection.unlock()
+            except secretstorage.exceptions.SecretStorageException:
+                # User cancelled or unlock failed
+                return None
+
+        # Search for items with our application attribute
+        items = collection.search_items({"application": "claude-code"})
+
+        for item in items:
+            # Check if this is the Claude Code credentials item
+            label = item.get_label()
+            # Use exact match for "Claude Code-credentials" to avoid false positives
+            if label == "Claude Code-credentials":
+                # Get the secret (stored as JSON string)
+                secret = item.get_secret()
+                if not secret:
+                    continue
+
+                try:
+                    # Explicitly decode bytes to string if needed
+                    if isinstance(secret, bytes):
+                        secret = secret.decode("utf-8")
+                    data = json.loads(secret)
+                    token = data.get("claudeAiOauth", {}).get("accessToken")
+
+                    if token and token.startswith("sk-ant-oat01-"):
+                        return token
+                except json.JSONDecodeError:
+                    continue
+
+        return None
+
+    except (
+        secretstorage.exceptions.SecretStorageException,
+        json.JSONDecodeError,
+        KeyError,
+        AttributeError,
+        TypeError,
+    ):
+        # Any error with secret-service, fall back to env var
+        return None
+
+
 def get_auth_token() -> str | None:
     """
     Get authentication token from environment variables or system credential store.
@@ -138,7 +225,7 @@ def get_auth_token() -> str | None:
     Checks multiple sources in priority order:
     1. CLAUDE_CODE_OAUTH_TOKEN (env var)
     2. ANTHROPIC_AUTH_TOKEN (CCR/proxy env var for enterprise setups)
-    3. System credential store (macOS Keychain, Windows Credential Manager)
+    3. System credential store (macOS Keychain, Windows Credential Manager, Linux Secret Service)
 
     NOTE: ANTHROPIC_API_KEY is intentionally NOT supported to prevent
     silent billing to user's API credits when OAuth is misconfigured.
@@ -171,7 +258,7 @@ def get_auth_token_source() -> str | None:
         elif system == "Windows":
             return "Windows Credential Files"
         else:
-            return "System Credential Store"
+            return "Linux Secret Service"
 
     return None
 
@@ -208,10 +295,12 @@ def require_auth_token() -> str:
                 "Check: %LOCALAPPDATA%\\Claude\\credentials.json"
             )
         else:
+            # Linux
             error_msg += (
                 "To authenticate:\n"
                 "  1. Run: claude setup-token\n"
-                "  2. Set CLAUDE_CODE_OAUTH_TOKEN in your .env file"
+                "  2. The token will be saved to the system secret service (gnome-keyring/kwallet)\n\n"
+                "If secret-service is not available, set CLAUDE_CODE_OAUTH_TOKEN in your .env file."
             )
         raise ValueError(error_msg)
     return token
diff --git a/apps/backend/requirements.txt b/apps/backend/requirements.txt
index aa5e2713..78331e01 100644
--- a/apps/backend/requirements.txt
+++ b/apps/backend/requirements.txt
@@ -5,6 +5,11 @@ python-dotenv>=1.0.0
 # TOML parsing fallback for Python < 3.11
 tomli>=2.0.0; python_version < "3.11"
 
+# Linux Secret Service support for credential storage
+# Provides access to the Freedesktop.org Secret Service API via DBus
+# Used on Linux to store OAuth tokens in gnome-keyring/kwallet
+secretstorage>=3.3.3; sys_platform == "linux"
+
 # Memory Integration - LadybugDB (embedded graph database)
 # Requires Python 3.12+ (no Docker required)
 real_ladybug>=0.13.0; python_version >= "3.12"
diff --git a/tests/test_auth.py b/tests/test_auth.py
index d7c5b80b..d5a41288 100644
--- a/tests/test_auth.py
+++ b/tests/test_auth.py
@@ -1,129 +1,608 @@
 #!/usr/bin/env python3
 """
-Tests for core/auth module.
+Tests for Authentication System
+================================
 
-Tests authentication token management and SDK environment variable handling,
-including the PYTHONPATH isolation fix for ACS-251.
+Tests the auth.py module functionality including:
+- Environment variable token resolution
+- System credential store integration (macOS, Windows, Linux)
+- Token source detection
+- Token validation and format checking
 """
 
