diff --git a/src/cleveragents/lsp/transport.py b/src/cleveragents/lsp/transport.py
index 841475b1c..41063d16f 100644
--- a/src/cleveragents/lsp/transport.py
+++ b/src/cleveragents/lsp/transport.py
@@ -27,6 +27,8 @@ from typing import Any
 
 import structlog
 
+from cleveragents.lsp.errors import LspError
+
 logger = structlog.get_logger(__name__)
 
 # Maximum time (seconds) to wait for a graceful process termination
@@ -112,36 +114,15 @@ class StdioTransport:
                 cwd=self._cwd,
             )
         except FileNotFoundError as exc:
-            self._process = None
-
-            from cleveragents.lsp.errors import LspError
-
             raise LspError(
                 f"LSP server command not found: {self._command}",
                 details={"command": self._command, "args": self._args},
             ) from exc
         except OSError as exc:
-            # Popen may have partially started the subprocess before
-            # raising (e.g. execve failure post-fork on some platforms).
-            # Ensure cleanup so the process does not leak into the caller's
-            # address space.
-            if self._process is not None:
-                self.stop()
-                self._process = None
-
-            from cleveragents.lsp.errors import LspError
-
             raise LspError(
                 f"Failed to start LSP server: {exc}",
                 details={"command": self._command, "error": str(exc)},
             ) from exc
-        except Exception:
-            # Catch-all for any other low-level OSError / resource-error
-            # variants that might leave a zombie process behind.
-            if self._process is not None:
-                self.stop()
-                self._process = None
-            raise
 
         logger.info(
             "lsp.transport.started",
@@ -239,7 +220,23 @@ class StdioTransport:
             return self._read_one_message(effective_timeout)
 
     def _read_one_message(self, timeout: float) -> dict[str, Any] | None:
-        """Parse a single ``Content-Length`` framed JSON-RPC message."""
+        """Parse a single ``Content-Length``-framed JSON-RPC message.
+
+        Header lines are decoded with ``errors="strict"``; any non-ASCII byte
+        raises :class:`~cleveragents.lsp.errors.LspError`.  An additional
+        printable-ASCII guard rejects decoded headers containing characters
+        outside the codepoint range 0x20 (space) to 0x7E (tilde).
+
+        Returns
+        -------
+        dict[str, Any] | None
+            Parsed JSON body, or ``None`` on EOF / timeout.
+
+        Raises
+        ------
+        LspError
+            If a header contains non-ASCII bytes or non-printable characters.
+        """
         assert self._process is not None
         assert self._process.stdout is not None
 
@@ -254,7 +251,21 @@ class StdioTransport:
             line = stdout.readline()
             if not line:
                 return None  # EOF — server exited
-            decoded = line.decode("ascii", errors="replace").strip()
+            try:
+                decoded = line.decode("ascii", errors="strict").strip()
+            except UnicodeDecodeError as exc:
+                raise LspError(
+                    f"LSP header contains non-ASCII bytes: {exc}",
+                    details={"raw_header": repr(line)},
+                ) from exc
+            # Printable-ASCII guard: reject control characters
+            # outside the range 0x20 (space) through 0x7E (tilde).
+            if not all(0x20 <= ord(c) <= 0x7E for c in decoded):
+                raise LspError(
+                    f"LSP header contains non-printable ASCII characters: "
+                    f"{decoded!r}",
+                    details={"raw_header": repr(line)},
+                )
             if not decoded:
                 break  # Empty line = end of headers
             if decoded.lower().startswith("content-length:"):