feat(oidc): a Serverpod module for signing in with OpenID Connect, SimpleLogin a preset (ADR 0023)

halkyon_oidc_server and its generated client halkyon_oidc_client: any
provider following OpenID Connect's code flow, beside Serverpod's own
providers, with a preset for SimpleLogin (no PKCE, no `typ`). Nothing
of Halkyon's is in it; BSD-3-Clause, `publish_to: none` for now.

- Attempts in `halkyon_oidc_attempt` (ADR 0020's D13): the ID
  PostgreSQL's gen_random_uuid(), a state and a nonce of 32 random
  bytes, PKCE's verifier where used, the caller if signed in; ten
  minutes, taken once. A host may begin the state with a marker.
- The code exchanged by the module itself, so a provider out of reach
  (a timeout, a 5xx) is told apart from a refused code; the ID token
  checked with dart_jsonwebtoken's algorithms (ADR 0020's D12): the
  provider's algorithms alone, the key its `kid` names, of the right
  type; issuer, audience and `azp`, expiry, time issued, `nbf`, a
  minute's leeway; the attempt's nonce; a `sub`.
- Keys kept an hour, fetched again for an unknown `kid` at most once
  a minute; discovery checked against the issuer; every request with
  a timeout.
- Identities in `halkyon_oidc_identity`: the provider's key and the
  `sub`, one per person and provider, deleted with the person.
- The stock endpoint (`OidcIdpBaseEndpoint`), and the parts for a host
  with rules of its own: begin, verify, keep and takeKept, link,
  accountOf, unlink; merging people; a limit per network.
- The forwarding page, moved here from halkyon_base in the next commit.

The package's main library is the code generator's, as halkyon_base's;
a host imports `package:halkyon_oidc_server/oidc.dart`. ADR 0023's
point 5 no longer names OAuth2PkceUtil, which the module doesn't use.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_oidc_client/.gitignore

@@ -0,0 +1,13 @@
+# Files and directories created by pub
+.dart_tool/
+.packages
+
+# Omit committing pubspec.lock for library packages:
+# https://dart.dev/guides/libraries/private-files#pubspeclock
+pubspec.lock
+
+# Conventional directory for build outputs
+build/
+
+# Directory created by dartdoc
+doc/api/

packages/halkyon_oidc_client/CHANGELOG.md

@@ -0,0 +1,3 @@
+## 0.1.0
+
+* The client of halkyon_oidc 0.1.0, generated by Serverpod.

packages/halkyon_oidc_client/LICENSE

@@ -0,0 +1,28 @@
+BSD 3-Clause License
+
+Copyright (c) 2026, the Project Halkyon authors
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+3. Neither the name of the copyright holder nor the names of its
+   contributors may be used to endorse or promote products derived from
+   this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

packages/halkyon_oidc_client/README.md

@@ -0,0 +1,9 @@
+# halkyon_oidc_client
+
+The client of [`halkyon_oidc`](../halkyon_oidc_server/README.md), generated by
+Serverpod: the models an app gets when it signs in with OpenID Connect (`OidcStart`,
+`OidcRefusedException` with its `OidcRefusal`), and the stock endpoint a host extends.
+Never edited by hand; run `serverpod --no-analytics generate` in `halkyon_oidc_server`
+instead.
+
+Under the BSD 3-Clause licence, Serverpod's own.

packages/halkyon_oidc_client/analysis_options.yaml

@@ -0,0 +1,16 @@
+# Defines a default set of lint rules enforced for
+# projects at Google. For details and rationale,
+# see https://github.com/dart-lang/pedantic#enabled-lints.
+
+# For lint rules and documentation, see http://dart-lang.github.io/linter/lints.
+# Uncomment to specify additional rules.
+# linter:
+#   rules:
+#     - camel_case_types
+
+analyzer:
+  exclude:
+    - lib/src/protocol/**
+
+formatter:
+  trailing_commas: preserve

packages/halkyon_oidc_client/lib/halkyon_oidc_client.dart

@@ -0,0 +1,3 @@
+export 'src/protocol/protocol.dart';
+
+export 'package:serverpod_client/serverpod_client.dart';

