diff --git a/app_dart/lib/src/model/common/presubmit_check_state.dart b/app_dart/lib/src/model/common/presubmit_check_state.dart
new file mode 100644
index 0000000000..64da23b7da
--- /dev/null
+++ b/app_dart/lib/src/model/common/presubmit_check_state.dart
@@ -0,0 +1,57 @@
+// Copyright 2025 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// @docImport 'presubmit_check_state.dart';
+library;
+
+import 'package:buildbucket/buildbucket_pb.dart' as bbv2;
+import 'package:cocoon_common/task_status.dart';
+
+import '../../service/luci_build_service/build_tags.dart';
+import '../bbv2_extension.dart';
+
+/// Represents the current state of a check run.
+class PresubmitCheckState {
+  final String buildName;
+  final TaskStatus status;
+  final int attemptNumber; //static int _currentAttempt(BuildTags buildTags)
+  final int? startTime;
+  final int? endTime;
+  final String? summary;
+
+  const PresubmitCheckState({
+    required this.buildName,
+    required this.status,
+    required this.attemptNumber,
+    this.startTime,
+    this.endTime,
+    this.summary,
+  });
+
+  /// Returns true if the build is waiting for backfill or in progress.
+  bool get isBuildInProgress =>
+      status == TaskStatus.waitingForBackfill ||
+      status == TaskStatus.inProgress;
+
+  /// Returns true if the build succeeded or was skipped.
+  bool get isBuildSuccessed =>
+      status == TaskStatus.succeeded || status == TaskStatus.skipped;
+
+  /// Returns true if the build failed, had an infra failure, or was cancelled.
+  bool get isBuildFailed =>
+      status == TaskStatus.failed ||
+      status == TaskStatus.infraFailure ||
+      status == TaskStatus.cancelled;
+}
+
+extension BuildToPresubmitCheckState on bbv2.Build {
+  PresubmitCheckState toPresubmitCheckState() => PresubmitCheckState(
+    buildName: builder.builder,
+    status: status.toTaskStatus(),
+    attemptNumber: BuildTags.fromStringPairs(tags).currentAttempt,
+    startTime: startTime.toDateTime().microsecondsSinceEpoch,
+    endTime: endTime.toDateTime().microsecondsSinceEpoch,
+    summary: summaryMarkdown,
+  );
+}
diff --git a/app_dart/lib/src/model/common/presubmit_guard_conclusion.dart b/app_dart/lib/src/model/common/presubmit_guard_conclusion.dart
new file mode 100644
index 0000000000..fe14c82b35
--- /dev/null
+++ b/app_dart/lib/src/model/common/presubmit_guard_conclusion.dart
@@ -0,0 +1,81 @@
+// Copyright 2025 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// @docImport 'presubmit_guard_conclusion.dart';
+library;
+
+/// Explains what happened when attempting to mark the conclusion of a check run
+/// using [PresubmitGuard.markConclusion].
+enum PresubmitGuardConclusionResult {
+  /// Check run update recorded successfully in the respective CI stage.
+  ///
+  /// It is OK to evaluate returned results for stage completeness.
+  ok,
+
+  /// The check run is not in the specified CI stage.
+  ///
+  /// Perhaps it's from a different CI stage.
+  missing,
+
+  /// An unexpected error happened, and the results of the conclusion are
+  /// undefined.
+  ///
+  /// Examples of situations that can lead to this result:
+  ///
+  /// * The Firestore document is missing.
+  /// * The contents of the Firestore document are inconsistent.
+  /// * An unexpected error happend while trying to read from/write to Firestore.
+  ///
+  /// When this happens, it's best to stop the current transaction, report the
+  /// error to the logs, and have someone investigate the issue.
+  internalError,
+}
+
+/// Results from attempting to mark a staging task as completed.
+///
+/// See: [PresubmitGuard.markConclusion]
+class PresubmitGuardConclusion {
+  final PresubmitGuardConclusionResult result;
+  final int remaining;
+  final int? checkRunId;
+  final int failed;
+  final String summary;
+  final String details;
+
+  const PresubmitGuardConclusion({
+    required this.result,
+    required this.remaining,
+    required this.checkRunId,
+    required this.failed,
+    required this.summary,
+    required this.details,
+  });
+
+  bool get isOk => result == PresubmitGuardConclusionResult.ok;
+
+  bool get isPending => isOk && remaining > 0;
+
+  bool get isFailed => isOk && !isPending && failed > 0;
+
+  bool get isComplete => isOk && !isPending && !isFailed;
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      (other is PresubmitGuardConclusion &&
+          other.result == result &&
+          other.remaining == remaining &&
+          other.checkRunId == checkRunId &&
+          other.failed == failed &&
+          other.summary == summary &&
+          other.details == details);
+
+  @override
+  int get hashCode =>
+      Object.hashAll([result, remaining, checkRunId, failed, summary, details]);
+
+  @override
+  String toString() =>
+      'BuildConclusion("$result", "$remaining", "$checkRunId", "$failed", "$summary", "$details")';
+}
diff --git a/app_dart/lib/src/model/firestore/base.dart b/app_dart/lib/src/model/firestore/base.dart
index d84275ee74..584fb6779f 100644
--- a/app_dart/lib/src/model/firestore/base.dart
+++ b/app_dart/lib/src/model/firestore/base.dart
@@ -7,6 +7,25 @@ import 'dart:convert';
 import 'package:googleapis/firestore/v1.dart' as g;
 import 'package:meta/meta.dart';
 
+/// Well-defined stages in the build infrastructure.
+enum CiStage implements Comparable<CiStage> {
+  /// Build engine artifacts
+  fusionEngineBuild('engine'),
+
+  /// All non-engine artifact tests (engine & framework)
+  fusionTests('fusion');
+
+  const CiStage(this.name);
+
+  final String name;
+
+  @override
+  int compareTo(CiStage other) => index - other.index;
+
+  @override
+  String toString() => name;
+}
+
 /// Defines the `documentId` for a given document [T].
 ///
 /// The path to a document in Firestore follows the pattern:
diff --git a/app_dart/lib/src/model/firestore/ci_staging.dart b/app_dart/lib/src/model/firestore/ci_staging.dart
index e55fc52573..e607b60377 100644
--- a/app_dart/lib/src/model/firestore/ci_staging.dart
+++ b/app_dart/lib/src/model/firestore/ci_staging.dart
@@ -479,25 +479,6 @@ enum TaskConclusion {
   bool get isSuccess => this == success;
 }
 
-/// Well-defined stages in the build infrastructure.
-enum CiStage implements Comparable<CiStage> {
-  /// Build engine artifacts
-  fusionEngineBuild('engine'),
-
-  /// All non-engine artifact tests (engine & framework)
-  fusionTests('fusion');
-
-  const CiStage(this.name);
-
-  final String name;
-
-  @override
-  int compareTo(CiStage other) => index - other.index;
-
-  @override
-  String toString() => name;
-}
-
 /// Explains what happened when attempting to mark the conclusion of a check run
 /// using [CiStaging.markConclusion].
 enum StagingConclusionResult {
diff --git a/app_dart/lib/src/model/firestore/presubmit_check.dart b/app_dart/lib/src/model/firestore/presubmit_check.dart
new file mode 100644
index 0000000000..80fa78b265
--- /dev/null
+++ b/app_dart/lib/src/model/firestore/presubmit_check.dart
@@ -0,0 +1,269 @@
+// Copyright 2025 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// @docImport 'presubmit_check.dart';
+library;
+
+import 'package:buildbucket/buildbucket_pb.dart' as bbv2;
+import 'package:cocoon_common/task_status.dart';
+import 'package:googleapis/firestore/v1.dart' hide Status;
+import 'package:meta/meta.dart';
+import 'package:path/path.dart' as p;
+
+import '../../service/firestore.dart';
+import '../bbv2_extension.dart';
+import 'base.dart';
+
+const String collectionId = 'presubmit_checks';
+
+@immutable
+final class PresubmitCheckId extends AppDocumentId<PresubmitCheck> {
+  PresubmitCheckId({
+    required this.checkRunId,
+    required this.buildName,
+    required this.attemptNumber,
+  }) {
+    if (checkRunId < 1) {
+      throw RangeError.value(checkRunId, 'checkRunId', 'Must be at least 1');
+    } else if (attemptNumber < 1) {
+      throw RangeError.value(
+        attemptNumber,
+        'attemptNumber',
+        'Must be at least 1',
+      );
+    }
+  }
+
+  /// Parse the inverse of [PresubmitCheckId.documentName].
+  factory PresubmitCheckId.parse(String documentName) {
+    final result = tryParse(documentName);
+    if (result == null) {
+      throw FormatException(
+        'Unexpected firestore presubmit check document name: "$documentName"',
+      );
+    }
+    return result;
+  }
+
+  /// Tries to parse the inverse of [PresubmitCheckId.documentName].
+  ///
+  /// If could not be parsed, returns `null`.
