diff --git a/lib/dartdoc.dart b/lib/dartdoc.dart
index b8bb3dcf87..521116bcc4 100644
--- a/lib/dartdoc.dart
+++ b/lib/dartdoc.dart
@@ -23,6 +23,7 @@ import 'package:dartdoc/src/logging.dart';
 import 'package:dartdoc/src/markdown_processor.dart' show markdownStats;
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/package_meta.dart';
+import 'package:dartdoc/src/tool_definition.dart';
 import 'package:dartdoc/src/tool_runner.dart';
 import 'package:dartdoc/src/tuple.dart';
 import 'package:dartdoc/src/utils.dart';
diff --git a/lib/src/dartdoc_options.dart b/lib/src/dartdoc_options.dart
index c0204f6eaf..0e21dd6ff3 100644
--- a/lib/src/dartdoc_options.dart
+++ b/lib/src/dartdoc_options.dart
@@ -24,10 +24,10 @@ import 'package:dartdoc/dartdoc.dart';
 import 'package:dartdoc/src/experiment_options.dart';
 import 'package:dartdoc/src/io_utils.dart';
 import 'package:dartdoc/src/source_linker.dart';
+import 'package:dartdoc/src/tool_definition.dart';
 import 'package:dartdoc/src/tool_runner.dart';
-import 'package:dartdoc/src/tuple.dart';
 import 'package:dartdoc/src/warnings.dart';
-import 'package:path/path.dart' as p show Context, canonicalize, extension;
+import 'package:path/path.dart' as p show Context, canonicalize;
 import 'package:yaml/yaml.dart';
 
 /// Constants to help with type checking, because T is int and so forth
@@ -39,7 +39,7 @@ const int _kIntVal = 0;
 const double _kDoubleVal = 0.0;
 const bool _kBoolVal = true;
 
-const String _kCompileArgsTagName = 'compile_args';
+const String compileArgsTagName = 'compile_args';
 
 int get _usageLineLength => stdout.hasTerminal ? stdout.terminalColumns : null;
 