packages/halkyon_oidc_client/lib/src/protocol/client.dart

@@ -0,0 +1,62 @@
+/* AUTOMATICALLY GENERATED CODE DO NOT MODIFY */
+/*   To generate run: "serverpod generate"    */
+
+// ignore_for_file: implementation_imports
+// ignore_for_file: library_private_types_in_public_api
+// ignore_for_file: non_constant_identifier_names
+// ignore_for_file: public_member_api_docs
+// ignore_for_file: type_literal_in_constant_pattern
+// ignore_for_file: use_super_parameters
+// ignore_for_file: invalid_use_of_internal_member
+
+// ignore_for_file: no_leading_underscores_for_library_prefixes
+import 'dart:async' as _ida;
+import 'package:halkyon_oidc_client/src/protocol/oidc_start.dart' as _innqwlkm;
+import 'package:serverpod_auth_core_client/serverpod_auth_core_client.dart'
+    as _iacc;
+import 'package:serverpod_auth_idp_client/serverpod_auth_idp_client.dart'
+    as _iaic;
+import 'package:serverpod_client/serverpod_client.dart' as _isc;
+
+/// The stock endpoint of OpenID Connect sign-in (ADR 0023, 6), as
+/// Serverpod's own providers have theirs: a host extends it in its own
+/// server (`class OidcIdpEndpoint extends OidcIdpBaseEndpoint {}`), which
+/// puts it into the host's client.
+///
+/// Its per-network limit counts by the request's `remoteInfo`, the connection's
+/// address, or the `Forwarded` or `X-Forwarded-For` header's first address
+/// where one came, whoever wrote it: behind a proxy that passes either on as
+/// the visitor sent it, a host overrides [begin] and counts by the address
+/// its proxy saw.
+/// {@category Endpoint}
+abstract class EndpointOidcIdpBase extends _iaic.EndpointIdpBase {
+  EndpointOidcIdpBase(_isc.EndpointCaller caller) : super(caller);
+
+  /// Begins a sign-in with [provider] (its key): signed in, to add the
+  /// identity to the caller ([link]); signed out, to sign in ([login]).
+  _ida.Future<_innqwlkm.OidcStart> begin({required String provider});
+
+  /// Signs in with the identity [code] proves for [attempt], begun signed
+  /// out: a session for its person, made for one nobody has.
+  _ida.Future<_iacc.AuthSuccess> login({
+    required _isc.UuidValue attempt,
+    required String code,
+  });
+
+  /// Links the identity [code] proves for [attempt], begun by the caller
+  /// signed in, to them.
+  _ida.Future<void> link({
+    required _isc.UuidValue attempt,
+    required String code,
+  });
+
+  @override
+  _ida.Future<bool> hasAccount();
+}
+
+class Caller extends _isc.ModuleEndpointCaller {
+  Caller(_isc.ServerpodClientShared client) : super(client) {}
+
+  @override
+  Map<String, _isc.EndpointRef> get endpointRefLookup => {};
+}

packages/halkyon_oidc_client/lib/src/protocol/oidc_refusal.dart

@@ -0,0 +1,75 @@
+/* AUTOMATICALLY GENERATED CODE DO NOT MODIFY */
+/*   To generate run: "serverpod generate"    */
+
+// ignore_for_file: implementation_imports
+// ignore_for_file: library_private_types_in_public_api
+// ignore_for_file: non_constant_identifier_names
+// ignore_for_file: public_member_api_docs
+// ignore_for_file: type_literal_in_constant_pattern
+// ignore_for_file: use_super_parameters
+// ignore_for_file: invalid_use_of_internal_member
+
+// ignore_for_file: no_leading_underscores_for_library_prefixes
+import 'package:serverpod_client/serverpod_client.dart' as _isc;
+
+/// Why a sign-in with an OpenID Connect provider was refused.
+enum OidcRefusal implements _isc.SerializableModel {
+  /// The host names no provider by this key.
+  unknownProvider,
+
+  /// No attempt has this ID: none began, it was ended already, or its time
+  /// ran out.
+  unknownAttempt,
+
+  /// The attempt was begun by someone else, or signed in where this call is
+  /// signed out, or the other way round.
+  wrongCaller,
+
+  /// The provider refused the code, or its answer didn't prove anyone: a
+  /// token not signed by its key, for another client or attempt, expired.
+  rejected,
+
+  /// The provider couldn't be reached, or answered nothing readable; trying
+  /// again later may work.
+  providerUnavailable,
+
+  /// The identity is another person's.
+  takenByAnotherPerson,
+
+  /// The person has another identity at this provider.
+  otherIdentityLinked,
+
+  /// Too many sign-ins were begun from this network; later.
+  tooManyAttempts;
+
+  static OidcRefusal fromJson(String name) {
+    switch (name) {
+      case 'unknownProvider':
+        return OidcRefusal.unknownProvider;
+      case 'unknownAttempt':
+        return OidcRefusal.unknownAttempt;
+      case 'wrongCaller':
+        return OidcRefusal.wrongCaller;
+      case 'rejected':
+        return OidcRefusal.rejected;
+      case 'providerUnavailable':
+        return OidcRefusal.providerUnavailable;
+      case 'takenByAnotherPerson':
+        return OidcRefusal.takenByAnotherPerson;
+      case 'otherIdentityLinked':
+        return OidcRefusal.otherIdentityLinked;
+      case 'tooManyAttempts':
+        return OidcRefusal.tooManyAttempts;
+      default:
+        throw ArgumentError(
+          'Value "$name" cannot be converted to "OidcRefusal"',
+        );
+    }
+  }
+
+  @override
+  String toJson() => name;
+
+  @override
+  String toString() => name;
+}