+  static PresubmitCheckId? tryParse(String documentName) {
+    if (_parseDocumentName.matchAsPrefix(documentName) case final match?) {
+      final checkRunId = int.tryParse(match.group(1)!);
+      final buildName = match.group(2)!;
+      final attemptNumber = int.tryParse(match.group(3)!);
+      if (checkRunId != null && attemptNumber != null) {
+        return PresubmitCheckId(
+          checkRunId: checkRunId,
+          buildName: buildName,
+          attemptNumber: attemptNumber,
+        );
+      }
+    }
+    return null;
+  }
+
+  /// Parses `{checkRunId}_{buildName}_{attemptNumber}`.
+  ///
+  /// [buildName] could also include underscores which led us to use regexp .
+  /// But we dont have build number at the moment of creating the document and
+  /// we need to query by checkRunId and buildName for updating the document.
+  static final _parseDocumentName = RegExp(r'([0-9]+)_(.*)_([0-9]+)$');
+
+  final int checkRunId;
+  final String buildName;
+  final int attemptNumber;
+
+  @override
+  String get documentId {
+    return [checkRunId, buildName, attemptNumber].join('_');
+  }
+
+  @override
+  AppDocumentMetadata<PresubmitCheck> get runtimeMetadata =>
+      PresubmitCheck.metadata;
+}
+
+final class PresubmitCheck extends AppDocument<PresubmitCheck> {
+  static const fieldCheckRunId = 'checkRunId';
+  static const fieldBuildName = 'buildName';
+  static const fieldBuildNumber = 'buildNumber';
+  static const fieldStatus = 'status';
+  static const fieldAttemptNumber = 'attemptNumber';
+  static const fieldCreationTime = 'creationTime';
+  static const fieldStartTime = 'startTime';
+  static const fieldEndTime = 'endTime';
+  static const fieldSummary = 'summary';
+
+  static AppDocumentId<PresubmitCheck> documentIdFor({
+    required int checkRunId,
+    required String buildName,
+    required int attemptNumber,
+  }) {
+    return PresubmitCheckId(
+      checkRunId: checkRunId,
+      buildName: buildName,
+      attemptNumber: attemptNumber,
+    );
+  }
+
+  /// Returns a firebase documentName used in [fromFirestore].
+  static String documentNameFor({
+    required int checkRunId,
+    required String buildName,
+    required int attemptNumber,
+  }) {
+    // Document names cannot cannot have '/' in the document id.
+    final docId = documentIdFor(
+      checkRunId: checkRunId,
+      buildName: buildName,
+      attemptNumber: attemptNumber,
+    );
+    return '$kDocumentParent/$collectionId/${docId.documentId}';
+  }
+
+  @override
+  AppDocumentMetadata<PresubmitCheck> get runtimeMetadata => metadata;
+
+  static final metadata = AppDocumentMetadata<PresubmitCheck>(
+    collectionId: collectionId,
+    fromDocument: PresubmitCheck.fromDocument,
+  );
+
+  static Future<PresubmitCheck> fromFirestore(
+    FirestoreService firestoreService,
+    AppDocumentId<PresubmitCheck> id,
+  ) async {
+    final document = await firestoreService.getDocument(
+      p.posix.join(kDatabase, 'documents', collectionId, id.documentId),
+    );
+    return PresubmitCheck.fromDocument(document);
+  }
+
+  factory PresubmitCheck({
+    required int checkRunId,
+    required String buildName,
+    required TaskStatus status,
+    required int attemptNumber,
+    required int creationTime,
+    int? buildNumber,
+    int? startTime,
+    int? endTime,
+    String? summary,
+  }) {
+    return PresubmitCheck._(
+      {
+        fieldCheckRunId: checkRunId.toValue(),
+        fieldBuildName: buildName.toValue(),
+        fieldBuildNumber: ?buildNumber?.toValue(),
+        fieldStatus: status.value.toValue(),
+        fieldAttemptNumber: attemptNumber.toValue(),
+        fieldCreationTime: creationTime.toValue(),
+        fieldStartTime: ?startTime?.toValue(),
+        fieldEndTime: ?endTime?.toValue(),
+        fieldSummary: ?summary?.toValue(),
+      },
+      name: documentNameFor(
+        checkRunId: checkRunId,
+        buildName: buildName,
+        attemptNumber: attemptNumber,
+      ),
+    );
+  }
+
+  factory PresubmitCheck.fromDocument(Document document) {
+    return PresubmitCheck._(document.fields!, name: document.name!);
+  }
+
+  factory PresubmitCheck.init({
+    required String buildName,
+    required int checkRunId,
+    required int creationTime,
+  }) {
+    return PresubmitCheck(
+      buildName: buildName,
+      attemptNumber: 1,
+      checkRunId: checkRunId,
+      creationTime: creationTime,
+      status: TaskStatus.waitingForBackfill,
+      buildNumber: null,
+      startTime: null,
+      endTime: null,
+      summary: null,
+    );
+  }
+
+  PresubmitCheck._(Map<String, Value> fields, {required String name}) {
+    this
+      ..fields = fields
+      ..name = name;
+  }
+
+  int get checkRunId => int.parse(fields[fieldCheckRunId]!.integerValue!);
+  String get buildName => fields[fieldBuildName]!.stringValue!;
+  int get attemptNumber => int.parse(fields[fieldAttemptNumber]!.integerValue!);
+  int get creationTime => int.parse(fields[fieldCreationTime]!.integerValue!);
+  int? get buildNumber => fields[fieldBuildNumber] != null
+      ? int.parse(fields[fieldBuildNumber]!.integerValue!)
+      : null;
+  int? get startTime => fields[fieldStartTime] != null
+      ? int.parse(fields[fieldStartTime]!.integerValue!)
+      : null;
+  int? get endTime => fields[fieldEndTime] != null
+      ? int.parse(fields[fieldEndTime]!.integerValue!)
+      : null;
+  String? get summary => fields[fieldSummary]?.stringValue;
+
+  TaskStatus get status {
+    final rawValue = fields[fieldStatus]!.stringValue!;
+    return TaskStatus.from(rawValue);
+  }
+
+  set status(TaskStatus status) {
+    fields[fieldStatus] = status.value.toValue();
+  }
+
+  set startTime(int startTime) {
+    fields[fieldStartTime] = startTime.toValue();
+  }
+
+  set endTime(int endTime) {
+    fields[fieldEndTime] = endTime.toValue();
+  }
+
+  set summary(String summary) {
+    fields[fieldSummary] = summary.toValue();
+  }
+
+  void updateFromBuild(bbv2.Build build) {
+    fields[fieldBuildNumber] = build.number.toValue();
+    fields[fieldCreationTime] = build.createTime
+        .toDateTime()
+        .millisecondsSinceEpoch
+        .toValue();
+
+    if (build.hasStartTime()) {
+      fields[fieldStartTime] = build.startTime
+          .toDateTime()
+          .millisecondsSinceEpoch
+          .toValue();
+    }
+
+    if (build.hasEndTime()) {
+      fields[fieldEndTime] = build.endTime
+          .toDateTime()
+          .millisecondsSinceEpoch
+          .toValue();
+    }
+    _setStatusFromLuciStatus(build);
+  }
+
+  void _setStatusFromLuciStatus(bbv2.Build build) {
+    if (status.isComplete) {
+      return;
+    }
+    status = build.status.toTaskStatus();
+  }
+}
diff --git a/app_dart/lib/src/model/firestore/presubmit_guard.dart b/app_dart/lib/src/model/firestore/presubmit_guard.dart
new file mode 100644
index 0000000000..c94cae0426
--- /dev/null
+++ b/app_dart/lib/src/model/firestore/presubmit_guard.dart
@@ -0,0 +1,234 @@
+// Copyright 2025 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// @docImport 'presubmit_guard.dart';
+library;
+
+import 'dart:convert';
+
+import 'package:cocoon_common/task_status.dart';
+import 'package:github/github.dart';
+import 'package:googleapis/firestore/v1.dart' hide Status;
+import 'package:path/path.dart' as p;
+
+import '../../../cocoon_service.dart';
+import '../../service/firestore.dart';
+import 'base.dart';
+
+final class PresubmitGuardId extends AppDocumentId<PresubmitGuard> {
+  PresubmitGuardId({
+    required this.slug,
+    required this.pullRequestId,
+    required this.checkRunId,
+    required this.stage,
+  });
+
+  /// The repository owner/name.
+  final RepositorySlug slug;
+
+  /// The pull request id.
+  final int pullRequestId;
+
+  /// The Check Run Id.
+  final int checkRunId;
+
+  /// The stage of the CI process.
+  final CiStage stage;
+
+  @override
+  String get documentId =>
+      [slug.owner, slug.name, pullRequestId, checkRunId, stage].join('_');
+
+  @override
+  AppDocumentMetadata<PresubmitGuard> get runtimeMetadata =>
+      PresubmitGuard.metadata;
+}
+
+final class PresubmitGuard extends AppDocument<PresubmitGuard> {
+  static const collectionId = 'presubmit_guards';
+  static const fieldCheckRun = 'check_run';
+  static const fieldCommitSha = 'commit_sha';
+  static const fieldAuthor = 'author';
+  static const fieldCreationTime = 'creation_time';
+  static const fieldRemainingBuilds = 'remaining_builds';
+  static const fieldFailedBuilds = 'failed_builds';
+  static const fieldBuilds = 'builds';
+
+  static AppDocumentId<PresubmitGuard> documentIdFor({
+    required RepositorySlug slug,
+    required int pullRequestId,
+    required int checkRunId,
+    required CiStage stage,
+  }) => PresubmitGuardId(
+    slug: slug,
+    pullRequestId: pullRequestId,
+    checkRunId: checkRunId,
+    stage: stage,
+  );
+
+  /// Returns a firebase documentName used in [fromFirestore].