@@ -114,219 +114,6 @@ class CategoryConfiguration {
   }
 }
 
-/// Defines the attributes of a tool in the options file, corresponding to
-/// the 'tools' keyword in the options file, and populated by the
-/// [ToolConfiguration] class.
-class ToolDefinition {
-  /// A list containing the command and options to be run for this tool. The
-  /// first argument in the command is the tool executable, and will have its
-  /// path evaluated relative to the `dartdoc_options.yaml` location. Must not
-  /// be an empty list, or be null.
-  final List<String> command;
-
-  /// A list containing the command and options to setup phase for this tool.
-  /// The first argument in the command is the tool executable, and will have
-  /// its path evaluated relative to the `dartdoc_options.yaml` location. May
-  /// be null or empty, in which case it will be ignored at setup time.
-  final List<String> setupCommand;
-
-  /// A description of the defined tool. Must not be null.
-  final String description;
-
-  /// If set, then the setup command has been run once for this tool definition.
-  bool setupComplete = false;
-
-  /// Returns true if the given executable path has an extension recognized as a
-  /// Dart extension (e.g. '.dart' or '.snapshot').
-  static bool isDartExecutable(String executable) {
-    var extension = p.extension(executable);
-    return extension == '.dart' || extension == '.snapshot';
-  }
-
-  /// Creates a ToolDefinition or subclass that is appropriate for the command
-  /// given.
-  factory ToolDefinition.fromCommand(
-      List<String> command,
-      List<String> setupCommand,
-      String description,
-      ResourceProvider resourceProvider,
-      {List<String> compileArgs}) {
-    assert(command != null);
-    assert(command.isNotEmpty);
-    assert(description != null);
-    if (isDartExecutable(command[0])) {
-      return DartToolDefinition(
-          command, setupCommand, description, resourceProvider,
-          compileArgs: compileArgs ?? const []);
-    } else {
-      if (compileArgs != null && compileArgs.isNotEmpty) {
-        throw DartdocOptionError(
-            'Compile arguments may only be specified for Dart tools, but '
-            '$_kCompileArgsTagName of $compileArgs were specified for '
-            '$command.');
-      }
-      return ToolDefinition(command, setupCommand, description);
-    }
-  }
-
-  ToolDefinition(this.command, this.setupCommand, this.description)
-      : assert(command != null),
-        assert(command.isNotEmpty),
-        assert(description != null);
-
-  @override
-  String toString() {
-    final commandString =
-        '${this is DartToolDefinition ? '(Dart) ' : ''}"${command.join(' ')}"';
-    if (setupCommand == null) {
-      return '$runtimeType: $commandString ($description)';
-    } else {
-      return '$runtimeType: $commandString, with setup command '
-          '"${setupCommand.join(' ')}" ($description)';
-    }
-  }
-}
-
-/// Manages the creation of a single snapshot file in a context where multiple
-/// async functions could be trying to use and/or create it.
-///
-/// To use:
-///
-/// var s = new Snapshot(...);
-///
-/// if (s.needsSnapshot) {
-///   // create s.snapshotFile, then call:
-///   s.snapshotCompleted();
-/// } else {
-///   await snapshotValid();
-///   // use existing s.snapshotFile;
-/// }
-///
-class Snapshot {
-  File _snapshotFile;
-
-  // TODO(srawlins): Deprecate this public getter; change private field to just
-  // be the absolute path.
-  File get snapshotFile => _snapshotFile;
-  final Completer<void> _snapshotCompleter = Completer();
-
-  Snapshot(Folder snapshotCache, String toolPath, int serial,
-      ResourceProvider resourceProvider) {
-    if (toolPath.endsWith('.snapshot')) {
-      _needsSnapshot = false;
-      _snapshotFile = resourceProvider.getFile(toolPath);
-      snapshotCompleted();
-    } else {
-      _snapshotFile = resourceProvider.getFile(resourceProvider.pathContext
-          .join(resourceProvider.pathContext.absolute(snapshotCache.path),
-              'snapshot_$serial'));
-    }
-  }
-
-  bool _needsSnapshot = true;
-
-  /// Will return true precisely once, unless [snapshotFile] was already a
-  /// snapshot.  In that case, will always return false.
-  bool get needsSnapshot {
-    if (_needsSnapshot == true) {
-      _needsSnapshot = false;
-      return true;
-    }
-    return _needsSnapshot;
-  }
-
-  Future<void> snapshotValid() => _snapshotCompleter.future;
-
-  void snapshotCompleted() => _snapshotCompleter.complete();
-}
-
-/// A singleton that keeps track of cached snapshot files. The [dispose]
-/// function must be called before process exit to clean up snapshots in the
-/// cache.
-class SnapshotCache {
-  static SnapshotCache _instance;
-
-  // TODO(srawlins): Make this final.
-  Folder snapshotCache;
-  final ResourceProvider _resourceProvider;
-  final Map<String, Snapshot> snapshots = {};
-  int _serial = 0;
-
-  SnapshotCache._(this._resourceProvider)
-      : snapshotCache =
-            _resourceProvider.createSystemTemp('dartdoc_snapshot_cache_');
-
-  static SnapshotCache get instance => _instance;
-
-  static SnapshotCache createInstance(ResourceProvider resourceProvider) =>
-      _instance ??= SnapshotCache._(resourceProvider);
-
-  Snapshot getSnapshot(String toolPath) {
-    if (snapshots.containsKey(toolPath)) {
-      return snapshots[toolPath];
-    }
-    snapshots[toolPath] =
-        Snapshot(snapshotCache, toolPath, _serial, _resourceProvider);
-    _serial++;
-    return snapshots[toolPath];
-  }
-
-  void dispose() {
-    _instance = null;
-    if (snapshotCache != null && snapshotCache.exists) {
-      return snapshotCache.delete();
-    }
-    return null;
-  }
-}
-
-/// A special kind of tool definition for Dart commands.
-class DartToolDefinition extends ToolDefinition {
-  final ResourceProvider _resourceProvider;
-
-  /// A list of arguments to add to the snapshot compilation arguments.
-  final List<String> compileArgs;
-
-  /// Takes a list of args to modify, and returns the name of the executable
-  /// to run. If no snapshot file existed, then create one and modify the args
-  /// so that if they are executed with dart, will result in the snapshot being
-  /// built.
-  Future<Tuple2<String, Function()>> modifyArgsToCreateSnapshotIfNeeded(
-      List<String> args) async {
-    assert(args[0] == command.first);
-    // Set up flags to create a new snapshot, if needed, and use the first run
-    // as the training run.
-    SnapshotCache.createInstance(_resourceProvider);
-    var snapshot = SnapshotCache.instance.getSnapshot(command.first);
-    var snapshotFile = snapshot.snapshotFile;
-    var needsSnapshot = snapshot.needsSnapshot;
-    if (needsSnapshot) {
-      args.insertAll(0, [
-        // TODO(jcollins-g): remove ignore and verbosity resets once
-        // https://dart-review.googlesource.com/c/sdk/+/181421 is safely
-        // in the rearview mirror in dev/Flutter.
-        '--ignore-unrecognized-flags',
-        '--verbosity=error',
-        '--snapshot=${_resourceProvider.pathContext.absolute(snapshotFile.path)}',
-        '--snapshot_kind=app-jit',
-        ...compileArgs,
-      ]);
-    } else {
-      await snapshot.snapshotValid();
-      // replace the first argument with the path to the snapshot.
-      args[0] = _resourceProvider.pathContext.absolute(snapshotFile.path);
-    }
-    return Tuple2(_resourceProvider.resolvedExecutable,
-        needsSnapshot ? snapshot.snapshotCompleted : null);
-  }
-
-  DartToolDefinition(List<String> command, List<String> setupCommand,
-      String description, this._resourceProvider,
-      {this.compileArgs = const []})
-      : assert(compileArgs != null),
-        super(command, setupCommand, description);
-}
-
 /// A configuration class that can interpret [ToolDefinition]s from a YAML map.
 class ToolConfiguration {
   final Map<String, ToolDefinition> tools;
@@ -396,17 +183,17 @@ class ToolConfiguration {
 
