diff --git a/etc/build.props b/etc/build.props
--- a/etc/build.props
+++ b/etc/build.props
@@ -1,298 +1,299 @@
 title = Isabelle/Scala
 module = $ISABELLE_HOME/lib/classes/isabelle.jar
 main = isabelle.jedit.Main
 resources = \
   lib/services/java.nio.charset.spi.CharsetProvider:META-INF/services/ \
   lib/logo/isabelle_transparent-32.gif:isabelle/ \
   lib/logo/isabelle_transparent.gif:isabelle/
 sources = \
   src/HOL/SPARK/Tools/spark.scala \
   src/HOL/Tools/ATP/system_on_tptp.scala \
   src/HOL/Tools/Mirabelle/mirabelle.scala \
   src/HOL/Tools/Nitpick/kodkod.scala \
   src/Pure/Admin/afp.scala \
   src/Pure/Admin/build_csdp.scala \
   src/Pure/Admin/build_cygwin.scala \
   src/Pure/Admin/build_doc.scala \
   src/Pure/Admin/build_e.scala \
   src/Pure/Admin/build_fonts.scala \
   src/Pure/Admin/build_history.scala \
   src/Pure/Admin/build_jcef.scala \
   src/Pure/Admin/build_jdk.scala \
   src/Pure/Admin/build_jedit.scala \
   src/Pure/Admin/build_log.scala \
   src/Pure/Admin/build_minisat.scala \
   src/Pure/Admin/build_polyml.scala \
   src/Pure/Admin/build_release.scala \
   src/Pure/Admin/build_spass.scala \
   src/Pure/Admin/build_sqlite.scala \
   src/Pure/Admin/build_status.scala \
   src/Pure/Admin/build_vampire.scala \
   src/Pure/Admin/build_verit.scala \
   src/Pure/Admin/build_zipperposition.scala \
   src/Pure/Admin/check_sources.scala \
   src/Pure/Admin/ci_profile.scala \
   src/Pure/Admin/isabelle_cronjob.scala \
   src/Pure/Admin/isabelle_devel.scala \
   src/Pure/Admin/jenkins.scala \
   src/Pure/Admin/other_isabelle.scala \
   src/Pure/Concurrent/consumer_thread.scala \
   src/Pure/Concurrent/counter.scala \
   src/Pure/Concurrent/delay.scala \
   src/Pure/Concurrent/event_timer.scala \
   src/Pure/Concurrent/future.scala \
   src/Pure/Concurrent/isabelle_thread.scala \
   src/Pure/Concurrent/mailbox.scala \
   src/Pure/Concurrent/par_list.scala \
   src/Pure/Concurrent/synchronized.scala \
   src/Pure/GUI/color_value.scala \
   src/Pure/GUI/desktop_app.scala \
   src/Pure/GUI/gui.scala \
   src/Pure/GUI/gui_thread.scala \
   src/Pure/GUI/popup.scala \
   src/Pure/GUI/wrap_panel.scala \
   src/Pure/General/antiquote.scala \
   src/Pure/General/bytes.scala \
   src/Pure/General/cache.scala \
   src/Pure/General/codepoint.scala \
   src/Pure/General/comment.scala \
   src/Pure/General/completion.scala \
   src/Pure/General/csv.scala \
   src/Pure/General/date.scala \
   src/Pure/General/exn.scala \
   src/Pure/General/file.scala \
   src/Pure/General/file_watcher.scala \
   src/Pure/General/graph.scala \
   src/Pure/General/graph_display.scala \
   src/Pure/General/graphics_file.scala \
   src/Pure/General/http.scala \
   src/Pure/General/json.scala \
+  src/Pure/General/json_api.scala \
   src/Pure/General/linear_set.scala \
   src/Pure/General/logger.scala \
   src/Pure/General/long_name.scala \
   src/Pure/General/mailman.scala \
   src/Pure/General/mercurial.scala \