+  static String documentNameFor({
+    required RepositorySlug slug,
+    required int pullRequestId,
+    required int checkRunId,
+    required CiStage stage,
+  }) {
+    // Document names cannot cannot have '/' in the document id.
+    final docId = documentIdFor(
+      slug: slug,
+      pullRequestId: pullRequestId,
+      checkRunId: checkRunId,
+      stage: stage,
+    );
+    return '$kDocumentParent/$collectionId/${docId.documentId}';
+  }
+
+  /// Returns the document ID for the given parameters.
+  // static String documentId({
+  //   required RepositorySlug slug,
+  //   required int pullRequestId,
+  //   required int checkRunId,
+  //   required CiStage stage,
+  // }) =>
+  //     '${slug.owner}_${slug.name}_${pullRequestId}_${checkRunId}_${stage.name}';
+
+  @override
+  AppDocumentMetadata<PresubmitGuard> get runtimeMetadata => metadata;
+
+  static final metadata = AppDocumentMetadata<PresubmitGuard>(
+    collectionId: collectionId,
+    fromDocument: PresubmitGuard.fromDocument,
+  );
+
+  factory PresubmitGuard.init({
+    required RepositorySlug slug,
+    required int pullRequestId,
+    required CheckRun checkRun,
+    required CiStage stage,
+    required String commitSha,
+    required int creationTime,
+    required String author,
+    required int buildCount,
+  }) {
+    return PresubmitGuard(
+      checkRun: checkRun,
+      commitSha: commitSha,
+      slug: slug,
+      pullRequestId: pullRequestId,
+      stage: stage,
+      author: author,
+      creationTime: creationTime,
+      remainingBuilds: buildCount,
+      failedBuilds: 0,
+    );
+  }
+
+  factory PresubmitGuard.fromDocument(Document document) {
+    return PresubmitGuard._(document.fields!, name: document.name!);
+  }
+
+  factory PresubmitGuard({
+    required CheckRun checkRun,
+    required String commitSha,
+    required RepositorySlug slug,
+    required int pullRequestId,
+    required CiStage stage,
+    required int creationTime,
+    required String author,
+    int? remainingBuilds,
+    int? failedBuilds,
+    Map<String, TaskStatus>? builds,
+  }) {
+    return PresubmitGuard._(
+      {
+        fieldCommitSha: commitSha.toValue(),
+        fieldCreationTime: creationTime.toValue(),
+        fieldAuthor: author.toValue(),
+        fieldCheckRun: json.encode(checkRun.toJson()).toValue(),
+        fieldRemainingBuilds: ?remainingBuilds?.toValue(),
+        fieldFailedBuilds: ?failedBuilds?.toValue(),
+        if (builds != null)
+          fieldBuilds: Value(
+            mapValue: MapValue(
+              fields: builds.map((k, v) => MapEntry(k, v.value.toValue())),
+            ),
+          ),
+      },
+      name: documentNameFor(
+        slug: slug,
+        pullRequestId: pullRequestId,
+        checkRunId: checkRun.id!,
+        stage: stage,
+      ),
+    );
+  }
+
+  PresubmitGuard._(Map<String, Value> fields, {required String name}) {
+    this.fields = fields;
+    this.name = name;
+  }
+
+  String get commitSha => fields[fieldCommitSha]!.stringValue!;
+  String get author => fields[fieldAuthor]!.stringValue!;
+  int get creationTime => int.parse(fields[fieldCreationTime]!.integerValue!);
+  int? get remainingBuilds => fields[fieldRemainingBuilds] != null
+      ? int.parse(fields[fieldRemainingBuilds]!.integerValue!)
+      : null;
+  int? get failedBuilds => fields[fieldFailedBuilds] != null
+      ? int.parse(fields[fieldFailedBuilds]!.integerValue!)
+      : null;
+  Map<String, TaskStatus>? get builds =>
+      fields[fieldBuilds]?.mapValue?.fields?.map<String, TaskStatus>(
+        (k, v) => MapEntry(k, TaskStatus.from(v.stringValue!)),
+      );
+  CheckRun get checkRun {
+    final jsonData =
+        jsonDecode(fields[fieldCheckRun]!.stringValue!) as Map<String, Object?>;
+    return CheckRun.fromJson(jsonData);
+  }
+
+  /// The repository that this stage is recorded for.
+  RepositorySlug get slug {
+    // Read it from the document name.
+    final [owner, repo, _, _, _] = p.posix.basename(name!).split('_');
+    return RepositorySlug(owner, repo);
+  }
+
+  /// The pull request for which this stage is recorded for.
+  int get pullRequestId {
+    // Read it from the document name.
+    final [_, _, pullRequestId, _, _] = p.posix.basename(name!).split('_');
+    return int.parse(pullRequestId);
+  }
+
+  /// Which commit this stage is recorded for.
+  int get checkRunId {
+    // Read it from the document name.
+    final [_, _, _, checkRunId, _] = p.posix.basename(name!).split('_');
+    return int.parse(checkRunId);
+  }
+
+  /// The stage of the CI process.
+  CiStage get stage {
+    // Read it from the document name.
+    final [_, _, _, _, stageName] = p.posix.basename(name!).split('_');
+    return CiStage.values.firstWhere((e) => e.name == stageName);
+  }
+
+  set remainingBuilds(int remainingBuilds) {
+    fields[fieldRemainingBuilds] = remainingBuilds.toValue();
+  }
+
+  set failedBuilds(int failedBuilds) {
+    fields[fieldFailedBuilds] = failedBuilds.toValue();
+  }
+
+  set builds(Map<String, TaskStatus> builds) {
+    fields[fieldBuilds] = Value(
+      mapValue: MapValue(
+        fields: builds.map((k, v) => MapEntry(k, v.value.toValue())),
+      ),
+    );
+  }
+}
diff --git a/app_dart/lib/src/request_handlers/get_engine_artifacts_ready.dart b/app_dart/lib/src/request_handlers/get_engine_artifacts_ready.dart
index b8a8cb81f3..b7c5cac4e9 100644
--- a/app_dart/lib/src/request_handlers/get_engine_artifacts_ready.dart
+++ b/app_dart/lib/src/request_handlers/get_engine_artifacts_ready.dart
@@ -7,6 +7,7 @@ import 'dart:io';
 import 'package:googleapis/firestore/v1.dart';
 
 import '../../cocoon_service.dart';
+import '../model/firestore/base.dart';
 import '../model/firestore/ci_staging.dart';
 import '../request_handling/exceptions.dart';
 
diff --git a/app_dart/lib/src/request_handlers/presubmit_luci_subscription.dart b/app_dart/lib/src/request_handlers/presubmit_luci_subscription.dart
index 42210ce9c6..941b95f920 100644
--- a/app_dart/lib/src/request_handlers/presubmit_luci_subscription.dart
+++ b/app_dart/lib/src/request_handlers/presubmit_luci_subscription.dart
@@ -158,10 +158,7 @@ final class PresubmitLuciSubscription extends SubscriptionHandler {
   /// It returns 1 if this is the first run.
   static int _nextAttempt(BuildTags buildTags) {
     final attempt = buildTags.getTagOfType<CurrentAttemptBuildTag>();
-    if (attempt == null) {
-      return 1;
-    }
-    return attempt.attemptNumber;
+    return attempt?.attemptNumber ?? 1;
   }
 
   Future<int> _getMaxAttempt(
diff --git a/app_dart/lib/src/service/firestore/unified_check_run.dart b/app_dart/lib/src/service/firestore/unified_check_run.dart
new file mode 100644
index 0000000000..981febb6db
--- /dev/null
+++ b/app_dart/lib/src/service/firestore/unified_check_run.dart
@@ -0,0 +1,393 @@
+// Copyright 2025 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// @docImport 'unified_check_run.dart';
+library;
+
+import 'package:cocoon_common/task_status.dart';
+import 'package:cocoon_server/logging.dart';
+import 'package:collection/collection.dart';
+import 'package:github/github.dart';
+import 'package:googleapis/firestore/v1.dart' hide Status;
+
+import '../../model/common/presubmit_check_state.dart';
+import '../../model/common/presubmit_guard_conclusion.dart';
+import '../../model/firestore/base.dart';
+import '../../model/firestore/ci_staging.dart';
+import '../../model/firestore/presubmit_check.dart';
+import '../../model/firestore/presubmit_guard.dart';
+import '../config.dart';
+import '../firestore.dart';
+
+final class UnifiedCheckRun {
+  static Future<void> initializeCiStagingDocument({
+    required FirestoreService firestoreService,
+    required RepositorySlug slug,
+    required String sha,
+    required CiStage stage,
+    required List<String> tasks,
+    required Config config,
+    PullRequest? pullRequest,
+    CheckRun? checkRun,
+  }) async {
+    if (checkRun != null &&
+        pullRequest != null &&
+        config.flags.isUnifiedCheckRunFlowEnabledForUser(
+          pullRequest.user!.login!,
+        )) {
+      log.info(
+        'Storing UnifiedCheckRun data for ${slug.fullName}#${pullRequest.number} as it enabled for user ${pullRequest.user!.login}.',
+      );
+      // Create the UnifiedCheckRun and UnifiedCheckRunBuilds.