         List<String> findArgs() {
           List<String> args;
-          if (toolMap.containsKey(_kCompileArgsTagName)) {
-            var compileArgs = toolMap[_kCompileArgsTagName];
+          if (toolMap.containsKey(compileArgsTagName)) {
+            var compileArgs = toolMap[compileArgsTagName];
             if (compileArgs is String) {
-              args = [toolMap[_kCompileArgsTagName].toString()];
+              args = [toolMap[compileArgsTagName].toString()];
             } else if (compileArgs is YamlList) {
               args =
                   compileArgs.map<String>((node) => node.toString()).toList();
             } else {
               throw DartdocOptionError(
                   'Tool compile arguments must be a list of strings. The tool '
-                  '$name has a $_kCompileArgsTagName entry that is a '
+                  '$name has a $compileArgsTagName entry that is a '
                   '${compileArgs.runtimeType}');
             }
           }
diff --git a/lib/src/tool_definition.dart b/lib/src/tool_definition.dart
new file mode 100644
index 0000000000..629f8bf326
--- /dev/null
+++ b/lib/src/tool_definition.dart
@@ -0,0 +1,247 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:analyzer/file_system/file_system.dart';
+import 'package:dartdoc/src/dartdoc_options.dart';
+import 'package:dartdoc/src/io_utils.dart';
+import 'package:path/path.dart' as p show extension;
+
+/// Defines the attributes of a tool in the options file, corresponding to
+/// the 'tools' keyword in the options file, and populated by the
+/// [ToolConfiguration] class.
+class ToolDefinition {
+  /// A list containing the command and options to be run for this tool. The
+  /// first argument in the command is the tool executable, and will have its
+  /// path evaluated relative to the `dartdoc_options.yaml` location. Must not
+  /// be an empty list, or be null.
+  final List<String> command;
+
+  /// A list containing the command and options to setup phase for this tool.
+  /// The first argument in the command is the tool executable, and will have
+  /// its path evaluated relative to the `dartdoc_options.yaml` location. May
+  /// be null or empty, in which case it will be ignored at setup time.
+  final List<String> setupCommand;
+
+  /// A description of the defined tool. Must not be null.
+  final String description;
+
+  /// If set, then the setup command has been run once for this tool definition.
+  bool setupComplete = false;
+
+  /// Returns true if the given executable path has an extension recognized as a
+  /// Dart extension (e.g. '.dart' or '.snapshot').
+  static bool isDartExecutable(String executable) {
+    var extension = p.extension(executable);
+    return extension == '.dart' || extension == '.snapshot';
+  }
+
+  /// Creates a ToolDefinition or subclass that is appropriate for the command
+  /// given.
+  factory ToolDefinition.fromCommand(
+      List<String> command,
+      List<String> setupCommand,
+      String description,
+      ResourceProvider resourceProvider,
+      {List<String> compileArgs}) {
+    assert(command != null);
+    assert(command.isNotEmpty);
+    assert(description != null);
+    if (isDartExecutable(command[0])) {
+      return DartToolDefinition(
+          command, setupCommand, description, resourceProvider,
+          compileArgs: compileArgs ?? const []);
+    } else {
+      if (compileArgs != null && compileArgs.isNotEmpty) {
+        throw DartdocOptionError(
+            'Compile arguments may only be specified for Dart tools, but '
+            '$compileArgsTagName of $compileArgs were specified for '
+            '$command.');
+      }
+      return ToolDefinition(command, setupCommand, description);
+    }
+  }
+
+  ToolDefinition(this.command, this.setupCommand, this.description)
+      : assert(command != null),
+        assert(command.isNotEmpty),
+        assert(description != null);
+
+  @override
+  String toString() {
+    final commandString =
+        '${this is DartToolDefinition ? '(Dart) ' : ''}"${command.join(' ')}"';
+    if (setupCommand == null) {
+      return '$runtimeType: $commandString ($description)';
+    } else {
+      return '$runtimeType: $commandString, with setup command '
+          '"${setupCommand.join(' ')}" ($description)';
+    }
+  }
+
+  Future<ToolStateForArgs> toolStateForArgs(List<String> args) async {
+    var commandPath = args.removeAt(0);
+    return ToolStateForArgs(commandPath, args, null);
+  }
+}
+
+/// A special kind of tool definition for Dart commands.
+class DartToolDefinition extends ToolDefinition {
+  final ResourceProvider _resourceProvider;
+
+  /// A list of arguments to add to the snapshot compilation arguments.
+  final List<String> compileArgs;
+
+  /// Takes a list of args to modify, and returns the name of the executable
+  /// to run.
+  ///
+  /// If no snapshot file existed, then creates one and modify the args
+  /// so that if they are executed with dart, will result in the snapshot being
+  /// built.
+  @override