+import json
+import os
 import platform
-from unittest.mock import patch
+from unittest.mock import MagicMock, Mock
 
-from core.auth import get_sdk_env_vars
+import pytest
+from core.auth import (
+    AUTH_TOKEN_ENV_VARS,
+    ensure_claude_code_oauth_token,
+    get_auth_token,
+    get_auth_token_source,
+    get_sdk_env_vars,
+    get_token_from_keychain,
+    require_auth_token,
+)
 
 
-class TestGetSdkEnvVars:
-    """Tests for get_sdk_env_vars() function."""
+class TestEnvVarTokenResolution:
+    """Tests for environment variable token resolution."""
 
-    def test_pythonpath_is_always_set_in_result(self, monkeypatch):
-        """
-        PYTHONPATH should always be present in result, even when empty.
+    @pytest.fixture(autouse=True)
+    def clear_env(self):
+        """Clear auth environment variables before each test."""
+        for var in AUTH_TOKEN_ENV_VARS:
+            os.environ.pop(var, None)
+        yield
+        # Cleanup after test
+        for var in AUTH_TOKEN_ENV_VARS:
+            os.environ.pop(var, None)
 
-        When no SDK env vars are set, PYTHONPATH is still explicitly set to
-        empty string to override any inherited value from the parent process.
-        """
-        # Clear all SDK env vars
-        for var in [
-            "ANTHROPIC_BASE_URL",
-            "ANTHROPIC_AUTH_TOKEN",
-            "ANTHROPIC_MODEL",
-            "NO_PROXY",
-            "DISABLE_TELEMETRY",
-        ]:
-            monkeypatch.delenv(var, raising=False)
+    def test_claude_oauth_token_from_env(self):
+        """Reads CLAUDE_CODE_OAUTH_TOKEN from environment."""
+        test_token = "sk-ant-oat01-test-token"
+        os.environ["CLAUDE_CODE_OAUTH_TOKEN"] = test_token
 
-        result = get_sdk_env_vars()
+        token = get_auth_token()
+        assert token == test_token
 
-        # PYTHONPATH should always be present, even if empty
-        assert "PYTHONPATH" in result
-        assert result["PYTHONPATH"] == ""
+    def test_anthropic_auth_token_from_env(self):
+        """Reads ANTHROPIC_AUTH_TOKEN from environment."""
+        test_token = "sk-ant-oat01-test-enterprise-token"
+        os.environ["ANTHROPIC_AUTH_TOKEN"] = test_token
 
-    def test_includes_anthropic_base_url_when_set(self, monkeypatch):
-        """Should include ANTHROPIC_BASE_URL when set."""
-        monkeypatch.setenv("ANTHROPIC_BASE_URL", "https://api.example.com")
+        token = get_auth_token()
+        assert token == test_token
 
-        result = get_sdk_env_vars()
+    def test_claude_oauth_takes_precedence(self):
+        """CLAUDE_CODE_OAUTH_TOKEN takes precedence over ANTHROPIC_AUTH_TOKEN."""
+        claude_token = "sk-ant-oat01-claude-token"
+        anthropic_token = "sk-ant-oat01-anthropic-token"
 
-        assert result.get("ANTHROPIC_BASE_URL") == "https://api.example.com"
+        os.environ["ANTHROPIC_AUTH_TOKEN"] = anthropic_token
+        os.environ["CLAUDE_CODE_OAUTH_TOKEN"] = claude_token
 
-    def test_includes_anthropic_auth_token_when_set(self, monkeypatch):
-        """Should include ANTHROPIC_AUTH_TOKEN when set."""
-        monkeypatch.setenv("ANTHROPIC_AUTH_TOKEN", "sk-test-token")
+        token = get_auth_token()
+        assert token == claude_token
 
-        result = get_sdk_env_vars()
+    def test_no_token_returns_none(self):
+        """Returns None when no auth token is configured."""
+        token = get_auth_token()
+        assert token is None
 
-        assert result.get("ANTHROPIC_AUTH_TOKEN") == "sk-test-token"
+    def test_token_source_from_env(self):
+        """Identifies environment variable as token source."""
+        os.environ["CLAUDE_CODE_OAUTH_TOKEN"] = "sk-ant-oat01-test-token"
 