+      final guard = PresubmitGuard(
+        checkRun: checkRun,
+        commitSha: sha,
+        slug: slug,
+        pullRequestId: pullRequest.number!,
+        stage: stage,
+        creationTime: pullRequest.createdAt!.microsecondsSinceEpoch,
+        author: pullRequest.user!.login!,
+        remainingBuilds: null,
+        failedBuilds: null,
+        builds: {for (final task in tasks) task: TaskStatus.waitingForBackfill},
+      );
+      final checks = [
+        for (final task in tasks)
+          PresubmitCheck.init(
+            buildName: task,
+            checkRunId: checkRun.id!,
+            creationTime: pullRequest.createdAt!.microsecondsSinceEpoch,
+          ),
+      ];
+      await firestoreService.writeViaTransaction(
+        documentsToWrites([...checks, guard], exists: false),
+      );
+    } else {
+      // Initialize the CiStaging document.
+      await CiStaging.initializeDocument(
+        firestoreService: firestoreService,
+        slug: slug,
+        sha: sha,
+        stage: stage,
+        tasks: tasks,
+        checkRunGuard: checkRun != null ? '$checkRun' : '',
+      );
+    }
+  }
+
+  /// Initializes a new document for the given [tasks] in Firestore so that stage-tracking can succeed.
+  ///
+  /// The list of tasks will be written as fields of a document with additional fields for tracking the creationTime
+  /// number of tasks, remaining count. It is required to include [commitSha] as a json encoded [CheckRun] as this
+  /// will be used to unlock any check runs blocking progress.
+  ///
+  /// Returns the created document or throws an error.
+  static Future<Document> initializePresubmitGuardDocument({
+    required FirestoreService firestoreService,
+
+    required RepositorySlug slug,
+    required int pullRequestId,
+    required CheckRun checkRun,
+    required CiStage stage,
+    required String commitSha,
+    required int creationTime,
+    required String author,
+    required List<String> tasks,
+  }) async {
+    final buildCount = tasks.length;
+    final logCrumb =
+        'initializeDocument(${slug.owner}_${slug.name}_${pullRequestId}_${checkRun.id}_$stage, $buildCount builds)';
+
+    final presubmitGuard = PresubmitGuard(
+      checkRun: checkRun,
+      commitSha: commitSha,
+      slug: slug,
+      pullRequestId: pullRequestId,
+      stage: stage,
+      creationTime: creationTime,
+      author: author,
+      remainingBuilds: buildCount,
+      failedBuilds: 0,
+      builds: {for (final task in tasks) task: TaskStatus.waitingForBackfill},
+    );
+
+    final presubmitGuardId = PresubmitGuard.documentIdFor(
+      slug: slug,
+      pullRequestId: pullRequestId,
+      checkRunId: checkRun.id!,
+      stage: stage,
+    );
+
+    try {
+      // Calling createDocument multiple times for the same documentId will return a 409 - ALREADY_EXISTS error;
+      // this is good because it means we don't have to do any transactions.
+      // curl -X POST -H "Content-Type: application/json" -H "Authorization: Bearer <TOKEN>" "https://firestore.googleapis.com/v1beta1/projects/flutter-dashboard/databases/cocoon/documents/presubmit_guard?documentId=foo_bar_baz" -d '{"fields": {"test": {"stringValue": "baz"}}}'
+      final newDoc = await firestoreService.createDocument(
+        presubmitGuard,
+        collectionId: presubmitGuard.runtimeMetadata.collectionId,
+        documentId: presubmitGuardId.documentId,
+      );
+      log.info('$logCrumb: document created');
+      return newDoc;
+    } catch (e) {
+      log.warn('$logCrumb: failed to create document', e);
+      rethrow;
+    }
+  }
+
+  /// Returns _all_ checks running against the specified github [checkRunId].
+  static Future<List<PresubmitCheck>> queryAllPresubmitChecksForGuard({
+    required FirestoreService firestoreService,
+    required int checkRunId,
+    TaskStatus? status,
+    String? buildName,
+    Transaction? transaction,
+  }) async {
+    return await _queryPresubmitChecks(
+      firestoreService: firestoreService,
+      checkRunId: checkRunId,
+      buildName: buildName,
+      status: status,
+      transaction: transaction,
+    );
+  }
+
+  /// Returns check for the specified github [checkRunId] and
+  /// [buildName] and [attemptNumber].
+  static Future<PresubmitCheck?> queryPresubmitCheck({
+    required FirestoreService firestoreService,
+    required int checkRunId,
+    required String buildName,
+    required int attemptNumber,
+    Transaction? transaction,
+  }) async {
+    return (await _queryPresubmitChecks(
+      firestoreService: firestoreService,
+      checkRunId: checkRunId,
+      buildName: buildName,
+      status: null,
+      attemptNumber: attemptNumber,
+      transaction: transaction,
+    )).firstOrNull;
+  }
+
+  static Future<List<PresubmitCheck>> _queryPresubmitChecks({
+    required FirestoreService firestoreService,
+    required int checkRunId,
+    String? buildName,
+    TaskStatus? status,
+    Transaction? transaction,
+    int? attemptNumber,
+  }) async {
+    final filterMap = {
+      '${PresubmitCheck.fieldCheckRunId} =': checkRunId,
+      '${PresubmitCheck.fieldBuildName} =': ?buildName,
+      '${PresubmitCheck.fieldStatus} =': ?status?.value,
+      '${PresubmitCheck.fieldAttemptNumber} =': ?attemptNumber,
+    };
+    // For tasks, therer is no reason to _not_ order this way.
+    final orderMap = {PresubmitCheck.fieldCreationTime: kQueryOrderDescending};
+    final documents = await firestoreService.query(
+      collectionId,
+      filterMap,
+      orderMap: orderMap,
+      transaction: transaction,
+    );
+    return [...documents.map(PresubmitCheck.fromDocument)];
+  }
+
+  /// Mark a [buildName] for a given [stage] with [conclusion].
+  ///
+  /// Returns a [PresubmitGuardConclusion] record or throws. If the check_run was
+  /// both valid and recorded successfully, the record's `remaining` value
+  /// signals how many more tests are running. Returns the record (valid: false)
+  /// otherwise.
+  static Future<PresubmitGuardConclusion> markConclusion({
+    required FirestoreService firestoreService,
+    required PresubmitGuardId guardId,
+    required PresubmitCheckState state,
+  }) async {
+    final changeCrumb =
+        '${guardId.slug.owner}_${guardId.slug.name}_${guardId.pullRequestId}_${guardId.checkRunId}';
+    final logCrumb =
+        'markConclusion(${changeCrumb}_${guardId.stage}, ${state.buildName}, ${state.status}, ${state.attemptNumber})';
+
+    // Marking needs to happen while in a transaction to ensure `remaining` is
+    // updated correctly. For that to happen correctly; we need to perform a
+    // read of the document in the transaction as well. So start the transaction
+    // first thing.
+    final transaction = await firestoreService.beginTransaction();
+
+    var remaining = -1;
+    var failed = -1;
+    var valid = false;
+
+    late final PresubmitGuard presubmitGuard;
+    late final PresubmitCheck presubmitCheck;
+    // transaction block
+    try {
+      // First: read the fields we want to change.
+      final presubmitGuardDocumentName = PresubmitGuard.documentNameFor(
+        slug: guardId.slug,
+        pullRequestId: guardId.pullRequestId,
+        checkRunId: guardId.checkRunId,
+        stage: guardId.stage,
+      );
+      final presubmitGuardDocument = await firestoreService.getDocument(
+        presubmitGuardDocumentName,
+        transaction: transaction,
+      );
+      presubmitGuard = PresubmitGuard.fromDocument(presubmitGuardDocument);
+
+      // Check if the build is present in the guard before trying to load it.