   src/Pure/General/multi_map.scala \
   src/Pure/General/output.scala \
   src/Pure/General/path.scala \
   src/Pure/General/position.scala \
   src/Pure/General/pretty.scala \
   src/Pure/General/properties.scala \
   src/Pure/General/rdf.scala \
   src/Pure/General/scan.scala \
   src/Pure/General/sha1.scala \
   src/Pure/General/sql.scala \
   src/Pure/General/ssh.scala \
   src/Pure/General/symbol.scala \
   src/Pure/General/time.scala \
   src/Pure/General/timing.scala \
   src/Pure/General/untyped.scala \
   src/Pure/General/url.scala \
   src/Pure/General/utf8.scala \
   src/Pure/General/uuid.scala \
   src/Pure/General/value.scala \
   src/Pure/General/word.scala \
   src/Pure/General/xz.scala \
   src/Pure/Isar/document_structure.scala \
   src/Pure/Isar/keyword.scala \
   src/Pure/Isar/line_structure.scala \
   src/Pure/Isar/outer_syntax.scala \
   src/Pure/Isar/parse.scala \
   src/Pure/Isar/token.scala \
   src/Pure/ML/ml_console.scala \
   src/Pure/ML/ml_lex.scala \
   src/Pure/ML/ml_process.scala \
   src/Pure/ML/ml_profiling.scala \
   src/Pure/ML/ml_statistics.scala \
   src/Pure/ML/ml_syntax.scala \
   src/Pure/PIDE/byte_message.scala \
   src/Pure/PIDE/command.scala \
   src/Pure/PIDE/command_span.scala \
   src/Pure/PIDE/document.scala \
   src/Pure/PIDE/document_id.scala \
   src/Pure/PIDE/document_status.scala \
   src/Pure/PIDE/editor.scala \
   src/Pure/PIDE/headless.scala \
   src/Pure/PIDE/line.scala \
   src/Pure/PIDE/markup.scala \
   src/Pure/PIDE/markup_tree.scala \
   src/Pure/PIDE/protocol.scala \
   src/Pure/PIDE/protocol_handlers.scala \
   src/Pure/PIDE/protocol_message.scala \
   src/Pure/PIDE/prover.scala \
   src/Pure/PIDE/query_operation.scala \
   src/Pure/PIDE/rendering.scala \
   src/Pure/PIDE/resources.scala \
   src/Pure/PIDE/session.scala \
   src/Pure/PIDE/text.scala \
   src/Pure/PIDE/xml.scala \
   src/Pure/PIDE/yxml.scala \
   src/Pure/ROOT.scala \
   src/Pure/System/bash.scala \
   src/Pure/System/command_line.scala \
   src/Pure/System/components.scala \
   src/Pure/System/executable.scala \
   src/Pure/System/getopts.scala \
   src/Pure/System/isabelle_charset.scala \
   src/Pure/System/isabelle_fonts.scala \
   src/Pure/System/isabelle_platform.scala \
   src/Pure/System/isabelle_process.scala \
   src/Pure/System/isabelle_system.scala \
   src/Pure/System/isabelle_tool.scala \
   src/Pure/System/java_statistics.scala \
   src/Pure/System/linux.scala \
   src/Pure/System/mingw.scala \
   src/Pure/System/numa.scala \
   src/Pure/System/options.scala \
   src/Pure/System/platform.scala \
   src/Pure/System/posix_interrupt.scala \
   src/Pure/System/process_result.scala \
   src/Pure/System/progress.scala \
   src/Pure/System/scala.scala \
   src/Pure/System/system_channel.scala \
   src/Pure/System/tty_loop.scala \
   src/Pure/Thy/bibtex.scala \
   src/Pure/Thy/document_build.scala \
   src/Pure/Thy/export.scala \
   src/Pure/Thy/export_theory.scala \