-    def test_pythonpath_isolation_from_parent_process(self, monkeypatch):
-        """
-        Test ACS-251 fix: PYTHONPATH from parent process should be overridden.
+        source = get_auth_token_source()
+        assert source == "CLAUDE_CODE_OAUTH_TOKEN"
 
-        This ensures that Auto-Claude's PYTHONPATH (which may point to Python 3.12
-        packages) doesn't pollute agent subprocess environments, preventing
-        failures when working on external projects with different Python versions.
-        """
-        # Simulate parent process having a PYTHONPATH set
-        monkeypatch.setenv(
-            "PYTHONPATH",
-            "/path/to/auto-claude/backend:/path/to/python3.12/site-packages",
+    def test_empty_token_ignored(self):
+        """Empty string tokens are ignored."""
+        os.environ["CLAUDE_CODE_OAUTH_TOKEN"] = ""
+        os.environ["ANTHROPIC_AUTH_TOKEN"] = "sk-ant-oat01-test-token"
+
+        token = get_auth_token()
+        # Should get ANTHROPIC_AUTH_TOKEN since CLAUDE_CODE_OAUTH_TOKEN is empty
+        assert token == "sk-ant-oat01-test-token"
+
+
+class TestMacOSKeychain:
+    """Tests for macOS keychain token retrieval."""
+
+    def test_macos_keychain_success(self, monkeypatch):
+        """Successfully retrieves token from macOS keychain."""
+        test_token = "sk-ant-oat01-macos-token"
+        credentials = json.dumps({"claudeAiOauth": {"accessToken": test_token}})
+
+        mock_result = Mock()
+        mock_result.returncode = 0
+        mock_result.stdout = credentials
+
+        monkeypatch.setattr(platform, "system", lambda: "Darwin")
+        monkeypatch.setattr("subprocess.run", Mock(return_value=mock_result))
+
+        token = get_token_from_keychain()
+        assert token == test_token
+
+    def test_macos_keychain_command_failure(self, monkeypatch):
+        """Returns None when security command fails."""
+        mock_result = Mock()
+        mock_result.returncode = 1
+
+        monkeypatch.setattr(platform, "system", lambda: "Darwin")
+        monkeypatch.setattr("subprocess.run", Mock(return_value=mock_result))
+
+        token = get_token_from_keychain()
+        assert token is None
+
+    def test_macos_keychain_invalid_json(self, monkeypatch):
+        """Returns None when keychain returns invalid JSON."""
+        mock_result = Mock()
+        mock_result.returncode = 0
+        mock_result.stdout = "invalid json"
+
+        monkeypatch.setattr(platform, "system", lambda: "Darwin")
+        monkeypatch.setattr("subprocess.run", Mock(return_value=mock_result))
+
+        token = get_token_from_keychain()
+        assert token is None
+
+    def test_macos_keychain_invalid_token_format(self, monkeypatch):
+        """Returns None when token doesn't start with sk-ant-oat01-."""
+        credentials = json.dumps({"claudeAiOauth": {"accessToken": "invalid-token"}})
+
+        mock_result = Mock()
+        mock_result.returncode = 0
+        mock_result.stdout = credentials
+
+        monkeypatch.setattr(platform, "system", lambda: "Darwin")
+        monkeypatch.setattr("subprocess.run", Mock(return_value=mock_result))
+
+        token = get_token_from_keychain()
+        assert token is None
+
+
+class TestWindowsCredentialFiles:
+    """Tests for Windows credential file token retrieval."""
+
+    def test_windows_credential_file_success(self, monkeypatch, tmp_path):
+        """Successfully retrieves token from Windows credential file."""
+        test_token = "sk-ant-oat01-windows-token"
+        credentials = json.dumps({"claudeAiOauth": {"accessToken": test_token}})
+
+        # Create a temporary credential file
+        cred_file = tmp_path / ".credentials.json"
+        cred_file.write_text(credentials)
+
+        monkeypatch.setattr(platform, "system", lambda: "Windows")
+        monkeypatch.setattr(
+            os.path, "expandvars", lambda p: str(cred_file).replace("\\", "/")
         )
 