+      if (presubmitGuard.builds?[state.buildName] == null) {
+        log.info(
+          '$logCrumb: ${state.buildName} with attemptNumber ${state.attemptNumber} not present for $transaction / ${presubmitGuardDocument.fields}',
+        );
+        await firestoreService.rollback(transaction);
+        return PresubmitGuardConclusion(
+          result: PresubmitGuardConclusionResult.missing,
+          remaining: presubmitGuard.remainingBuilds ?? -1,
+          checkRunId: guardId.checkRunId,
+          failed: presubmitGuard.failedBuilds ?? -1,
+          summary:
+              'Check run "${state.buildName}" not present in ${guardId.stage} CI stage',
+          details: 'Change $changeCrumb',
+        );
+      }
+
+      final checkDocName = PresubmitCheck.documentNameFor(
+        checkRunId: guardId.checkRunId,
+        buildName: state.buildName,
+        attemptNumber: state.attemptNumber,
+      );
+      final presubmitCheckDocument = await firestoreService.getDocument(
+        checkDocName,
+        transaction: transaction,
+      );
+      presubmitCheck = PresubmitCheck.fromDocument(presubmitCheckDocument);
+
+      remaining = presubmitGuard.remainingBuilds!;
+      failed = presubmitGuard.failedBuilds!;
+      final builds = presubmitGuard.builds;
+
+      // If build is in progress, we should only update appropriate checks with
+      // that [TaskStatus]
+      if (state.isBuildInProgress) {
+        presubmitCheck.startTime = state.startTime!;
+      } else {
+        // "remaining" should go down if buildSuccessed of any attempt
+        // or buildFailed a first attempt.
+        // "failed_count" can go up or down depending on:
+        //   attemptNumber > 1 && buildSuccessed: down (-1)
+        //   attemptNumber = 1 && buildFailed: up (+1)
+        // So if the test existed and either remaining or failed_count is changed;
+        // the response is valid.
+
+        if (state.isBuildSuccessed ||
+            state.attemptNumber == 1 && state.isBuildFailed) {
+          // Guard against going negative and log enough info so we can debug.
+          if (remaining == 0) {
+            throw '$logCrumb: field "${PresubmitGuard.fieldRemainingBuilds}" is already zero for $transaction / ${presubmitGuardDocument.fields}';
+          }
+          remaining = remaining - 1;
+          valid = true;
+        }
+
+        // Only rollback the "failed" counter if this is a successful test run,
+        // i.e. the test failed, the user requested a rerun, and now it passes.
+        if (state.attemptNumber > 1 && state.isBuildSuccessed) {
+          log.info(
+            '$logCrumb: conclusion flipped to positive - assuming test was re-run',
+          );
+          if (failed == 0) {
+            throw '$logCrumb: field "${PresubmitGuard.fieldFailedBuilds}" is already zero for $transaction / ${presubmitGuardDocument.fields}';
+          }
+          valid = true;
+          failed = failed - 1;
+        }
+
+        // Only increment the "failed" counter if the conclusion failed for the first attempt.
+        if (state.attemptNumber == 1 && state.isBuildFailed) {
+          log.info('$logCrumb: test failed');
+          valid = true;
+          failed = failed + 1;
+        }
+
+        // All checks pass. "valid" is only set to true if there was a change in either the remaining or failed count.
+        log.info(
+          '$logCrumb: setting remaining to $remaining, failed to $failed',
+        );
+        presubmitGuard.remainingBuilds = remaining;
+        presubmitGuard.failedBuilds = failed;
+        presubmitCheck.endTime = state.endTime!;
+      }
+      builds![state.buildName] = state.status;
+      presubmitGuard.builds = builds;
+      presubmitCheck.status = state.status;
+    } on DetailedApiRequestError catch (e, stack) {
+      if (e.status == 404) {
+        // An attempt to read a document not in firestore should not be retried.
+        log.info(
+          '$logCrumb: $collectionId document not found for $transaction',
+        );
+        await firestoreService.rollback(transaction);
+        return PresubmitGuardConclusion(
+          result: PresubmitGuardConclusionResult.internalError,
+          remaining: -1,
+          checkRunId: null,
+          failed: failed,
+          summary: 'Internal server error',
+          details:
+              '''
+$collectionId document not found for stage "${guardId.stage}" for $changeCrumb. Got 404 from Firestore.
+Error: ${e.toString()}
+$stack
+''',
+        );
+      }
+      // All other errors should bubble up and be retried.
+      await firestoreService.rollback(transaction);
+      rethrow;
+    } catch (e) {
+      // All other errors should bubble up and be retried.
+      await firestoreService.rollback(transaction);
+      rethrow;
+    }
+    // Commit this write firebase and if no one else was writing at the same time, return success.
+    // If this commit fails, that means someone else modified firestore and the caller should try again.
+    // We do not need to rollback the transaction; firebase documentation says a failed commit takes care of that.
+    try {
+      final response = await firestoreService.commit(
+        transaction,
+        documentsToWrites([presubmitGuard, presubmitCheck], exists: true),
+      );
+      log.info(
+        '$logCrumb: results = ${response.writeResults?.map((e) => e.toJson())}',
+      );
+      return PresubmitGuardConclusion(
+        result: valid
+            ? PresubmitGuardConclusionResult.ok
+            : PresubmitGuardConclusionResult.internalError,
+        remaining: remaining,
+        checkRunId: guardId.checkRunId,
+        failed: failed,
+        summary: valid
+            ? 'Successfully updated presubmit guard status'
+            : 'Not a valid state transition for ${state.buildName}',
+        details: valid
+            ? '''
+For CI stage ${guardId.stage}:
+  Pending: $remaining
+  Failed: $failed
+'''
+            : 'Attempted to set the state of check run ${state.buildName} '
+                  'to "${state.status.name}".',
+      );
+    } catch (e) {
+      log.info('$logCrumb: failed to update presubmit check', e);
+      rethrow;
+    }
+  }
+}
diff --git a/app_dart/lib/src/service/luci_build_service/build_tags.dart b/app_dart/lib/src/service/luci_build_service/build_tags.dart
index 93c41d2645..e472f6544b 100644
--- a/app_dart/lib/src/service/luci_build_service/build_tags.dart
+++ b/app_dart/lib/src/service/luci_build_service/build_tags.dart
@@ -75,6 +75,20 @@ final class BuildTags {
   List<bbv2.StringPair> toStringPairs() {
     return buildTags.map((e) => e.toStringPair()).toList();
   }
+
+  /// the current reschedule attempt.
+  ///
+  /// It returns 1 if this is the first run.
+  int get currentAttempt {
+    final attempt = getTagOfType<CurrentAttemptBuildTag>();
+    return attempt?.attemptNumber ?? 1;
+  }
+
+  /// GitHub Pull Request Number
+  int get pullRequestNumber {
+    final prTag = getTagOfType<GitHubPullRequestBuildTag>();
+    return prTag!.pullRequestNumber;
+  }
 }
 
 /// Valid tags for [bbv2.ScheduleBuildRequest.tags].
@@ -320,6 +334,16 @@ final class GitHubCheckRunIdBuildTag extends BuildTag {
   final int checkRunId;
 }
 
+/// A link back to the GitHub checkRun for this build.
+final class GuardCheckRunIdBuildTag extends BuildTag {
+  static const _keyName = 'guard_checkrun';
+  GuardCheckRunIdBuildTag({required this.checkRunId})
+    : super(_keyName, '$checkRunId');
+
+  /// ID of the checkRun.
+  final int checkRunId;
+}
+
 /// A build tag that specifies the ID of the scheduling job.
 ///
 /// For Flutter, this is always `flutter/{Build Target}`.
