Harden MCP transport error handling

Normalize timeout output to text, convert tool exceptions into tool errors, and keep stdio running after response serialization failures.

1 files changed, 15 insertions, 5 deletions

diff --git a/src/sailfish_devel_mcp/runner.py b/src/sailfish_devel_mcp/runner.py
index b0b88bf..74748f4 100644
--- a/src/sailfish_devel_mcp/runner.py
+++ b/src/sailfish_devel_mcp/runner.py
@@ -4,7 +4,7 @@ from dataclasses import dataclass
 from pathlib import Path
 import shlex
 import subprocess
-from typing import Iterable, Sequence
+from typing import Any, Iterable, Sequence
 
 from .config import DeviceConfig
 
@@ -39,6 +39,16 @@ def truncate(text: str, limit: int) -> tuple[str, bool]:
     return text[:limit] + f"\n[truncated after {limit} characters]", True
 
 
+def ensure_text(value: Any) -> str:
+    if value is None:
+        return ""
+    if isinstance(value, str):
+        return value
+    if isinstance(value, bytes):
+        return value.decode("utf-8", errors="replace")
+    return str(value)
+
+
 def run(
     argv: Sequence[str],
     *,
@@ -57,15 +67,15 @@ def run(
         return CommandResult(
             argv=tuple(str(arg) for arg in argv),
             returncode=completed.returncode,
-            stdout=completed.stdout,
-            stderr=completed.stderr,
+            stdout=ensure_text(completed.stdout),
+            stderr=ensure_text(completed.stderr),
         )
     except subprocess.TimeoutExpired as exc:
         return CommandResult(
             argv=tuple(str(arg) for arg in argv),
             returncode=124,
-            stdout=exc.stdout or "",
-            stderr=(exc.stderr or "") + f"\ncommand timed out after {timeout}s",
+            stdout=ensure_text(exc.stdout),
+            stderr=ensure_text(exc.stderr) + f"\ncommand timed out after {timeout}s",
         )