packages/halkyon_oidc_client/lib/src/protocol/oidc_refused_exception.dart

@@ -0,0 +1,77 @@
+/* AUTOMATICALLY GENERATED CODE DO NOT MODIFY */
+/*   To generate run: "serverpod generate"    */
+
+// ignore_for_file: implementation_imports
+// ignore_for_file: library_private_types_in_public_api
+// ignore_for_file: non_constant_identifier_names
+// ignore_for_file: public_member_api_docs
+// ignore_for_file: type_literal_in_constant_pattern
+// ignore_for_file: use_super_parameters
+// ignore_for_file: invalid_use_of_internal_member
+
+// ignore_for_file: no_leading_underscores_for_library_prefixes
+import 'package:serverpod_client/serverpod_client.dart' as _isc;
+import 'oidc_refusal.dart' as _i8gvcysd;
+
+/// A sign-in with an OpenID Connect provider was refused; nothing changed.
+abstract class OidcRefusedException
+    implements
+        _isc.SerializableException,
+        _isc.SerializableModel,
+        _isc.ProtocolSerialization {
+  OidcRefusedException._({required this.reason});
+
+  factory OidcRefusedException({required _i8gvcysd.OidcRefusal reason}) =
+      _OidcRefusedExceptionImpl;
+
+  factory OidcRefusedException.fromJson(
+    Map<String, dynamic> jsonSerialization,
+  ) {
+    return OidcRefusedException(
+      reason: _i8gvcysd.OidcRefusal.fromJson(
+        (jsonSerialization['reason'] as String),
+      ),
+    );
+  }
+
+  /// Why it was refused.
+  _i8gvcysd.OidcRefusal reason;
+
+  /// Returns a shallow copy of this [OidcRefusedException]
+  /// with some or all fields replaced by the given arguments.
+  @_isc.useResult
+  OidcRefusedException copyWith({_i8gvcysd.OidcRefusal? reason});
+  @override
+  Map<String, dynamic> toJson() {
+    return {
+      '__className__': 'halkyon_oidc.OidcRefusedException',
+      'reason': reason.toJson(),
+    };
+  }
+
+  @override
+  Map<String, dynamic> toJsonForProtocol() {
+    return {
+      '__className__': 'halkyon_oidc.OidcRefusedException',
+      'reason': reason.toJson(),
+    };
+  }
+
+  @override
+  String toString() {
+    return 'OidcRefusedException(reason: $reason)';
+  }
+}
+
+class _OidcRefusedExceptionImpl extends OidcRefusedException {
+  _OidcRefusedExceptionImpl({required _i8gvcysd.OidcRefusal reason})
+    : super._(reason: reason);
+
+  /// Returns a shallow copy of this [OidcRefusedException]
+  /// with some or all fields replaced by the given arguments.
+  @_isc.useResult
+  @override
+  OidcRefusedException copyWith({_i8gvcysd.OidcRefusal? reason}) {
+    return OidcRefusedException(reason: reason ?? this.reason);
+  }
+}