-        result = get_sdk_env_vars()
+        token = get_token_from_keychain()
+        assert token == test_token
 
-        # PYTHONPATH should be explicitly overridden to empty string
-        # This prevents the SDK from inheriting the parent's PYTHONPATH
-        assert "PYTHONPATH" in result
-        assert result["PYTHONPATH"] == ""
+    def test_windows_credential_file_not_found(self, monkeypatch):
+        """Returns None when credential file doesn't exist."""
+        monkeypatch.setattr(platform, "system", lambda: "Windows")
+        monkeypatch.setattr(os.path, "exists", lambda x: False)
 
-    def test_skips_empty_env_vars(self, monkeypatch):
-        """Should not include env vars with empty values."""
-        monkeypatch.setenv("ANTHROPIC_BASE_URL", "")
-        monkeypatch.setenv("ANTHROPIC_AUTH_TOKEN", "")
+        token = get_token_from_keychain()
+        assert token is None
 
-        result = get_sdk_env_vars()
+    def test_windows_credential_file_invalid_json(self, monkeypatch, tmp_path):
+        """Returns None when credential file contains invalid JSON."""
+        cred_file = tmp_path / ".credentials.json"
+        cred_file.write_text("invalid json")
 
-        # Empty vars should not be in result (except PYTHONPATH which is explicitly set)
-        assert "ANTHROPIC_BASE_URL" not in result
-        assert "ANTHROPIC_AUTH_TOKEN" not in result
-        # PYTHONPATH should still be present as explicit override
-        assert result["PYTHONPATH"] == ""
+        monkeypatch.setattr(platform, "system", lambda: "Windows")
+        monkeypatch.setattr(
+            os.path, "expandvars", lambda p: str(cred_file).replace("\\", "/")
+        )
+        monkeypatch.setattr(os.path, "exists", lambda x: str(x).endswith(".json"))
 
-    def test_on_windows_auto_detects_git_bash_path(self, monkeypatch):
-        """On Windows, should auto-detect git-bash path if not set."""
-        # Mock platform.system to simulate Windows for cross-platform testing
-        with patch.object(platform, "system", return_value="Windows"):
-            # Mock _find_git_bash_path to return a path
-            with patch(
-                "core.auth._find_git_bash_path",
-                return_value="C:/Program Files/Git/bin/bash.exe",
-            ):
-                monkeypatch.delenv("CLAUDE_CODE_GIT_BASH_PATH", raising=False)
+        token = get_token_from_keychain()
+        assert token is None
 
-                result = get_sdk_env_vars()
 
-                assert (
-                    result.get("CLAUDE_CODE_GIT_BASH_PATH")
-                    == "C:/Program Files/Git/bin/bash.exe"
-                )
+class TestLinuxSecretService:
+    """Tests for Linux Secret Service token retrieval."""
 
-    def test_preserves_existing_git_bash_path_on_windows(self, monkeypatch):
-        """On Windows, should preserve existing CLAUDE_CODE_GIT_BASH_PATH."""
-        # Mock platform.system to simulate Windows for cross-platform testing
-        with patch.object(platform, "system", return_value="Windows"):
-            monkeypatch.setenv("CLAUDE_CODE_GIT_BASH_PATH", "C:/Custom/bash.exe")
+    def test_linux_secret_service_not_installed(self, monkeypatch):
+        """Returns None when secretstorage is not installed."""
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        monkeypatch.setattr("core.auth.secretstorage", None)
 
-            result = get_sdk_env_vars()
+        token = get_token_from_keychain()
+        assert token is None
 
-            assert result.get("CLAUDE_CODE_GIT_BASH_PATH") == "C:/Custom/bash.exe"
+    def test_linux_secret_service_dbus_not_available(self, monkeypatch):
+        """Returns None when DBus is not available."""
+        mock_ss = MagicMock()
+        mock_ss.exceptions = MagicMock()
+        mock_ss.exceptions.SecretServiceNotAvailableException = Exception
 
