diff --git a/src/opensquilla/engine/runtime.py b/src/opensquilla/engine/runtime.py
index f850b790b..99e310809 100644
--- a/src/opensquilla/engine/runtime.py
+++ b/src/opensquilla/engine/runtime.py
@@ -228,7 +228,7 @@
 from opensquilla.observability.trace import TraceContext, TraceEvent, write_trace_event
 from opensquilla.observability.turn_call_log import TurnCallLogger, is_turn_call_log_enabled
 from opensquilla.paths import media_root_from_config
-from opensquilla.process_tree import task_process_scope
+from opensquilla.process_tree import default_command_shell, task_process_scope
 from opensquilla.provider import (
     ErrorEvent as ProviderErrorEvent,
 )
@@ -8680,7 +8680,7 @@ def _assemble_prompt(
             if restricted_tool_boundary
             else {
                 "os": os_name,
-                "shell": os.environ.get("SHELL", ""),
+                "shell": default_command_shell(),
                 "workspace_dir": str(workspace_dir or bootstrap_workspace_dir),
             }
         )
diff --git a/src/opensquilla/process_tree.py b/src/opensquilla/process_tree.py
index c2f6b84f0..67c82f1c0 100644
--- a/src/opensquilla/process_tree.py
+++ b/src/opensquilla/process_tree.py
@@ -2269,10 +2269,23 @@ async def create_owned_subprocess_exec(*argv: str, **kwargs: Any) -> Any:
     raise ProcessTreeOwnershipError(f"unsupported process-tree platform: {os.name}")
 
 
+def default_command_shell() -> str:
+    """Name the interpreter that runs agent shell commands on this host.
+
+    Windows never sets ``SHELL``, so reading it there yields an empty string.
+    ``create_owned_subprocess_shell`` execs ``COMSPEC`` on Windows, which makes
+    that the honest answer. POSIX keeps reporting the login shell it always has.
+    """
+    if os.name == "nt":
+        return os.environ.get("COMSPEC", "cmd.exe")
+    return os.environ.get("SHELL", "")
+
+
 async def create_owned_subprocess_shell(command: str, **kwargs: Any) -> Any:
     if os.name == "nt":
-        comspec = os.environ.get("COMSPEC", "cmd.exe")
-        return await create_owned_subprocess_exec(comspec, "/d", "/s", "/c", command, **kwargs)
+        return await create_owned_subprocess_exec(
+            default_command_shell(), "/d", "/s", "/c", command, **kwargs
+        )
     if os.name != "posix":
         raise ProcessTreeOwnershipError(f"unsupported process-tree platform: {os.name}")
     child_kwargs = dict(kwargs)
@@ -3001,6 +3014,7 @@ async def _wait_direct_process(process: Any, timeout: float) -> bool:
     "create_owned_popen",
     "create_owned_subprocess_exec",
     "create_owned_subprocess_shell",
+    "default_command_shell",
     "main",
     "reconcile_persisted_processes",
     "task_process_scope",
diff --git a/tests/test_process_tree.py b/tests/test_process_tree.py
index 3fe6e39a3..9a68999f5 100644
--- a/tests/test_process_tree.py
+++ b/tests/test_process_tree.py
@@ -3060,3 +3060,41 @@ async def test_windows_job_kills_descendant_after_direct_leader_exits(tmp_path)
         assert kernel32.WaitForSingleObject(child_handle, 5000) == 0
     finally:
         kernel32.CloseHandle(child_handle)
+
+
+def test_default_command_shell_reports_comspec_on_windows(monkeypatch) -> None:
+    """On Windows the reported shell must name the interpreter that actually runs
+    commands. create_owned_subprocess_shell execs COMSPEC there, so SHELL --
+    which Windows never sets -- left the field empty."""
+    monkeypatch.setattr(os, "name", "nt")
+    monkeypatch.delenv("SHELL", raising=False)
+    monkeypatch.setenv("COMSPEC", r"C:\WINDOWS\system32\cmd.exe")
+
+    assert process_tree.default_command_shell() == r"C:\WINDOWS\system32\cmd.exe"
+
+
+def test_default_command_shell_falls_back_when_comspec_missing(monkeypatch) -> None:
+    """COMSPEC is effectively always set on Windows, but the field must never be
+    empty even if it is not."""
+    monkeypatch.setattr(os, "name", "nt")
+    monkeypatch.delenv("SHELL", raising=False)
+    monkeypatch.delenv("COMSPEC", raising=False)
+
+    assert process_tree.default_command_shell() == "cmd.exe"
+
+
+def test_default_command_shell_keeps_posix_shell(monkeypatch) -> None:
+    """Control: POSIX behaviour is unchanged -- still the login shell."""
+    monkeypatch.setattr(os, "name", "posix")
+    monkeypatch.setenv("SHELL", "/bin/zsh")
+
+    assert process_tree.default_command_shell() == "/bin/zsh"
+
+
+def test_default_command_shell_is_empty_when_posix_shell_unset(monkeypatch) -> None:
+    """Control: the POSIX empty case is pre-existing behaviour, not a regression
+    this change is expected to fix."""
+    monkeypatch.setattr(os, "name", "posix")
+    monkeypatch.delenv("SHELL", raising=False)
+
+    assert process_tree.default_command_shell() == ""