packages/halkyon_oidc_client/lib/src/protocol/oidc_start.dart

@@ -0,0 +1,114 @@
+/* AUTOMATICALLY GENERATED CODE DO NOT MODIFY */
+/*   To generate run: "serverpod generate"    */
+
+// ignore_for_file: implementation_imports
+// ignore_for_file: library_private_types_in_public_api
+// ignore_for_file: non_constant_identifier_names
+// ignore_for_file: public_member_api_docs
+// ignore_for_file: type_literal_in_constant_pattern
+// ignore_for_file: use_super_parameters
+// ignore_for_file: invalid_use_of_internal_member
+
+// ignore_for_file: no_leading_underscores_for_library_prefixes
+import 'package:serverpod_client/serverpod_client.dart' as _isc;
+
+/// What a device needs to send a person to their provider: the attempt it
+/// ends the sign-in with, which only the device holds, and where to go.
+abstract class OidcStart
+    implements _isc.SerializableModel, _isc.ProtocolSerialization {
+  OidcStart._({
+    required this.attempt,
+    required this.authorizationUrl,
+    required this.state,
+  });
+
+  factory OidcStart({
+    required _isc.UuidValue attempt,
+    required Uri authorizationUrl,
+    required String state,
+  }) = _OidcStartImpl;
+
+  factory OidcStart.fromJson(Map<String, dynamic> jsonSerialization) {
+    return OidcStart(
+      attempt: _isc.UuidValueJsonExtension.fromJson(
+        jsonSerialization['attempt'],
+      ),
+      authorizationUrl: _isc.UriJsonExtension.fromJson(
+        jsonSerialization['authorizationUrl'],
+      ),
+      state: jsonSerialization['state'] as String,
+    );
+  }
+
+  /// The attempt's ID, which ends it (`verify`); never shown, never sent
+  /// anywhere but to this server.
+  _isc.UuidValue attempt;
+
+  /// The provider's authorization address, with everything it needs.
+  Uri authorizationUrl;
+
+  /// The state the provider sends back with the code, which the device
+  /// compares with what comes back before it goes on.
+  String state;
+
+  /// Returns a shallow copy of this [OidcStart]
+  /// with some or all fields replaced by the given arguments.
+  @_isc.useResult
+  OidcStart copyWith({
+    _isc.UuidValue? attempt,
+    Uri? authorizationUrl,
+    String? state,
+  });
+  @override
+  Map<String, dynamic> toJson() {
+    return {
+      '__className__': 'halkyon_oidc.OidcStart',
+      'attempt': attempt.toJson(),
+      'authorizationUrl': authorizationUrl.toJson(),
+      'state': state,
+    };
+  }
+
+  @override
+  Map<String, dynamic> toJsonForProtocol() {
+    return {
+      '__className__': 'halkyon_oidc.OidcStart',
+      'attempt': attempt.toJson(),
+      'authorizationUrl': authorizationUrl.toJson(),
+      'state': state,
+    };
+  }
+
+  @override
+  String toString() {
+    return _isc.SerializationManager.encode(this);
+  }
+}
+
+class _OidcStartImpl extends OidcStart {
+  _OidcStartImpl({
+    required _isc.UuidValue attempt,
+    required Uri authorizationUrl,
+    required String state,
+  }) : super._(
+         attempt: attempt,
+         authorizationUrl: authorizationUrl,
+         state: state,
+       );
+
+  /// Returns a shallow copy of this [OidcStart]
+  /// with some or all fields replaced by the given arguments.
+  @_isc.useResult
+  @override
+  OidcStart copyWith({
+    _isc.UuidValue? attempt,
+    Uri? authorizationUrl,
+    String? state,
+  }) {
+    return OidcStart(
+      attempt: attempt ?? this.attempt,
+      authorizationUrl: authorizationUrl ?? this.authorizationUrl,
+      state: state ?? this.state,
+    );
+  }
+}