   src/Pure/Thy/file_format.scala \
   src/Pure/Thy/html.scala \
   src/Pure/Thy/latex.scala \
   src/Pure/Thy/presentation.scala \
   src/Pure/Thy/sessions.scala \
   src/Pure/Thy/thy_element.scala \
   src/Pure/Thy/thy_header.scala \
   src/Pure/Thy/thy_syntax.scala \
   src/Pure/Tools/build.scala \
   src/Pure/Tools/build_docker.scala \
   src/Pure/Tools/build_job.scala \
   src/Pure/Tools/check_keywords.scala \
   src/Pure/Tools/debugger.scala \
   src/Pure/Tools/doc.scala \
   src/Pure/Tools/dump.scala \
   src/Pure/Tools/flarum.scala \
   src/Pure/Tools/fontforge.scala \
   src/Pure/Tools/java_monitor.scala \
   src/Pure/Tools/logo.scala \
   src/Pure/Tools/mkroot.scala \
   src/Pure/Tools/phabricator.scala \
   src/Pure/Tools/print_operation.scala \
   src/Pure/Tools/profiling_report.scala \
   src/Pure/Tools/scala_build.scala \
   src/Pure/Tools/scala_project.scala \
   src/Pure/Tools/server.scala \
   src/Pure/Tools/server_commands.scala \
   src/Pure/Tools/simplifier_trace.scala \
   src/Pure/Tools/spell_checker.scala \
   src/Pure/Tools/task_statistics.scala \
   src/Pure/Tools/update.scala \
   src/Pure/Tools/update_cartouches.scala \
   src/Pure/Tools/update_comments.scala \
   src/Pure/Tools/update_header.scala \
   src/Pure/Tools/update_then.scala \
   src/Pure/Tools/update_theorems.scala \
   src/Pure/library.scala \
   src/Pure/pure_thy.scala \
   src/Pure/term.scala \
   src/Pure/term_xml.scala \
   src/Pure/thm_name.scala \
   src/Tools/Graphview/graph_file.scala \
   src/Tools/Graphview/graph_panel.scala \
   src/Tools/Graphview/graphview.scala \
   src/Tools/Graphview/layout.scala \
   src/Tools/Graphview/main_panel.scala \
   src/Tools/Graphview/metrics.scala \
   src/Tools/Graphview/model.scala \
   src/Tools/Graphview/mutator.scala \
   src/Tools/Graphview/mutator_dialog.scala \
   src/Tools/Graphview/mutator_event.scala \
   src/Tools/Graphview/popups.scala \
   src/Tools/Graphview/shapes.scala \
   src/Tools/Graphview/tree_panel.scala \
   src/Tools/VSCode/src/build_vscode.scala \
   src/Tools/VSCode/src/channel.scala \
   src/Tools/VSCode/src/dynamic_output.scala \
   src/Tools/VSCode/src/language_server.scala \
   src/Tools/VSCode/src/lsp.scala \
   src/Tools/VSCode/src/preview_panel.scala \
   src/Tools/VSCode/src/state_panel.scala \
   src/Tools/VSCode/src/textmate_grammar.scala \
   src/Tools/VSCode/src/vscode_model.scala \
   src/Tools/VSCode/src/vscode_rendering.scala \
   src/Tools/VSCode/src/vscode_resources.scala \
   src/Tools/VSCode/src/vscode_spell_checker.scala \
   src/Tools/jEdit/src/active.scala \
   src/Tools/jEdit/src/base_plugin.scala \
   src/Tools/jEdit/src/completion_popup.scala \
   src/Tools/jEdit/src/context_menu.scala \
   src/Tools/jEdit/src/debugger_dockable.scala \
   src/Tools/jEdit/src/dockable.scala \
   src/Tools/jEdit/src/document_model.scala \
   src/Tools/jEdit/src/document_view.scala \
   src/Tools/jEdit/src/documentation_dockable.scala \