diff --git a/app_dart/lib/src/service/scheduler.dart b/app_dart/lib/src/service/scheduler.dart
index 881a7d1da4..9acbb720af 100644
--- a/app_dart/lib/src/service/scheduler.dart
+++ b/app_dart/lib/src/service/scheduler.dart
@@ -20,6 +20,7 @@ import '../foundation/utils.dart';
 import '../model/ci_yaml/ci_yaml.dart';
 import '../model/ci_yaml/target.dart';
 import '../model/commit_ref.dart';
+import '../model/firestore/base.dart';
 import '../model/firestore/ci_staging.dart';
 import '../model/firestore/commit.dart' as fs;
 import '../model/firestore/pr_check_runs.dart';
diff --git a/app_dart/test/model/firestore/ci_staging_test.dart b/app_dart/test/model/firestore/ci_staging_test.dart
index e189baf4fe..a364f3403d 100644
--- a/app_dart/test/model/firestore/ci_staging_test.dart
+++ b/app_dart/test/model/firestore/ci_staging_test.dart
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:cocoon_server_test/test_logging.dart';
+import 'package:cocoon_service/src/model/firestore/base.dart';
 import 'package:cocoon_service/src/model/firestore/ci_staging.dart';
 import 'package:cocoon_service/src/service/firestore.dart';
 import 'package:github/github.dart';
diff --git a/app_dart/test/model/firestore/presubmit_check_test.dart b/app_dart/test/model/firestore/presubmit_check_test.dart
new file mode 100644
index 0000000000..8548f209a4
--- /dev/null
+++ b/app_dart/test/model/firestore/presubmit_check_test.dart
@@ -0,0 +1,174 @@
+// Copyright 2025 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:buildbucket/buildbucket_pb.dart' as bbv2;
+import 'package:cocoon_common/task_status.dart';
+import 'package:cocoon_server_test/test_logging.dart';
+import 'package:cocoon_service/src/model/firestore/presubmit_check.dart';
+import 'package:fixnum/fixnum.dart';
+import 'package:googleapis/firestore/v1.dart';
+import 'package:test/test.dart';
+
+import '../../src/service/fake_firestore_service.dart';
+
+void main() {
+  useTestLoggerPerTest();
+
+  group('PresubmitCheckId', () {
+    test('validates checkRunId', () {
+      expect(
+        () => PresubmitCheckId(
+          checkRunId: 0,
+          buildName: 'linux',
+          attemptNumber: 1,
+        ),
+        throwsA(isA<RangeError>()),
+      );
+    });
+
+    test('validates attemptNumber', () {
+      expect(
+        () => PresubmitCheckId(
+          checkRunId: 1,
+          buildName: 'linux',
+          attemptNumber: 0,
+        ),
+        throwsA(isA<RangeError>()),
+      );
+    });
+
+    test('generates correct documentId', () {
+      final id = PresubmitCheckId(
+        checkRunId: 123,
+        buildName: 'linux_test',
+        attemptNumber: 2,
+      );
+      expect(id.documentId, '123_linux_test_2');
+    });
+
+    test('parses valid documentName', () {
+      final id = PresubmitCheckId.parse('123_linux_test_2');
+      expect(id.checkRunId, 123);
+      expect(id.buildName, 'linux_test');
+      expect(id.attemptNumber, 2);
+    });
+
+    test('tryParse returns null for invalid format', () {
+      expect(PresubmitCheckId.tryParse('invalid'), isNull);
+      expect(PresubmitCheckId.tryParse('123_linux'), isNull);
+    });
+  });
+
+  group('PresubmitCheck', () {
+    late FakeFirestoreService firestoreService;
+
+    setUp(() {
+      firestoreService = FakeFirestoreService();
+    });
+
+    test('init creates correct initial state', () {
+      final check = PresubmitCheck.init(
+        buildName: 'linux',
+        checkRunId: 123,
+        creationTime: 1000,
+      );
+
+      expect(check.buildName, 'linux');
+      expect(check.checkRunId, 123);
+      expect(check.creationTime, 1000);
+      expect(check.attemptNumber, 1);
+      expect(check.status, TaskStatus.waitingForBackfill);
+      expect(check.buildNumber, isNull);
+      expect(check.startTime, isNull);
+      expect(check.endTime, isNull);
+      expect(check.summary, isNull);
+    });
+
+    test('fromFirestore loads document correctly', () async {
+      final check = PresubmitCheck(
+        checkRunId: 123,
+        buildName: 'linux',
+        status: TaskStatus.succeeded,
+        attemptNumber: 1,
+        creationTime: 1000,
+        buildNumber: 456,
+        startTime: 2000,
+        endTime: 3000,
+        summary: 'Success',
+      );
+
+      // Use the helper to get the correct document name
+      final docName = PresubmitCheck.documentNameFor(
+        checkRunId: 123,
+        buildName: 'linux',
+        attemptNumber: 1,
+      );
+
+      // Manually ensuring the name is set for the fake service, usually done by `putDocument`
+      // but we should verify the `PresubmitCheck` object has it right via the factory or init.
+      // Actually `PresubmitCheck` constructor sets name.
+      firestoreService.putDocument(
+        Document(name: docName, fields: check.fields),
+      );
+
+      final loadedCheck = await PresubmitCheck.fromFirestore(
+        firestoreService,
+        PresubmitCheckId(checkRunId: 123, buildName: 'linux', attemptNumber: 1),
+      );
+
+      expect(loadedCheck.checkRunId, 123);
+      expect(loadedCheck.buildName, 'linux');
+      expect(loadedCheck.status, TaskStatus.succeeded);
+      expect(loadedCheck.attemptNumber, 1);
+      expect(loadedCheck.creationTime, 1000);
+      expect(loadedCheck.buildNumber, 456);
+      expect(loadedCheck.startTime, 2000);
+      expect(loadedCheck.endTime, 3000);
+      expect(loadedCheck.summary, 'Success');
+    });
+
+    test('updateFromBuild updates fields', () {
+      final check = PresubmitCheck.init(
+        buildName: 'linux',
+        checkRunId: 123,
+        creationTime: 1000,
+      );
+
+      final build = bbv2.Build(
+        number: 456,
+        createTime: bbv2.Timestamp(seconds: Int64(2000)),
+        startTime: bbv2.Timestamp(seconds: Int64(2100)),
+        endTime: bbv2.Timestamp(seconds: Int64(2200)),
+        status: bbv2.Status.SUCCESS,
+      );
+
+      check.updateFromBuild(build);
+
+      expect(check.buildNumber, 456);
+      expect(check.creationTime, 2000000); // seconds to millis
+      expect(check.startTime, 2100000);
+      expect(check.endTime, 2200000);
+      expect(check.status, TaskStatus.succeeded);
+    });
+
+    test('updateFromBuild does not update status if already complete', () {
+      final check = PresubmitCheck.init(
+        buildName: 'linux',
+        checkRunId: 123,
+        creationTime: 1000,
+      );
+      check.status = TaskStatus.succeeded;
+
+      final build = bbv2.Build(
+        number: 456,
+        createTime: bbv2.Timestamp(seconds: Int64(2000)),
+        status: bbv2.Status.STARTED,
+      );
+
+      check.updateFromBuild(build);
+
+      expect(check.status, TaskStatus.succeeded);
+    });
+  });
+}
diff --git a/app_dart/test/model/firestore/presubmit_guard_test.dart b/app_dart/test/model/firestore/presubmit_guard_test.dart
new file mode 100644
index 0000000000..e74d8a24df
--- /dev/null
+++ b/app_dart/test/model/firestore/presubmit_guard_test.dart
@@ -0,0 +1,127 @@
+// Copyright 2025 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:cocoon_common/task_status.dart';
+import 'package:cocoon_server_test/test_logging.dart';
+import 'package:cocoon_service/src/model/firestore/base.dart';
+import 'package:cocoon_service/src/model/firestore/presubmit_guard.dart';
+
+import 'package:github/github.dart';
+import 'package:googleapis/firestore/v1.dart';
+import 'package:test/test.dart';
+
+void main() {
+  useTestLoggerPerTest();
+
+  group('PresubmitGuardId', () {
+    test('generates correct documentId', () {
+      final id = PresubmitGuardId(
+        slug: RepositorySlug('flutter', 'flutter'),
+        pullRequestId: 123,
+        checkRunId: 456,
+        stage: CiStage.fusionEngineBuild,
+      );
+      expect(id.documentId, 'flutter_flutter_123_456_engine');
+    });
+  });
+
+  group('PresubmitGuard', () {
+    final slug = RepositorySlug('flutter', 'flutter');
+    final checkRun = CheckRun.fromJson({
+      'id': 456,
+      'name': 'Merge Queue Guard',
+      'head_sha': 'abc',
+      'started_at': DateTime.now().toIso8601String(),
+      'check_suite': {'id': 789},
+    });
+
+    test('init creates correct initial state', () {
+      final guard = PresubmitGuard.init(
+        slug: slug,
+        pullRequestId: 123,
+        checkRun: checkRun,
+        stage: CiStage.fusionEngineBuild,
+        commitSha: 'abc',
+        creationTime: 1000,
+        author: 'author',
+        buildCount: 2,
+      );
+
+      expect(guard.slug, slug);
+      expect(guard.pullRequestId, 123);
+      expect(guard.checkRunId, 456);
+      expect(guard.stage, CiStage.fusionEngineBuild);
+      expect(guard.commitSha, 'abc');
+      expect(guard.creationTime, 1000);
+      expect(guard.author, 'author');
+      expect(guard.remainingBuilds, 2);
+      expect(guard.failedBuilds, 0);
+      expect(guard.checkRun.id, 456);
+    });
+
+    test('fromDocument loads correct state', () {
+      final guard = PresubmitGuard.init(
+        slug: slug,
+        pullRequestId: 123,
+        checkRun: checkRun,
+        stage: CiStage.fusionEngineBuild,