packages/halkyon_oidc_client/lib/src/protocol/protocol.dart

@@ -0,0 +1,231 @@
+/* AUTOMATICALLY GENERATED CODE DO NOT MODIFY */
+/*   To generate run: "serverpod generate"    */
+
+// ignore_for_file: implementation_imports
+// ignore_for_file: library_private_types_in_public_api
+// ignore_for_file: non_constant_identifier_names
+// ignore_for_file: public_member_api_docs
+// ignore_for_file: type_literal_in_constant_pattern
+// ignore_for_file: use_super_parameters
+// ignore_for_file: invalid_use_of_internal_member
+// ignore_for_file: dead_code, unnecessary_type_check
+
+// ignore_for_file: no_leading_underscores_for_library_prefixes
+import 'package:serverpod_auth_core_client/serverpod_auth_core_client.dart'
+    as _iacc;
+import 'package:serverpod_auth_idp_client/serverpod_auth_idp_client.dart'
+    as _iaic;
+import 'package:serverpod_client/serverpod_client.dart' as _isc;
+import 'oidc_refusal.dart' as _i8gvcysd;
+import 'oidc_refused_exception.dart' as _i70wwg4b;
+import 'oidc_start.dart' as _i4fk0f61;
+export 'oidc_refusal.dart';
+export 'oidc_refused_exception.dart';
+export 'oidc_start.dart';
+export 'client.dart';
+
+class Protocol extends _isc.SerializationManager {
+  Protocol._();
+
+  factory Protocol() => _instance;
+
+  static final Protocol _instance = Protocol._();
+
+  final Set<_isc.SerializationManager> _hostProtocols = {};
+
+  void registerHostProtocol(
+    String projectName,
+    _isc.SerializationManager protocol,
+  ) {
+    _hostProtocols.add(protocol);
+  }
+
+  static String? getClassNameFromObjectJson(dynamic data) {
+    if (data is! Map) return null;
+    final className = data['__className__'] as String?;
+    if (className == null) return null;
+    if (!className.startsWith('halkyon_oidc.')) return className;
+    return className.substring(13);
+  }
+
+  @override
+  T deserialize<T>(
+    dynamic data, [
+    Type? t,
+  ]) {
+    t ??= T;
+
+    final dataClassName = getClassNameFromObjectJson(data);
+    if (dataClassName != null && dataClassName != getClassNameForType(t)) {
+      try {
+        return deserializeByClassName({
+          'className': dataClassName,
+          'data': data,
+        });
+      } on _isc.DeserializationClassNameNotFoundException catch (_) {
+        // If the className is not recognized (e.g., older client receiving
+        // data with a new subtype), fall back to deserializing without the
+        // className, using the expected type T.
+      }
+    }
+
+    if (t == _i8gvcysd.OidcRefusal) {
+      return _i8gvcysd.OidcRefusal.fromJson(data) as T;
+    }
+    if (t == _i70wwg4b.OidcRefusedException) {
+      return _i70wwg4b.OidcRefusedException.fromJson(data) as T;
+    }
+    if (t == _i4fk0f61.OidcStart) {
+      return _i4fk0f61.OidcStart.fromJson(data) as T;
+    }
+    if (t == _isc.getType<_i8gvcysd.OidcRefusal?>()) {
+      return (data != null ? _i8gvcysd.OidcRefusal.fromJson(data) : null) as T;
+    }
+    if (t == _isc.getType<_i70wwg4b.OidcRefusedException?>()) {
+      return (data != null
+              ? _i70wwg4b.OidcRefusedException.fromJson(data)
+              : null)
+          as T;
+    }
+    if (t == _isc.getType<_i4fk0f61.OidcStart?>()) {
+      return (data != null ? _i4fk0f61.OidcStart.fromJson(data) : null) as T;
+    }
+    try {
+      return _iaic.Protocol().deserialize<T>(data, t);
+    } on _isc.DeserializationTypeNotFoundException catch (_) {}
+    try {
+      return _iacc.Protocol().deserialize<T>(data, t);
+    } on _isc.DeserializationTypeNotFoundException catch (_) {}
+    return super.deserialize<T>(data, t);
+  }
+
+  static String? getClassNameForType(Type type) {
+    return switch (type) {
+      _i8gvcysd.OidcRefusal => 'OidcRefusal',
+      _i70wwg4b.OidcRefusedException => 'OidcRefusedException',
+      _i4fk0f61.OidcStart => 'OidcStart',
+      _ => null,
+    };
+  }
+
+  @override
+  String? getClassNameForObject(Object? data) {
+    String? className = super.getClassNameForObject(data);
+    if (className != null) return className;
+
+    if (data is Map<String, dynamic> && data['__className__'] is String) {
+      return (data['__className__'] as String).replaceFirst(
+        'halkyon_oidc.',
+        '',
+      );
+    }
+
+    switch (data) {
+      case _i8gvcysd.OidcRefusal():
+        return 'OidcRefusal';
+      case _i70wwg4b.OidcRefusedException():
+        return 'OidcRefusedException';
+      case _i4fk0f61.OidcStart():
+        return 'OidcStart';
+    }
+    return null;
+  }
+
+  @override
+  dynamic deserializeByClassName(Map<String, dynamic> data) {
+    var dataClassName = data['className'];
+    if (dataClassName is! String) {
+      return super.deserializeByClassName(data);
+    }
+    if (dataClassName == 'OidcRefusal') {
+      return deserialize<_i8gvcysd.OidcRefusal>(data['data']);
+    }
+    if (dataClassName == 'OidcRefusedException') {
+      return deserialize<_i70wwg4b.OidcRefusedException>(data['data']);
+    }
+    if (dataClassName == 'OidcStart') {
+      return deserialize<_i4fk0f61.OidcStart>(data['data']);
+    }
+    return super.deserializeByClassName(data);
+  }
+
+  @override
+  Object? dynamicFieldToJson(
+    Object? object, {
+    bool forProtocol = false,
+  }) {
+    if ((object is List || object is Set || object is Map) ||
+        getClassNameForObject(object) != null) {
+      return super.dynamicFieldToJson(object, forProtocol: forProtocol);
+    }
+    for (final protocol in _hostProtocols) {
+      final className = protocol.getClassNameForObject(object);
+      if (className == null) continue;
+      final host = protocol.getModuleName();
+      final wrapped = {
+        'className': className.contains('.') ? className : '$host.$className',
+        'data': object,
+      };
+      return forProtocol
+          ? _isc.SerializationManager.toEncodableForProtocol(wrapped)
+          : _isc.SerializationManager.toEncodable(wrapped);
+    }
+    return super.dynamicFieldToJson(object, forProtocol: forProtocol);
+  }
+
+  @override
+  dynamic deserializeDynamicFieldValue(Object? value) {
+    if (value == null) return null;
+    if (value is! Map<String, dynamic> || value['className'] is! String) {
+      throw FormatException(
+        'Dynamic fields are encoded as a Map with className and data, but got '
+        '${value.runtimeType} instead.',
+      );
+    }
+    final className = value['className'] as String;
+    for (final protocol in _hostProtocols) {
+      final host = protocol.getModuleName();
+      final hostPrefix = '$host.';
+      if (className.startsWith(hostPrefix)) {
+        final strippedClassName = className.substring(hostPrefix.length);
+        if (strippedClassName.contains('.')) {
+          throw FormatException(
+            'Dynamic field className must not use multiple prefixes: $className',
+          );
+        }
+        final hostData = Map<String, dynamic>.from(value);
+        hostData['className'] = strippedClassName;
+        return protocol.deserializeByClassName(hostData);
+      }
+    }
+    if (className.contains('.')) {
+      for (final protocol in _hostProtocols) {
+        try {
+          return protocol.deserializeByClassName(value);
+        } on _isc.DeserializationClassNameNotFoundException catch (_) {}
+      }
+    }
+    return deserializeByClassName(value);
+  }
+
+  @override
+  String getModuleName() => 'halkyon_oidc';
+
+  /// Maps any `Record`s known to this [Protocol] to their JSON representation
+  ///
+  /// Throws in case the record type is not known.
+  ///
+  /// This method will return `null` (only) for `null` inputs.
+  Map<String, dynamic>? mapRecordToJson(Record? record) {
+    if (record == null) {
+      return null;
+    }
+    try {
+      return _iaic.Protocol().mapRecordToJson(record);
+    } catch (_) {}
+    try {
+      return _iacc.Protocol().mapRecordToJson(record);
+    } catch (_) {}
+    throw Exception('Unsupported record type ${record.runtimeType}');
+  }
+}

packages/halkyon_oidc_client/pubspec.yaml

@@ -0,0 +1,16 @@
+name: halkyon_oidc_client
+description: >-
+  The client side of halkyon_oidc, generated by Serverpod: what an app calls
+  to sign in with an OpenID Connect provider.
+version: 0.1.0
+# In the Halkyon repository until it runs on learn.halkyon.eu (ADR 0023, D3).
+publish_to: none
+
+environment:
+  sdk: ^3.13.0
+
+resolution: workspace
+
+dependencies:
+  serverpod_auth_idp_client: 4.0.4
+  serverpod_client: 4.0.4