diff --git a/Plugins/GRPCProtobufGeneratorCommand/CommandConfig.swift b/Plugins/GRPCProtobufGeneratorCommand/CommandConfig.swift
index 2a5e3cb..2ac25dd 100644
--- a/Plugins/GRPCProtobufGeneratorCommand/CommandConfig.swift
+++ b/Plugins/GRPCProtobufGeneratorCommand/CommandConfig.swift
@@ -42,10 +42,8 @@ struct CommandConfig {
 }
 
 extension CommandConfig {
-  static func parse(
-    argumentExtractor argExtractor: inout ArgumentExtractor,
-    pluginWorkDirectory: URL
-  ) throws -> CommandConfig {
+  static func parse(args: [String]) throws -> CommandConfig {
+    var argExtractor = ArgumentExtractor(args)
     var config = CommandConfig.defaults
 
     for flag in OptionsAndFlags.allCases {
@@ -131,9 +129,7 @@ extension CommandConfig {
         config.common.protocPath = argExtractor.extractSingleOption(named: flag.rawValue)
 
       case .outputPath:
-        config.common.outputPath =
-          argExtractor.extractSingleOption(named: flag.rawValue)
-          ?? pluginWorkDirectory.absoluteStringNoScheme
+        config.common.outputPath = argExtractor.extractSingleOption(named: flag.rawValue) ?? "."
 
       case .verbose:
         let verbose = argExtractor.extractFlag(named: flag.rawValue)
diff --git a/Plugins/GRPCProtobufGeneratorCommand/CommandPluginError.swift b/Plugins/GRPCProtobufGeneratorCommand/CommandPluginError.swift
index a09d4a7..8d0f27c 100644
--- a/Plugins/GRPCProtobufGeneratorCommand/CommandPluginError.swift
+++ b/Plugins/GRPCProtobufGeneratorCommand/CommandPluginError.swift
@@ -21,13 +21,12 @@ enum CommandPluginError: Error {
   case unknownAccessLevel(String)
   case unknownFileNamingStrategy(String)
   case conflictingFlags(String, String)
+  case invalidInputFiles([String])
   case generationFailure(
-    errorDescription: String,
     executable: String,
     arguments: [String],
     stdErr: String?
   )
-  case tooManyParameterSeparators
 }
 
 extension CommandPluginError: CustomStringConvertible {
@@ -45,21 +44,52 @@ extension CommandPluginError: CustomStringConvertible {
       return "Provided file naming strategy is unknown: \(value)."
     case .conflictingFlags(let flag1, let flag2):
       return "Provided flags conflict: '\(flag1)' and '\(flag2)'."
-    case .generationFailure(let errorDescription, let executable, let arguments, let stdErr):
-      var message = """
-        Code generation failed with: \(errorDescription).
-        \tExecutable: \(executable)
-        \tArguments: \(arguments.joined(separator: " "))
-        """
+
+    case .invalidInputFiles(let files):
+      var lines: [String] = []
+      lines.append("Invalid input file(s)")
+      lines.append("")
+      lines.append("Found \(files.count) input(s) not ending in '.proto':")
+      for file in files {
+        lines.append("- \(file)")
+      }
+      lines.append("")
+      lines.append("All options must be before '--', and all input files must be")
+      lines.append("after '--'. Input files must end in '.proto'.")
+      lines.append("")
+      lines.append("See --help for more information.")
+      return lines.joined(separator: "\n")
+
+    case .generationFailure(let executable, let arguments, let stdErr):
+      var lines: [String] = []
+      lines.append("protoc failed to generate code")
+      lines.append("")
+      lines.append(String(repeating: "-", count: 80))
+      lines.append("Command run:")
+      lines.append("")
+      lines.append("\(executable) \\")
+      var iterator = arguments.makeIterator()
+      var current = iterator.next()
+      while let currentArg = current {
+        var nextArg = iterator.next()
+        defer { current = nextArg }
+
+        if nextArg != nil {
+          lines.append("  \(currentArg) \\")
+        } else {
+          lines.append("  \(currentArg)")
+        }
+      }
+
       if let stdErr {
-        message += """
-          \n\tprotoc error output:
-          \t\(stdErr)
-          """
+        lines.append("")
+        lines.append(String(repeating: "-", count: 80))
+        lines.append("Error output (stderr):")
+        lines.append("")
+        lines.append(stdErr)
       }
-      return message
-    case .tooManyParameterSeparators:
-      return "Unexpected parameter structure, too many '--' separators."
+
+      return lines.joined(separator: "\n")
     }
   }
 }
diff --git a/Plugins/GRPCProtobufGeneratorCommand/Plugin.swift b/Plugins/GRPCProtobufGeneratorCommand/Plugin.swift
index 0c525d2..29af0c2 100644
--- a/Plugins/GRPCProtobufGeneratorCommand/Plugin.swift
+++ b/Plugins/GRPCProtobufGeneratorCommand/Plugin.swift
@@ -50,45 +50,29 @@ struct GRPCProtobufGeneratorCommandPlugin {
     tool: (String) throws -> PluginContext.Tool,
     pluginWorkDirectoryURL: URL
   ) throws {
-    let flagsAndOptions: [String]
-    let inputFiles: [String]
-
-    let separatorCount = arguments.count { $0 == CommandConfig.parameterGroupSeparator }
-    switch separatorCount {
-    case 0:
-      var argExtractor = ArgumentExtractor(arguments)
-      // check if help requested
-      if argExtractor.extractFlag(named: OptionsAndFlags.help.rawValue) > 0 {
-        OptionsAndFlags.printHelp(requested: true)
-        return
-      }
-
-      inputFiles = arguments
-      flagsAndOptions = []
+    // Check for help.
+    if arguments.isEmpty || arguments.contains(where: { $0 == "--help" || $0 == "-h" }) {
+      OptionsAndFlags.printHelp(requested: true)
+      return
+    }
 