-    def test_on_non_windows_git_bash_not_added(self, monkeypatch):
-        """On non-Windows platforms, CLAUDE_CODE_GIT_BASH_PATH should not be auto-added."""
-        # Mock platform.system to simulate Linux for cross-platform testing
-        # When platform is not Windows, _find_git_bash_path is never called
-        with patch.object(platform, "system", return_value="Linux"):
-            monkeypatch.delenv("CLAUDE_CODE_GIT_BASH_PATH", raising=False)
+        # Make get_default_collection raise exception
+        mock_ss.get_default_collection.side_effect = Exception("DBus not available")
 
-            result = get_sdk_env_vars()
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        monkeypatch.setattr("core.auth.secretstorage", mock_ss)
 
-            # Should not have CLAUDE_CODE_GIT_BASH_PATH
-            assert "CLAUDE_CODE_GIT_BASH_PATH" not in result
+        token = get_token_from_keychain()
+        assert token is None
+
+    def test_linux_secret_service_success(self, monkeypatch):
+        """Successfully retrieves token from Linux secret service."""
+        test_token = "sk-ant-oat01-linux-token"
+        credentials = json.dumps({"claudeAiOauth": {"accessToken": test_token}})
+
+        # Mock secretstorage
+        mock_ss = MagicMock()
+        mock_ss.exceptions = MagicMock()
+        mock_ss.exceptions.SecretServiceNotAvailableException = Exception
+        mock_ss.exceptions.SecretStorageException = Exception
+
+        # Mock collection
+        mock_collection = MagicMock()
+        mock_collection.is_locked.return_value = False
+        mock_collection.unlock.return_value = None
+
+        # Mock item
+        mock_item = MagicMock()
+        mock_item.get_label.return_value = "Claude Code-credentials"
+        mock_item.get_secret.return_value = credentials
+        mock_item.is_locked.return_value = False
+
+        mock_collection.search_items.return_value = [mock_item]
+        mock_ss.get_default_collection.return_value = mock_collection
+
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        monkeypatch.setattr("core.auth.secretstorage", mock_ss)
+
+        token = get_token_from_keychain()
+        assert token == test_token
+
+    def test_linux_secret_service_exact_label_match_only(self, monkeypatch):
+        """Only matches exact 'Claude Code-credentials' label."""
+        test_token = "sk-ant-oat01-linux-token"
+        credentials = json.dumps({"claudeAiOauth": {"accessToken": test_token}})
+
+        mock_ss = MagicMock()
+        mock_ss.exceptions = MagicMock()
+        mock_ss.exceptions.SecretServiceNotAvailableException = Exception
+        mock_ss.exceptions.SecretStorageException = Exception
+
+        mock_collection = MagicMock()
+        mock_collection.is_locked.return_value = False
+
+        # Mock item with similar but not exact label
+        mock_item = MagicMock()
+        mock_item.get_label.return_value = (
+            "Some-Claude-Code-Thing"  # Similar but not exact
+        )
+        mock_item.get_secret.return_value = credentials
+
+        mock_collection.search_items.return_value = [mock_item]
+        mock_ss.get_default_collection.return_value = mock_collection
+
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        monkeypatch.setattr("core.auth.secretstorage", mock_ss)
+
+        token = get_token_from_keychain()
+        # Should return None because label doesn't match exactly
+        assert token is None
+
+    def test_linux_secret_service_locked_collection_unlock_fails(self, monkeypatch):
+        """Returns None when collection is locked and unlock fails."""
+
+        mock_ss = MagicMock()
+        mock_ss.exceptions = MagicMock()
+        mock_ss.exceptions.SecretServiceNotAvailableException = Exception
+        mock_ss.exceptions.SecretStorageException = Exception
+
+        mock_collection = MagicMock()
+        mock_collection.is_locked.return_value = True
+        mock_collection.unlock.side_effect = Exception("Unlock failed")
+
+        mock_ss.get_default_collection.return_value = mock_collection
+
