diff --git a/tests/tools/test_terminal_sandbox_builder_dispatch.py b/tests/tools/test_terminal_sandbox_builder_dispatch.py
new file mode 100644
index 0000000000..5a84eaf5e5
--- /dev/null
+++ b/tests/tools/test_terminal_sandbox_builder_dispatch.py
@@ -0,0 +1,68 @@
+"""Regression coverage for sandbox backend builder dispatch (#112715)."""
+
+import pytest
+
+from tools import terminal_tool_backends as backends
+
+
+@pytest.mark.parametrize(
+    ("env_type", "expects_image"),
+    [
+        ("singularity", True),
+        ("daytona", True),
+        ("vercel_sandbox", False),
+    ],
+)
+def test_create_environment_dispatches_each_sandbox_builder_once(
+    monkeypatch, env_type, expects_image
+):
+    """The bound backend name must not collide with the generic dispatcher kwarg."""
+    received = {}
+
+    class FakeEnvironment:
+        def __init__(self, **kwargs):
+            received.update(kwargs)
+
+    monkeypatch.setitem(
+        backends._SANDBOX_ROWS,
+        env_type,
+        (lambda: FakeEnvironment, expects_image, lambda _cc, _kwargs: {}),
+    )
+
+    environment = backends._create_environment(
+        env_type=env_type,
+        image="test-image",
+        cwd="/workspace",
+        timeout=42,
+        container_config={},
+        task_id="test-task",
+    )
+
+    assert isinstance(environment, FakeEnvironment)
+    assert received["cwd"] == "/workspace"
+    assert received["timeout"] == 42
+    assert received["task_id"] == "test-task"
+    assert ("image" in received) is expects_image
+
+
+def test_create_environment_keeps_env_type_for_plugin_backends(monkeypatch):
+    """Unknown backends still receive their name for plugin provider lookup."""
+    received = {}
+
+    def build_plugin_env(**kwargs):
+        received.update(kwargs)
+        return object()
+
+    monkeypatch.setattr(backends, "_build_plugin_env", build_plugin_env)
+
+    backends._create_environment(
+        env_type="plugin-sandbox",
+        image="test-image",
+        cwd="/workspace",
+        timeout=42,
+        container_config={},
+        task_id="test-task",
+    )
+
+    assert received["env_type"] == "plugin-sandbox"
+    assert received["image"] == "test-image"
diff --git a/tools/terminal_tool_backends.py b/tools/terminal_tool_backends.py
index 8ce7bfd76c..7bcb060449 100644
--- a/tools/terminal_tool_backends.py
+++ b/tools/terminal_tool_backends.py
@@ -240,9 +240,12 @@ def _create_environment(env_type: str, image: str, cwd: str, timeout: int,
     for local/ssh/vercel; ``container_config`` carries the container_*/docker_* resource keys; ``host_cwd`` is
     the host dir bound into Docker when cwd mounting is enabled. ``probe_only`` asks ssh for a throwaway
     connection with no remote setup/sync (the prompt-time probe). Unknown types fall through to plugin backends."""
-    builder = _ENV_BUILDERS.get(env_type, _build_plugin_env)
-    return builder(env_type=env_type, image=image, cwd=cwd, timeout=timeout, cc=container_config or {},
-                   task_id=task_id, ssh_config=ssh_config, host_cwd=host_cwd, probe_only=probe_only)
+    builder = _ENV_BUILDERS.get(env_type)
+    kwargs = dict(image=image, cwd=cwd, timeout=timeout, cc=container_config or {}, task_id=task_id,
+                  ssh_config=ssh_config, host_cwd=host_cwd, probe_only=probe_only)
+    if builder is not None:
+        return builder(**kwargs)
+    return _build_plugin_env(env_type=env_type, **kwargs)
 
 
 # --- Requirement checkers: one generic path driven by _BACKEND_SPECS; optional fields, checked in order: