adding fallback to --system-prompt-file when --system-prompt is too long

Author: M-Alshoura

src/claude_agent_sdk/_internal/transport/subprocess_cli.py

@@ -335,6 +335,28 @@ def _build_command(self) -> list[str]:
 
         # Check if command line is too long (Windows limitation)
         cmd_str = " ".join(cmd)
+        if len(cmd_str) > _CMD_LENGTH_LIMIT and "--system-prompt" in cmd:
+            # Handle large --system-prompt by switching to --system-prompt-file
+            try:
+                sp_idx = cmd.index("--system-prompt")
+                sp_value = cmd[sp_idx + 1]
+                if len(sp_value) > _CMD_LENGTH_LIMIT // 2:
+                    temp_file = tempfile.NamedTemporaryFile(
+                        mode="w", suffix=".txt", delete=False, encoding="utf-8"
+                    )
+                    temp_file.write(sp_value)
+                    temp_file.close()
+                    self._temp_files.append(temp_file.name)
+                    # Replace --system-prompt with --system-prompt-file
+                    cmd[sp_idx] = "--system-prompt-file"
+                    cmd[sp_idx + 1] = temp_file.name
+                    logger.info(
+                        f"System prompt length ({len(sp_value)}) exceeds limit. "
+                        f"Using --system-prompt-file: {temp_file.name}"
+                    )
+            except (ValueError, IndexError) as e:
+                logger.warning(f"Failed to optimize system prompt length: {e}")
+
         if len(cmd_str) > _CMD_LENGTH_LIMIT and self._options.agents:
             # Command is too long - use temp file for agents
             # Find the --agents argument and replace its value with @filepath