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        monkeypatch.setattr("core.auth.secretstorage", mock_ss)
+
+        token = get_token_from_keychain()
+        assert token is None
+
+    def test_linux_secret_service_no_matching_item(self, monkeypatch):
+        """Returns None when no matching credential found."""
+        mock_ss = MagicMock()
+        mock_ss.exceptions = MagicMock()
+        mock_ss.exceptions.SecretServiceNotAvailableException = Exception
+        mock_ss.exceptions.SecretStorageException = Exception
+
+        mock_collection = MagicMock()
+        mock_collection.is_locked.return_value = False
+        mock_collection.search_items.return_value = []  # No items found
+
+        mock_ss.get_default_collection.return_value = mock_collection
+
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        monkeypatch.setattr("core.auth.secretstorage", mock_ss)
+
+        token = get_token_from_keychain()
+        assert token is None
+
+    def test_linux_secret_service_invalid_json(self, monkeypatch):
+        """Returns None when stored secret contains invalid JSON."""
+        mock_ss = MagicMock()
+        mock_ss.exceptions = MagicMock()
+        mock_ss.exceptions.SecretServiceNotAvailableException = Exception
+        mock_ss.exceptions.SecretStorageException = Exception
+
+        mock_collection = MagicMock()
+        mock_collection.is_locked.return_value = False
+
+        mock_item = MagicMock()
+        mock_item.get_label.return_value = "Claude Code-credentials"
+        mock_item.get_secret.return_value = "invalid json"
+
+        mock_collection.search_items.return_value = [mock_item]
+        mock_ss.get_default_collection.return_value = mock_collection
+
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        monkeypatch.setattr("core.auth.secretstorage", mock_ss)
+
+        token = get_token_from_keychain()
+        assert token is None
+
+    def test_linux_secret_service_invalid_token_format(self, monkeypatch):
+        """Returns None when token doesn't start with sk-ant-oat01-."""
+        credentials = json.dumps({"claudeAiOauth": {"accessToken": "invalid-token"}})
+
+        mock_ss = MagicMock()
+        mock_ss.exceptions = MagicMock()
+        mock_ss.exceptions.SecretServiceNotAvailableException = Exception
+        mock_ss.exceptions.SecretStorageException = Exception
+
+        mock_collection = MagicMock()
+        mock_collection.is_locked.return_value = False
+
+        mock_item = MagicMock()
+        mock_item.get_label.return_value = "Claude Code-credentials"
+        mock_item.get_secret.return_value = credentials
+
+        mock_collection.search_items.return_value = [mock_item]
+        mock_ss.get_default_collection.return_value = mock_collection
+
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        monkeypatch.setattr("core.auth.secretstorage", mock_ss)
+
+        token = get_token_from_keychain()
+        assert token is None
+
+
+class TestRequireAuthToken:
+    """Tests for require_auth_token function."""
+
+    @pytest.fixture(autouse=True)
+    def clear_env(self):
+        """Clear auth environment variables before each test."""
+        for var in AUTH_TOKEN_ENV_VARS:
+            os.environ.pop(var, None)
+        yield
+        # Cleanup after test
+        for var in AUTH_TOKEN_ENV_VARS:
+            os.environ.pop(var, None)
+
+    def test_require_token_returns_valid_token(self):
+        """Returns token when valid token exists."""
+        test_token = "sk-ant-oat01-test-token"
+        os.environ["CLAUDE_CODE_OAUTH_TOKEN"] = test_token
+
+        token = require_auth_token()
+        assert token == test_token
+
+    def test_require_token_raises_when_missing(self):
+        """Raises ValueError when no token is configured."""
+        with pytest.raises(ValueError, match="No OAuth token found"):
+            require_auth_token()
+
+    def test_error_message_includes_macos_instructions(self, monkeypatch):
+        """Error message includes macOS setup instructions."""
+        monkeypatch.setattr(platform, "system", lambda: "Darwin")
+
+        with pytest.raises(ValueError) as exc_info:
+            require_auth_token()