-    case 1:
-      let splitIndex = arguments.firstIndex(of: CommandConfig.parameterGroupSeparator)!
-      flagsAndOptions = Array(arguments[..<splitIndex])
-      inputFiles = Array(arguments[splitIndex.advanced(by: 1)...])
+    // Split the input into options and input files.
+    let (flagsAndOptions, inputFiles) = self.splitArgs(arguments)
 
-    default:
-      throw CommandPluginError.tooManyParameterSeparators
+    // Check the input files all look like protos
+    let nonProtoInputs = inputFiles.filter { !$0.hasSuffix(".proto") }
+    if !nonProtoInputs.isEmpty {
+      throw CommandPluginError.invalidInputFiles(nonProtoInputs)
     }
 
-    var argExtractor = ArgumentExtractor(flagsAndOptions)
-    // help requested
-    if argExtractor.extractFlag(named: OptionsAndFlags.help.rawValue) > 0 {
-      OptionsAndFlags.printHelp(requested: true)
-      return
+    if inputFiles.isEmpty {
+      throw CommandPluginError.missingInputFile
     }
 
-    // MARK: Configuration
+    // Parse the config.
     let commandConfig: CommandConfig
     do {
-      commandConfig = try CommandConfig.parse(
-        argumentExtractor: &argExtractor,
-        pluginWorkDirectory: pluginWorkDirectoryURL
-      )
+      commandConfig = try CommandConfig.parse(args: flagsAndOptions)
     } catch {
       throw error
     }
@@ -159,6 +143,22 @@ struct GRPCProtobufGeneratorCommandPlugin {
       }
     }
   }
+
+  private func splitArgs(_ args: [String]) -> (options: [String], inputs: [String]) {
+    let inputs: [String]
+    let options: [String]
+
+    if let index = args.firstIndex(of: "--") {
+      let nextIndex = args.index(after: index)
+      inputs = Array(args[nextIndex...])
+      options = Array(args[..<index])
+    } else {
+      options = []
+      inputs = args
+    }
+
+    return (options, inputs)
+  }
 }
 
 /// Execute a single invocation of `protoc`, printing output and if in verbose mode the invocation
@@ -202,7 +202,6 @@ func executeProtocInvocation(
       stdErr = nil
     }
     throw CommandPluginError.generationFailure(
-      errorDescription: "\(error)",
       executable: executableURL.absoluteStringNoScheme,
       arguments: arguments,
       stdErr: stdErr
@@ -224,7 +223,6 @@ func executeProtocInvocation(
   }
   let problem = "\(process.terminationReason):\(process.terminationStatus)"
   throw CommandPluginError.generationFailure(
-    errorDescription: problem,
     executable: executableURL.absoluteStringNoScheme,
     arguments: arguments,
     stdErr: stdErr