+        commitSha: 'abc',
+        creationTime: 1000,
+        author: 'author',
+        buildCount: 2,
+      );
+      guard.builds = {'linux': TaskStatus.succeeded};
+
+      final doc = Document(name: guard.name, fields: guard.fields);
+
+      final loadedGuard = PresubmitGuard.fromDocument(doc);
+
+      expect(loadedGuard.slug, slug);
+      expect(loadedGuard.pullRequestId, 123);
+      expect(loadedGuard.checkRunId, 456);
+      expect(loadedGuard.stage, CiStage.fusionEngineBuild);
+      expect(loadedGuard.builds, {'linux': TaskStatus.succeeded});
+      expect(loadedGuard.remainingBuilds, 2);
+    });
+
+    test('updates fields correctly', () {
+      final guard = PresubmitGuard.init(
+        slug: slug,
+        pullRequestId: 123,
+        checkRun: checkRun,
+        stage: CiStage.fusionEngineBuild,
+        commitSha: 'abc',
+        creationTime: 1000,
+        author: 'author',
+        buildCount: 2,
+      );
+
+      guard.remainingBuilds = 1;
+      guard.failedBuilds = 1;
+      guard.builds = {'linux': TaskStatus.failed};
+
+      expect(guard.remainingBuilds, 1);
+      expect(guard.failedBuilds, 1);
+      expect(guard.builds, {'linux': TaskStatus.failed});
+    });
+
+    test('parses properties from document name', () {
+      // flutter_flutter_123_456_fusionEngineBuild
+      final guard = PresubmitGuard(
+        checkRun: checkRun,
+        commitSha: 'abc',
+        slug: slug,
+        pullRequestId: 123,
+        stage: CiStage.fusionEngineBuild,
+        creationTime: 1000,
+        author: 'author',
+      );
+
+      expect(guard.slug, slug);
+      expect(guard.pullRequestId, 123);
+      expect(guard.checkRunId, 456);
+      expect(guard.stage, CiStage.fusionEngineBuild);
+    });
+  });
+}
diff --git a/app_dart/test/request_handlers/get_engine_artifacts_ready_test.dart b/app_dart/test/request_handlers/get_engine_artifacts_ready_test.dart
index dbd0df8cab..9d2484c506 100644
--- a/app_dart/test/request_handlers/get_engine_artifacts_ready_test.dart
+++ b/app_dart/test/request_handlers/get_engine_artifacts_ready_test.dart
@@ -6,6 +6,7 @@ import 'dart:convert';
 
 import 'package:cocoon_server_test/test_logging.dart';
 import 'package:cocoon_service/src/model/common/firestore_extensions.dart';
+import 'package:cocoon_service/src/model/firestore/base.dart';
 import 'package:cocoon_service/src/model/firestore/ci_staging.dart';
 import 'package:cocoon_service/src/request_handlers/get_engine_artifacts_ready.dart';
 import 'package:cocoon_service/src/request_handling/exceptions.dart';
diff --git a/app_dart/test/request_handlers/github/webhook_subscription_test.dart b/app_dart/test/request_handlers/github/webhook_subscription_test.dart
index 75e3157058..bd11cbfb81 100644
--- a/app_dart/test/request_handlers/github/webhook_subscription_test.dart
+++ b/app_dart/test/request_handlers/github/webhook_subscription_test.dart
@@ -11,6 +11,7 @@ import 'package:cocoon_common_test/cocoon_common_test.dart';
 import 'package:cocoon_server/logging.dart';
 import 'package:cocoon_server_test/mocks.dart';
 import 'package:cocoon_server_test/test_logging.dart';
+import 'package:cocoon_service/src/model/firestore/base.dart';
 import 'package:cocoon_service/src/model/firestore/ci_staging.dart';
 import 'package:cocoon_service/src/model/firestore/commit.dart' as fs;
 import 'package:cocoon_service/src/model/firestore/pr_check_runs.dart';
diff --git a/app_dart/test/service/firestore/unified_check_run_test.dart b/app_dart/test/service/firestore/unified_check_run_test.dart
new file mode 100644
index 0000000000..ed01d1be10
--- /dev/null
+++ b/app_dart/test/service/firestore/unified_check_run_test.dart
@@ -0,0 +1,286 @@
+// Copyright 2025 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:cocoon_common/task_status.dart';
+import 'package:cocoon_server_test/test_logging.dart';
+import 'package:cocoon_service/src/model/common/presubmit_check_state.dart';
+import 'package:cocoon_service/src/model/common/presubmit_guard_conclusion.dart';
+import 'package:cocoon_service/src/model/firestore/base.dart';
+import 'package:cocoon_service/src/model/firestore/presubmit_check.dart';
+import 'package:cocoon_service/src/model/firestore/presubmit_guard.dart';
+import 'package:cocoon_service/src/service/firestore.dart';
+import 'package:cocoon_service/src/service/firestore/unified_check_run.dart';
+import 'package:cocoon_service/src/service/flags/dynamic_config.dart';
+import 'package:github/github.dart';
+import 'package:googleapis/firestore/v1.dart';
+import 'package:test/test.dart';
+
+import '../../src/fake_config.dart';
+import '../../src/service/fake_firestore_service.dart';
+import '../../src/utilities/entity_generators.dart';
+
+void main() {
+  useTestLoggerPerTest();
+
+  late FakeConfig config;
+  late FakeFirestoreService firestoreService;
+
+  setUp(() {
+    config = FakeConfig();
+    firestoreService = FakeFirestoreService();
+  });
+
+  group('UnifiedCheckRun', () {
+    final slug = RepositorySlug('flutter', 'flutter');
+    final checkRun = CheckRun.fromJson({
+      'id': 123,
+      'name': 'check_run',
+      'head_sha': 'context_sha',
+      'started_at': DateTime.now().toIso8601String(),
+      'check_suite': {'id': 456},
+    });
+    final pullRequest = generatePullRequest(
+      id: 789,
+      authorLogin: 'dash',
+      number: 1,
+    );
+    const sha = 'sha';
+
+    group('initializeCiStagingDocument', () {
+      test('creates PresubmitGuard and Checks when enabled for user', () async {
+        config.dynamicConfig = DynamicConfig.fromJson({
+          'unifiedCheckRunFlow': {
+            'useForUsers': ['dash'],
+          },
+        });
+
+        await UnifiedCheckRun.initializeCiStagingDocument(
+          firestoreService: firestoreService,
+          slug: slug,
+          sha: sha,
+          stage: CiStage.fusionEngineBuild,
+          tasks: ['linux', 'mac'],
+          config: config,
+          pullRequest: pullRequest,
+          checkRun: checkRun,
+        );
+
+        final guardId = PresubmitGuard.documentIdFor(
+          slug: slug,
+          pullRequestId: 1,
+          checkRunId: 123,
+          stage: CiStage.fusionEngineBuild,
+        );
+        final guardDoc = await firestoreService.getDocument(
+          'projects/flutter-dashboard/databases/cocoon/documents/presubmit_guards/${guardId.documentId}',
+        );
+        expect(guardDoc.name, endsWith(guardId.documentId));
+
+        final checkId = PresubmitCheck.documentIdFor(
+          checkRunId: 123,
+          buildName: 'linux',
+          attemptNumber: 1,
+        );
+        final checkDoc = await firestoreService.getDocument(
+          'projects/flutter-dashboard/databases/cocoon/documents/presubmit_checks/${checkId.documentId}',
+        );
+        expect(checkDoc.name, endsWith(checkId.documentId));
+      });
+
+      test('initializes CiStagingDocument when NOT enabled for user', () async {
+        config.dynamicConfig = DynamicConfig.fromJson({
+          'unifiedCheckRunFlow': {'useForUsers': <String>[]},
+        });
+
+        await UnifiedCheckRun.initializeCiStagingDocument(
+          firestoreService: firestoreService,
+          slug: slug,
+          sha: sha,
+          stage: CiStage.fusionEngineBuild,
+          tasks: ['linux', 'mac'],
+          config: config,
+          pullRequest: pullRequest,
+          checkRun: checkRun,
+        );
+
+        // Verify PresubmitGuard is NOT created
+        final guardId = PresubmitGuard.documentIdFor(
+          slug: slug,
+          pullRequestId: 1,
+          checkRunId: 123,
+          stage: CiStage.fusionEngineBuild,
+        );
+        expect(
+          () => firestoreService.getDocument(
+            'projects/flutter-dashboard/databases/cocoon/documents/presubmit_guards/${guardId.documentId}',
+          ),
+          throwsA(isA<Exception>()),
+        );
+      });
+    });
+
+    group('markConclusion', () {
+      late PresubmitGuardId guardId;
+
+      setUp(() async {
+        guardId = PresubmitGuardId(
+          slug: slug,
+          pullRequestId: 1,
+          checkRunId: 123,
+          stage: CiStage.fusionEngineBuild,
+        );
+
+        // Initialize documents
+        await UnifiedCheckRun.initializePresubmitGuardDocument(
+          firestoreService: firestoreService,
+          slug: slug,
+          pullRequestId: 1,
+          checkRun: checkRun,
+          stage: CiStage.fusionEngineBuild,
+          commitSha: sha,
+          creationTime: 1000,
+          author: 'dash',
+          tasks: ['linux', 'mac'],
+        );
+
+        final check1 = PresubmitCheck.init(
+          buildName: 'linux',
+          checkRunId: 123,
+          creationTime: 1000,
+        );
+        final check2 = PresubmitCheck.init(
+          buildName: 'mac',
+          checkRunId: 123,
+          creationTime: 1000,
+        );
+
+        await firestoreService.writeViaTransaction(
+          documentsToWrites([
+            Document(name: check1.name, fields: check1.fields),