+
+        error_msg = str(exc_info.value)
+        assert "macOS Keychain" in error_msg
+        assert "claude setup-token" in error_msg
+
+    def test_error_message_includes_windows_instructions(self, monkeypatch):
+        """Error message includes Windows setup instructions."""
+        monkeypatch.setattr(platform, "system", lambda: "Windows")
+
+        with pytest.raises(ValueError) as exc_info:
+            require_auth_token()
+
+        error_msg = str(exc_info.value)
+        assert "Windows Credential Manager" in error_msg
+        assert "claude setup-token" in error_msg
+
+    def test_error_message_includes_linux_instructions(self, monkeypatch):
+        """Error message includes Linux setup instructions."""
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+
+        with pytest.raises(ValueError) as exc_info:
+            require_auth_token()
+
+        error_msg = str(exc_info.value)
+        assert "secret service" in error_msg.lower()
+        assert "gnome-keyring" in error_msg.lower()
+        assert "claude setup-token" in error_msg
+
+
+class TestEnsureClaudeCodeOAuthToken:
+    """Tests for ensure_claude_code_oauth_token function."""
+
+    @pytest.fixture(autouse=True)
+    def clear_env(self):
+        """Clear auth environment variables before each test."""
+        for var in AUTH_TOKEN_ENV_VARS:
+            os.environ.pop(var, None)
+        os.environ.pop("CLAUDE_CODE_OAUTH_TOKEN", None)
+        yield
+        # Cleanup after test
+        for var in AUTH_TOKEN_ENV_VARS:
+            os.environ.pop(var, None)
+        os.environ.pop("CLAUDE_CODE_OAUTH_TOKEN", None)
+
+    def test_does_nothing_when_already_set(self):
+        """Doesn't modify env var when CLAUDE_CODE_OAUTH_TOKEN is already set."""
+        existing_token = "sk-ant-oat01-existing-token"
+        os.environ["CLAUDE_CODE_OAUTH_TOKEN"] = existing_token
+
+        ensure_claude_code_oauth_token()
+
+        assert os.environ["CLAUDE_CODE_OAUTH_TOKEN"] == existing_token
+
+    def test_copies_from_anthropic_auth_token(self):
+        """Copies ANTHROPIC_AUTH_TOKEN to CLAUDE_CODE_OAUTH_TOKEN."""
+        anthropic_token = "sk-ant-oat01-anthropic-token"
+        os.environ["ANTHROPIC_AUTH_TOKEN"] = anthropic_token
+
+        ensure_claude_code_oauth_token()
+
+        assert os.environ["CLAUDE_CODE_OAUTH_TOKEN"] == anthropic_token
+
+    def test_does_nothing_when_no_token_available(self, monkeypatch):
+        """Doesn't set env var when no auth token is available."""
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        # Ensure keychain returns None
+        monkeypatch.setattr("core.auth.get_token_from_keychain", lambda: None)
+
+        ensure_claude_code_oauth_token()
+
+        assert "CLAUDE_CODE_OAUTH_TOKEN" not in os.environ
+
+
+class TestTokenSourceDetection:
+    """Tests for get_auth_token_source function."""
+
+    @pytest.fixture(autouse=True)
+    def clear_env(self):
+        """Clear auth environment variables before each test."""
+        for var in AUTH_TOKEN_ENV_VARS:
+            os.environ.pop(var, None)
+        yield
+        # Cleanup after test
+        for var in AUTH_TOKEN_ENV_VARS:
+            os.environ.pop(var, None)
+
+    def test_source_env_var_claude_oauth(self):
+        """Identifies CLAUDE_CODE_OAUTH_TOKEN as source."""
+        os.environ["CLAUDE_CODE_OAUTH_TOKEN"] = "sk-ant-oat01-test-token"
+
+        source = get_auth_token_source()
+        assert source == "CLAUDE_CODE_OAUTH_TOKEN"
+
+    def test_source_env_var_anthropic_auth(self):
+        """Identifies ANTHROPIC_AUTH_TOKEN as source."""
+        os.environ["ANTHROPIC_AUTH_TOKEN"] = "sk-ant-oat01-test-token"
+
+        source = get_auth_token_source()
+        assert source == "ANTHROPIC_AUTH_TOKEN"
+
+    def test_source_macos_keychain(self, monkeypatch):
+        """Identifies macOS Keychain as source."""
+        test_token = "sk-ant-oat01-macos-token"
+        credentials = json.dumps({"claudeAiOauth": {"accessToken": test_token}})
+
+        mock_result = Mock()
+        mock_result.returncode = 0
+        mock_result.stdout = credentials
+
