diff --git a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
index 58abc438d..6fbdceb95 100644
--- a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
+++ b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
@@ -510,9 +510,17 @@ def _build_settings_value(self) -> str | None:
                 else:
                     logger.warning(f"Settings file not found: {settings_path}")
 
-        # Merge sandbox settings
+        # Merge sandbox settings. The CLI reads sandbox.failIfUnavailable as
+        # `?? false`, so an enabled sandbox that cannot start degrades to a
+        # warning and every command then runs unsandboxed. Default it to true
+        # for an enabled sandbox, matching the TypeScript SDK, so callers who
+        # asked for a sandbox fail loudly instead of silently losing it.
         if has_sandbox:
-            settings_obj["sandbox"] = self._options.sandbox
+            assert self._options.sandbox is not None
+            sandbox: dict[str, Any] = dict(self._options.sandbox)
+            if sandbox.get("enabled") is True and "failIfUnavailable" not in sandbox:
+                sandbox["failIfUnavailable"] = True
+            settings_obj["sandbox"] = sandbox
 
         return json.dumps(settings_obj)
 
diff --git a/src/claude_agent_sdk/types.py b/src/claude_agent_sdk/types.py
index b0f5ce5d5..803c42636 100644
--- a/src/claude_agent_sdk/types.py
+++ b/src/claude_agent_sdk/types.py
@@ -888,6 +888,10 @@ class SandboxSettings(TypedDict, total=False):
 
     Attributes:
         enabled: Enable bash sandboxing (macOS/Linux only). Default: False
+        failIfUnavailable: Exit with an error at startup if ``enabled`` is True but the
+            sandbox cannot start. When False the CLI only warns and commands run
+            unsandboxed. The SDK defaults this to True for an enabled sandbox,
+            matching the TypeScript SDK.
         autoAllowBashIfSandboxed: Auto-approve bash commands when sandboxed. Default: True
         excludedCommands: Commands that should run outside the sandbox (e.g., ["git", "docker"])
         allowUnsandboxedCommands: Allow commands to bypass sandbox via dangerouslyDisableSandbox.
@@ -912,6 +916,7 @@ class SandboxSettings(TypedDict, total=False):
     """
 
     enabled: bool
+    failIfUnavailable: bool
     autoAllowBashIfSandboxed: bool
     excludedCommands: list[str]
     allowUnsandboxedCommands: bool
diff --git a/tests/test_transport.py b/tests/test_transport.py
index 2a0ed35bf..a5bb19178 100644
--- a/tests/test_transport.py
+++ b/tests/test_transport.py
@@ -1841,7 +1841,86 @@ def test_build_command_sandbox_minimal(self):
         settings_value = cmd[settings_idx + 1]
 
         parsed = json.loads(settings_value)
-        assert parsed == {"sandbox": {"enabled": True}}
+        assert parsed == {"sandbox": {"enabled": True, "failIfUnavailable": True}}
+
+    def test_sandbox_enabled_defaults_to_fail_closed(self):
+        """An enabled sandbox defaults failIfUnavailable to True.
+
+        The CLI reads this setting as ``?? false``, so without it a sandbox
+        that cannot start only warns and every command runs unsandboxed. The
+        TypeScript SDK injects True here; the Python SDK must match so the
+        same options do not silently fail open.
+        """
+        import json
+
+        from claude_agent_sdk import SandboxSettings
+
+        sandbox: SandboxSettings = {"enabled": True, "excludedCommands": ["git"]}
+
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(sandbox=sandbox),
+        )
+
+        cmd = transport._build_command()
+        parsed = json.loads(cmd[cmd.index("--settings") + 1])
+
+        assert parsed["sandbox"]["failIfUnavailable"] is True
+        assert parsed["sandbox"]["excludedCommands"] == ["git"]
+
+    def test_sandbox_explicit_fail_if_unavailable_is_preserved(self):
+        """An explicit failIfUnavailable is never overridden."""
+        import json
+
+        from claude_agent_sdk import SandboxSettings
+
+        sandbox: SandboxSettings = {"enabled": True, "failIfUnavailable": False}
+
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(sandbox=sandbox),
+        )
+
+        cmd = transport._build_command()
+        parsed = json.loads(cmd[cmd.index("--settings") + 1])
+
+        assert parsed["sandbox"]["failIfUnavailable"] is False
+
+    def test_sandbox_not_enabled_gets_no_fail_if_unavailable(self):
+        """A sandbox that is not enabled is passed through untouched."""
+        import json
+
+        from claude_agent_sdk import SandboxSettings
+
+        sandbox: SandboxSettings = {"enabled": False}
+
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(sandbox=sandbox),
+        )
+
+        cmd = transport._build_command()
+        parsed = json.loads(cmd[cmd.index("--settings") + 1])
+
+        assert parsed == {"sandbox": {"enabled": False}}
+
+    def test_sandbox_options_dict_is_not_mutated(self):
+        """Building the command does not mutate the caller's sandbox dict."""
+        import json
+
+        from claude_agent_sdk import SandboxSettings
+
+        sandbox: SandboxSettings = {"enabled": True}
+
+        transport = SubprocessCLITransport(
+            prompt="test",
+            options=make_options(sandbox=sandbox),
+        )
+
+        cmd = transport._build_command()
+        json.loads(cmd[cmd.index("--settings") + 1])
+
+        assert sandbox == {"enabled": True}
 
     def test_sandbox_network_config(self):
         """Test sandbox with full network configuration."""