runtime-guard

Implementation Reference

• src/tools/file_tools.py:20-58 (handler)

  The `read_file` tool handler implementation, which includes policy checks (path and file size), auditing, and the actual file read operation.

  def read_file(path: str, ctx: Context | None = None) -> str:
      context_tokens = activate_runtime_context(ctx)
      path = str(pathlib.Path(WORKSPACE_ROOT) / path) if not os.path.isabs(path) else path
  
      try:
          path_check = check_path_policy(path, tool="read_file")
          if path_check:
              result = PolicyResult(allowed=False, reason=path_check[0], decision_tier="blocked", matched_rule=path_check[1])
          else:
              result = PolicyResult(allowed=True, reason="allowed", decision_tier="allowed", matched_rule=None)
  
          if result.allowed:
              max_mb = POLICY.get("allowed", {}).get("max_file_size_mb", 10)
              try:
                  size_mb = os.path.getsize(path) / (1024 * 1024)
                  if size_mb > max_mb:
                      result = PolicyResult(
                          allowed=False,
                          reason=f"File size {size_mb:.1f} MB exceeds the policy limit of {max_mb} MB (allowed.max_file_size_mb)",
                          decision_tier="blocked",
                          matched_rule="allowed.max_file_size_mb",
                      )
              except (FileNotFoundError, OSError):
                  pass
  
          append_log_entry(build_log_entry("read_file", result, path=path))
          if not result.allowed:
              return f"[POLICY BLOCK] {result.reason}"
  
          try:
              with open(path, "r", errors="replace") as f:
                  return f.read()
          except FileNotFoundError:
              return f"Error: file not found: {path}"
          except OSError as e:
              return f"Error reading file: {e}"
      finally:
          reset_runtime_context(context_tokens)