+        monkeypatch.setattr(platform, "system", lambda: "Darwin")
+        monkeypatch.setattr("subprocess.run", Mock(return_value=mock_result))
+
+        source = get_auth_token_source()
+        assert source == "macOS Keychain"
+
+    def test_source_windows_credential_files(self, monkeypatch, tmp_path):
+        """Identifies Windows Credential Files as source."""
+        test_token = "sk-ant-oat01-windows-token"
+        credentials = json.dumps({"claudeAiOauth": {"accessToken": test_token}})
+
+        cred_file = tmp_path / ".credentials.json"
+        cred_file.write_text(credentials)
+
+        monkeypatch.setattr(platform, "system", lambda: "Windows")
+        monkeypatch.setattr(
+            os.path, "expandvars", lambda p: str(cred_file).replace("\\", "/")
+        )
+
+        source = get_auth_token_source()
+        assert source == "Windows Credential Files"
+
+    def test_source_linux_secret_service(self, monkeypatch):
+        """Identifies Linux Secret Service as source."""
+        test_token = "sk-ant-oat01-linux-token"
+        credentials = json.dumps({"claudeAiOauth": {"accessToken": test_token}})
+
+        mock_ss = MagicMock()
+        mock_ss.exceptions = MagicMock()
+        mock_ss.exceptions.SecretServiceNotAvailableException = Exception
+        mock_ss.exceptions.SecretStorageException = Exception
+
+        mock_collection = MagicMock()
+        mock_collection.is_locked.return_value = False
+
+        mock_item = MagicMock()
+        mock_item.get_label.return_value = "Claude Code-credentials"
+        mock_item.get_secret.return_value = credentials
+
+        mock_collection.search_items.return_value = [mock_item]
+        mock_ss.get_default_collection.return_value = mock_collection
+
+        monkeypatch.setattr(platform, "system", lambda: "Linux")
+        monkeypatch.setattr("core.auth.secretstorage", mock_ss)
+
+        source = get_auth_token_source()
+        assert source == "Linux Secret Service"
+
+    def test_source_none_when_not_found(self):
+        """Returns None when no token source is found."""
+        source = get_auth_token_source()
+        assert source is None
+
+
+class TestSdkEnvVars:
+    """Tests for get_sdk_env_vars function."""
+
+    def test_returns_non_empty_vars(self, monkeypatch):
+        """Only returns non-empty environment variables."""
+        monkeypatch.setenv("ANTHROPIC_BASE_URL", "https://api.anthropic.com")
+        monkeypatch.setenv("ANTHROPIC_MODEL", "")  # Empty, should be excluded
+        monkeypatch.setenv("DISABLE_TELEMETRY", "1")
+
+        env = get_sdk_env_vars()
+
+        assert "ANTHROPIC_BASE_URL" in env
+        assert env["ANTHROPIC_BASE_URL"] == "https://api.anthropic.com"
+        assert "ANTHROPIC_MODEL" not in env  # Empty value excluded
+        assert "DISABLE_TELEMETRY" in env
+        assert env["DISABLE_TELEMETRY"] == "1"
+
+    def test_includes_claude_git_bash_on_windows(self, monkeypatch):
+        """Auto-detects git-bash path on Windows."""
+        monkeypatch.setattr(platform, "system", lambda: "Windows")
+        monkeypatch.setattr(
+            "core.auth._find_git_bash_path",
+            lambda: "C:\\Program Files\\Git\\bin\\bash.exe",
+        )
+
+        env = get_sdk_env_vars()
+
+        assert "CLAUDE_CODE_GIT_BASH_PATH" in env
+        assert "Git\\bin\\bash.exe" in env["CLAUDE_CODE_GIT_BASH_PATH"]
+
+    def test_does_not_include_git_bash_on_non_windows(self, monkeypatch):
+        """Doesn't include git-bash path on non-Windows platforms."""
+        monkeypatch.setattr(platform, "system", lambda: "Darwin")
+
+        env = get_sdk_env_vars()
+
+        assert "CLAUDE_CODE_GIT_BASH_PATH" not in env
+
+    def test_does_not_overwrite_existing_git_bash_path(self, monkeypatch):
+        """Respects existing CLAUDE_CODE_GIT_BASH_PATH environment variable."""
+        existing_path = "/custom/bash.exe"
+        monkeypatch.setenv("CLAUDE_CODE_GIT_BASH_PATH", existing_path)
+
+        monkeypatch.setattr(platform, "system", lambda: "Windows")
+
+        env = get_sdk_env_vars()
+
+        assert env["CLAUDE_CODE_GIT_BASH_PATH"] == existing_path