fix: return debugger errors without a stack trace (#118)

## What this changes

Tool calls that fail with a `DebuggerError` (timeouts, a debugger that
exited, failed connects) now reach the caller as the error message
alone. Unexpected exceptions still include the stack trace.

## Why

`_dispatch_tool` appended `traceback.format_exc()` to every error. The
last line of that trace repeats the exception message, so since #114 a
timeout carrying partial output returned it twice: about 130 KB against
a real kernel target, instead of the 64 KiB cap. The trace itself tells
the model nothing it can act on.

## How it was verified

- New hermetic test through the registered tool handler (cdb and kd):
fails before the fix, passes after.
- Full suite: 132 passed, 2 skipped.
- Against a real KDNET target (TESTBOX-1): both kernel scenarios pass,
and `!process 0 7` with an 8 s timeout now returns 61 KB with the
partial output once and no traceback.

CHANGELOG.md

@@ -24,6 +24,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   instead of every pending command, `wait_for_break`, or session open sitting out
   its full timeout and then misreporting a timeout (#115). Thanks to
   @RamanaIntel for the fix.
+- Debugger errors (timeouts, a debugger that exited, failed connects) reach the
+  caller as their message alone, without a Python stack trace. The trace
+  repeated the message, so a timeout with partial output returned that output
+  twice.
 
 ## [1.3.0] - 2026-09-10
 

src/mcp_windbg/server.py

@@ -9,7 +9,7 @@ from typing import Dict, Optional
 from contextlib import asynccontextmanager
 
 from .cdb_session import CDBSession
-from .debug_session import DEFAULT_WAIT_FOR_BREAK_TIMEOUT
+from .debug_session import DEFAULT_WAIT_FOR_BREAK_TIMEOUT, DebuggerError
 from .kd_session import KDSession
 from .filter_script import FilterScript, load_filter_script
 from .prompts import load_prompt
@@ -517,6 +517,10 @@ def _create_server(
 
         except MCPError:
             raise
+        except DebuggerError as e:
+            # An expected debugger failure: its message is the whole story, and a
+            # stack trace would repeat it (partial output included) a second time.
+            raise MCPError(INTERNAL_ERROR, f"Error executing tool {name}: {e}")
         except Exception as e:
             traceback_str = traceback.format_exc()
             raise MCPError(INTERNAL_ERROR, f"Error executing tool {name}: {str(e)}\n{traceback_str}")

src/mcp_windbg/tests/test_tool_concurrency.py

@@ -9,6 +9,7 @@ import pytest
 from mcp.types import CallToolRequestParams
 
 from mcp_windbg import server as server_module
+from mcp_windbg.debug_session import DebuggerError
 
 
 @pytest.fixture
@@ -278,3 +279,38 @@ async def test_break_in_racing_with_close_reports_debugger_error(tool_handler, k
                 name="send_ctrl_break", arguments={"session_id": "test-session"}
             ),
         )
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize("kind", ["cdb", "kd"])
+async def test_debugger_error_reaches_caller_without_traceback(tool_handler, kind):
+    class TimingOutSession:
+        is_live_session = True
+
+        def send_command(self, command, timeout):
+            raise DebuggerError(
+                "Command timed out after 8 seconds: !process 0 7\n"
+                "Partial output before timeout:\nPROCESS ffffc48d14520040",
+                partial_output=["PROCESS ffffc48d14520040"],
+            )
+
+    server_module._sessions["test-session"] = {
+        "session": TimingOutSession(),
+        "kind": kind,
+        "label": "test target",
+    }
+    with pytest.raises(server_module.MCPError) as exc:
+        await tool_handler(
+            None,
+            CallToolRequestParams(
+                name=f"run_{kind}_command",
+                arguments={"session_id": "test-session", "command": "!process 0 7"},
+            ),
+        )
+
+    # The message is for the model: a stack trace adds nothing, and its last
+    # line would repeat the (possibly 64 KiB of) partial output a second time.
+    message = exc.value.message
+    assert "Command timed out" in message
+    assert "Traceback" not in message
+    assert message.count("PROCESS ffffc48d14520040") == 1