+            Document(name: check2.name, fields: check2.fields),
+          ], exists: false),
+        );
+      });
+
+      test('updates check status and remaining count on success', () async {
+        final state = const PresubmitCheckState(
+          buildName: 'linux',
+          status: TaskStatus.succeeded,
+          attemptNumber: 1,
+          startTime: 2000,
+          endTime: 3000,
+        );
+
+        final result = await UnifiedCheckRun.markConclusion(
+          firestoreService: firestoreService,
+          guardId: guardId,
+          state: state,
+        );
+
+        expect(result.result, PresubmitGuardConclusionResult.ok);
+        expect(result.remaining, 1);
+        expect(result.failed, 0);
+
+        final checkDoc = await PresubmitCheck.fromFirestore(
+          firestoreService,
+          PresubmitCheckId(
+            checkRunId: 123,
+            buildName: 'linux',
+            attemptNumber: 1,
+          ),
+        );
+        expect(checkDoc.status, TaskStatus.succeeded);
+        expect(checkDoc.endTime, 3000);
+      });
+
+      test(
+        'update all check status to succeeded lead to complete guard',
+        () async {
+          final result1 = await UnifiedCheckRun.markConclusion(
+            firestoreService: firestoreService,
+            guardId: guardId,
+            state: const PresubmitCheckState(
+              buildName: 'linux',
+              status: TaskStatus.succeeded,
+              attemptNumber: 1,
+              startTime: 2000,
+              endTime: 3000,
+            ),
+          );
+
+          expect(result1.remaining, 1);
+          expect(result1.failed, 0);
+          expect(result1.isOk, true);
+          expect(result1.isComplete, false);
+          expect(result1.isPending, true);
+
+          final result2 = await UnifiedCheckRun.markConclusion(
+            firestoreService: firestoreService,
+            guardId: guardId,
+            state: const PresubmitCheckState(
+              buildName: 'mac',
+              status: TaskStatus.succeeded,
+              attemptNumber: 1,
+              startTime: 2000,
+              endTime: 3000,
+            ),
+          );
+
+          expect(result2.remaining, 0);
+          expect(result2.failed, 0);
+          expect(result2.isOk, true);
+          expect(result2.isComplete, true);
+          expect(result2.isPending, false);
+
+          final checkDoc = await PresubmitCheck.fromFirestore(
+            firestoreService,
+            PresubmitCheckId(
+              checkRunId: 123,
+              buildName: 'linux',
+              attemptNumber: 1,
+            ),
+          );
+          expect(checkDoc.status, TaskStatus.succeeded);
+          expect(checkDoc.endTime, 3000);
+        },
+      );
+
+      test('updates check status and failed count on failure', () async {
+        final state = const PresubmitCheckState(
+          buildName: 'linux',
+          status: TaskStatus.failed,
+          attemptNumber: 1,
+          startTime: 2000,
+          endTime: 3000,
+        );
+
+        final result = await UnifiedCheckRun.markConclusion(
+          firestoreService: firestoreService,
+          guardId: guardId,
+          state: state,
+        );
+
+        expect(result.result, PresubmitGuardConclusionResult.ok);
+        expect(result.remaining, 1);
+        expect(result.failed, 1);
+      });
+
+      test('handles missing check gracefully', () async {
+        final state = const PresubmitCheckState(
+          buildName: 'windows', // Missing
+          status: TaskStatus.succeeded,
+          attemptNumber: 1,
+        );
+
+        final result = await UnifiedCheckRun.markConclusion(
+          firestoreService: firestoreService,
+          guardId: guardId,
+          state: state,
+        );
+
+        expect(result.result, PresubmitGuardConclusionResult.missing);
+      });
+    });
+  });
+}
diff --git a/app_dart/test/service/scheduler_test.dart b/app_dart/test/service/scheduler_test.dart
index 8ffdfd6cf4..ac1b1d1784 100644
--- a/app_dart/test/service/scheduler_test.dart
+++ b/app_dart/test/service/scheduler_test.dart
@@ -12,6 +12,7 @@ import 'package:cocoon_server_test/test_logging.dart';
 import 'package:cocoon_service/cocoon_service.dart';
 import 'package:cocoon_service/src/model/ci_yaml/ci_yaml.dart';
 import 'package:cocoon_service/src/model/ci_yaml/target.dart';
+import 'package:cocoon_service/src/model/firestore/base.dart';
 import 'package:cocoon_service/src/model/firestore/ci_staging.dart';
 import 'package:cocoon_service/src/model/firestore/commit.dart' as fs;
 import 'package:cocoon_service/src/model/firestore/pr_check_runs.dart';
diff --git a/app_dart/test/src/utilities/entity_generators.dart b/app_dart/test/src/utilities/entity_generators.dart
index 60d1cab273..71a2d42ad0 100644
--- a/app_dart/test/src/utilities/entity_generators.dart
+++ b/app_dart/test/src/utilities/entity_generators.dart
@@ -215,6 +215,7 @@ github.PullRequest generatePullRequest({
   String title = 'example message',
   int number = 123,
   DateTime? mergedAt,
+  DateTime? createdAt,
   String headSha = 'abc',
   String baseSha = 'def',
   bool merged = true,
@@ -222,11 +223,13 @@ github.PullRequest generatePullRequest({
   int changedFilesCount = 1,
 }) {
   mergedAt ??= DateTime.fromMillisecondsSinceEpoch(1);
+  createdAt ??= DateTime.fromMillisecondsSinceEpoch(1);
   return github.PullRequest(
     id: id,
     title: title,
     number: number,
     mergedAt: mergedAt,
+    createdAt: createdAt,
     base: github.PullRequestHead(
       ref: branch,
       sha: baseSha,
diff --git a/app_dart/test/src/utilities/mocks.mocks.dart b/app_dart/test/src/utilities/mocks.mocks.dart
index b0cb822145..afa6778535 100644
--- a/app_dart/test/src/utilities/mocks.mocks.dart
+++ b/app_dart/test/src/utilities/mocks.mocks.dart
@@ -12,10 +12,9 @@ import 'package:buildbucket/buildbucket_pb.dart' as _i6;
 import 'package:cocoon_common/rpc_model.dart' as _i19;
 import 'package:cocoon_service/cocoon_service.dart' as _i17;
 import 'package:cocoon_service/src/foundation/github_checks_util.dart' as _i10;
-import 'package:cocoon_service/src/model/ci_yaml/ci_yaml.dart' as _i41;
+import 'package:cocoon_service/src/model/ci_yaml/ci_yaml.dart' as _i40;
 import 'package:cocoon_service/src/model/ci_yaml/target.dart' as _i28;
 import 'package:cocoon_service/src/model/commit_ref.dart' as _i32;
-import 'package:cocoon_service/src/model/firestore/ci_staging.dart' as _i40;
 import 'package:cocoon_service/src/model/firestore/commit.dart' as _i38;
 import 'package:cocoon_service/src/model/firestore/task.dart' as _i33;
 import 'package:cocoon_service/src/model/github/checks.dart' as _i31;
@@ -37,7 +36,7 @@ import 'package:cocoon_service/src/service/luci_build_service/pending_task.dart'
 import 'package:cocoon_service/src/service/luci_build_service/user_data.dart'
     as _i30;
 import 'package:cocoon_service/src/service/scheduler/process_check_run_result.dart'
-    as _i42;
+    as _i41;
 import 'package:fixnum/fixnum.dart' as _i34;
 import 'package:github/github.dart' as _i7;
 import 'package:github/hooks.dart' as _i22;
@@ -5555,7 +5554,7 @@ class MockScheduler extends _i1.Mock implements _i17.Scheduler {
     String? baseRef,
     _i7.RepositorySlug? slug,
     String? headSha,
-    _i40.CiStage? stage,
+    dynamic stage,
   ) =>
       (super.noSuchMethod(
             Invocation.method(#getMergeGroupTargetsForStage, [
@@ -5573,7 +5572,7 @@ class MockScheduler extends _i1.Mock implements _i17.Scheduler {
     String? baseRef,
     _i7.RepositorySlug? slug,
     String? headSha, {
-    _i41.CiType? type = _i41.CiType.any,
+    _i40.CiType? type = _i40.CiType.any,
   }) =>
       (super.noSuchMethod(
             Invocation.method(
@@ -5665,7 +5664,7 @@ class MockScheduler extends _i1.Mock implements _i17.Scheduler {
   @override
   _i13.Future<List<_i28.Target>> getPresubmitTargets(
     _i7.PullRequest? pullRequest, {
-    _i41.CiType? type = _i41.CiType.any,
+    _i40.CiType? type = _i40.CiType.any,
   }) =>
       (super.noSuchMethod(
             Invocation.method(
@@ -5718,19 +5717,19 @@ class MockScheduler extends _i1.Mock implements _i17.Scheduler {
           as _i13.Future<void>);
 
   @override
-  _i13.Future<_i42.ProcessCheckRunResult> processCheckRun(
+  _i13.Future<_i41.ProcessCheckRunResult> processCheckRun(
     _i31.CheckRunEvent? checkRunEvent,
   ) =>
       (super.noSuchMethod(
             Invocation.method(#processCheckRun, [checkRunEvent]),
-            returnValue: _i13.Future<_i42.ProcessCheckRunResult>.value(
-              _i20.dummyValue<_i42.ProcessCheckRunResult>(
+            returnValue: _i13.Future<_i41.ProcessCheckRunResult>.value(
+              _i20.dummyValue<_i41.ProcessCheckRunResult>(
                 this,
                 Invocation.method(#processCheckRun, [checkRunEvent]),
               ),
             ),
           )
-          as _i13.Future<_i42.ProcessCheckRunResult>);
+          as _i13.Future<_i41.ProcessCheckRunResult>);
 
   @override
   _i7.CheckRun checkRunFromString(String? input) =>