+  Future<ToolStateForArgs> toolStateForArgs(List<String> args) async {
+    assert(args[0] == command.first);
+    // Set up flags to create a new snapshot, if needed, and use the first run
+    // as the training run.
+    SnapshotCache.createInstance(_resourceProvider);
+    var snapshot = SnapshotCache.instance.getSnapshot(command.first);
+    var snapshotPath =
+        _resourceProvider.pathContext.absolute(snapshot._snapshotFile.path);
+    var needsSnapshot = snapshot.needsSnapshot;
+    if (needsSnapshot) {
+      return ToolStateForArgs(
+          _resourceProvider.resolvedExecutable,
+          [
+            // TODO(jcollins-g): remove ignore and verbosity resets once
+            // https://dart-review.googlesource.com/c/sdk/+/181421 is safely
+            // in the rearview mirror in dev/Flutter.
+            '--ignore-unrecognized-flags',
+            '--verbosity=error',
+            '--snapshot=$snapshotPath',
+            '--snapshot_kind=app-jit',
+            ...compileArgs,
+            ...args,
+          ],
+          snapshot._snapshotCompleted);
+    } else {
+      await snapshot._snapshotValid();
+      // replace the first argument with the path to the snapshot.
+      args[0] = snapshotPath;
+      return ToolStateForArgs(_resourceProvider.resolvedExecutable, args, null);
+    }
+  }
+
+  DartToolDefinition(List<String> command, List<String> setupCommand,
+      String description, this._resourceProvider,
+      {this.compileArgs = const []})
+      : assert(compileArgs != null),
+        super(command, setupCommand, description);
+}
+
+/// Manages the creation of a single snapshot file in a context where multiple
+/// async functions could be trying to use and/or create it.
+///
+/// To use:
+///
+/// ```dart
+/// var s = new Snapshot(...);
+///
+/// if (s.needsSnapshot) {
+///   // create s.snapshotFile, then call:
+///   s.snapshotCompleted();
+/// } else {
+///   await snapshotValid();
+///   // use existing s.snapshotFile;
+/// }
+/// ```
+///
+class _Snapshot {
+  final File _snapshotFile;
+
+  final Completer<void> _snapshotCompleter = Completer();
+
+  factory _Snapshot(Folder snapshotCache, String toolPath, int serial,
+      ResourceProvider resourceProvider) {
+    if (toolPath.endsWith('.snapshot')) {
+      return _Snapshot.existing(toolPath, resourceProvider);
+    } else {
+      return _Snapshot.create(resourceProvider.getFile(
+          resourceProvider.pathContext.join(
+              resourceProvider.pathContext.absolute(snapshotCache.path),
+              'snapshot_$serial')));
+    }
+  }
+
+  _Snapshot.existing(String toolPath, ResourceProvider resourceProvider)
+      : _needsSnapshot = false,
+        _snapshotFile = resourceProvider.getFile(toolPath) {
+    _snapshotCompleted();
+  }
+
+  _Snapshot.create(this._snapshotFile);
+
+  bool _needsSnapshot = true;
+
+  /// Will return true precisely once, unless [snapshotFile] was already a
+  /// snapshot.  In that case, will always return false.
+  bool get needsSnapshot {
+    if (_needsSnapshot == true) {
+      _needsSnapshot = false;
+      return true;
+    }
+    return _needsSnapshot;
+  }
+
+  Future<void> _snapshotValid() => _snapshotCompleter.future;
+
+  void _snapshotCompleted() => _snapshotCompleter.complete();
+}
+
+/// A singleton that keeps track of cached snapshot files. The [dispose]
+/// function must be called before process exit to clean up snapshots in the
+/// cache.
+class SnapshotCache {
+  static SnapshotCache _instance;
+
+  final Folder snapshotCache;
+  final ResourceProvider _resourceProvider;
+  final Map<String, _Snapshot> snapshots = {};
+  int _serial = 0;
+
+  SnapshotCache._(this._resourceProvider)
+      : snapshotCache =
+            _resourceProvider.createSystemTemp('dartdoc_snapshot_cache_');
+
+  static SnapshotCache get instance => _instance;
+
+  static SnapshotCache createInstance(ResourceProvider resourceProvider) =>
+      _instance ??= SnapshotCache._(resourceProvider);
+
+  _Snapshot getSnapshot(String toolPath) {
+    if (snapshots.containsKey(toolPath)) {
+      return snapshots[toolPath];
+    }
+    snapshots[toolPath] =
+        _Snapshot(snapshotCache, toolPath, _serial, _resourceProvider);
+    _serial++;
+    return snapshots[toolPath];
+  }
+
+  void dispose() {
+    _instance = null;
+    if (snapshotCache != null && snapshotCache.exists) {
+      return snapshotCache.delete();
+    }
+    return null;
+  }
+}
+
+class ToolStateForArgs {
+  final String commandPath;
+  final List<String> args;
+  final void Function() onProcessComplete;
+
+  ToolStateForArgs(this.commandPath, this.args, this.onProcessComplete);
+}
diff --git a/lib/src/tool_runner.dart b/lib/src/tool_runner.dart
index b28be74c43..ab05d206b5 100644
--- a/lib/src/tool_runner.dart
+++ b/lib/src/tool_runner.dart
@@ -8,6 +8,7 @@ import 'dart:io' show Process, ProcessException;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:dartdoc/src/io_utils.dart';
+import 'package:dartdoc/src/tool_definition.dart';
 import 'package:path/path.dart' as p;
 import 'dartdoc_options.dart';
 
@@ -206,16 +207,10 @@ class ToolRunner {
     }
 
     argsWithInput = toolArgs + argsWithInput;
-    String commandPath;
-    void Function() callCompleter;
-    if (toolDefinition is DartToolDefinition) {
-      var modified = await toolDefinition
-          .modifyArgsToCreateSnapshotIfNeeded(argsWithInput);
-      commandPath = modified.item1;
-      callCompleter = modified.item2;
-    } else {
-      commandPath = argsWithInput.removeAt(0);
-    }
+    var toolStateForArgs = await toolDefinition.toolStateForArgs(argsWithInput);
+    var commandPath = toolStateForArgs.commandPath;
+    argsWithInput = toolStateForArgs.args;
+    var callCompleter = toolStateForArgs.onProcessComplete;
 
     if (callCompleter != null) {
       return _runProcess(tool, content, commandPath, argsWithInput,
diff --git a/test/tool_runner_test.dart b/test/tool_runner_test.dart
index f49af7840e..562d3faa23 100644
--- a/test/tool_runner_test.dart
+++ b/test/tool_runner_test.dart
@@ -8,6 +8,7 @@ import 'dart:io';
 
 import 'package:dartdoc/src/dartdoc_options.dart';
 import 'package:dartdoc/src/package_meta.dart';
+import 'package:dartdoc/src/tool_definition.dart';
 import 'package:dartdoc/src/tool_runner.dart';
 import 'package:path/path.dart' as path;
 import 'package:test/test.dart';