   src/Tools/jEdit/src/fold_handling.scala \
   src/Tools/jEdit/src/font_info.scala \
   src/Tools/jEdit/src/graphview_dockable.scala \
   src/Tools/jEdit/src/info_dockable.scala \
   src/Tools/jEdit/src/isabelle.scala \
   src/Tools/jEdit/src/isabelle_encoding.scala \
   src/Tools/jEdit/src/isabelle_export.scala \
   src/Tools/jEdit/src/isabelle_options.scala \
   src/Tools/jEdit/src/isabelle_session.scala \
   src/Tools/jEdit/src/isabelle_vfs.scala \
   src/Tools/jEdit/src/jedit_bibtex.scala \
   src/Tools/jEdit/src/jedit_editor.scala \
   src/Tools/jEdit/src/jedit_lib.scala \
   src/Tools/jEdit/src/jedit_options.scala \
   src/Tools/jEdit/src/jedit_plugins.scala \
   src/Tools/jEdit/src/jedit_rendering.scala \
   src/Tools/jEdit/src/jedit_resources.scala \
   src/Tools/jEdit/src/jedit_sessions.scala \
   src/Tools/jEdit/src/jedit_spell_checker.scala \
   src/Tools/jEdit/src/keymap_merge.scala \
   src/Tools/jEdit/src/main.scala \
   src/Tools/jEdit/src/main_plugin.scala \
   src/Tools/jEdit/src/monitor_dockable.scala \
   src/Tools/jEdit/src/output_dockable.scala \
   src/Tools/jEdit/src/pide_docking_framework.scala \
   src/Tools/jEdit/src/pretty_text_area.scala \
   src/Tools/jEdit/src/pretty_tooltip.scala \
   src/Tools/jEdit/src/process_indicator.scala \
   src/Tools/jEdit/src/protocol_dockable.scala \
   src/Tools/jEdit/src/query_dockable.scala \
   src/Tools/jEdit/src/raw_output_dockable.scala \
   src/Tools/jEdit/src/rich_text_area.scala \
   src/Tools/jEdit/src/session_build.scala \
   src/Tools/jEdit/src/simplifier_trace_dockable.scala \
   src/Tools/jEdit/src/simplifier_trace_window.scala \
   src/Tools/jEdit/src/sledgehammer_dockable.scala \
   src/Tools/jEdit/src/state_dockable.scala \
   src/Tools/jEdit/src/status_widget.scala \
   src/Tools/jEdit/src/symbols_dockable.scala \
   src/Tools/jEdit/src/syntax_style.scala \
   src/Tools/jEdit/src/syslog_dockable.scala \
   src/Tools/jEdit/src/text_overview.scala \
   src/Tools/jEdit/src/text_structure.scala \
   src/Tools/jEdit/src/theories_dockable.scala \
   src/Tools/jEdit/src/timing_dockable.scala \
   src/Tools/jEdit/src/token_markup.scala
 services = \
   isabelle.Bash$Handler \
   isabelle.Bibtex$File_Format \
   isabelle.Document_Build$Build_Engine \
   isabelle.Document_Build$LuaLaTeX_Engine \
   isabelle.Document_Build$PDFLaTeX_Engine \
   isabelle.ML_Statistics$Handler \
   isabelle.Print_Operation$Handler \
   isabelle.Scala$Handler \
   isabelle.Scala_Functions \
   isabelle.Server_Commands \
   isabelle.Sessions$File_Format \
   isabelle.Simplifier_Trace$Handler \
   isabelle.Tools \
   isabelle.jedit.JEdit_Plugin0 \
   isabelle.jedit.JEdit_Plugin1 \
   isabelle.nitpick.Kodkod$Handler \
   isabelle.nitpick.Scala_Functions \
   isabelle.spark.SPARK$Load_Command1 \
   isabelle.spark.SPARK$Load_Command2
diff --git a/src/Pure/General/json_api.scala b/src/Pure/General/json_api.scala
new file mode 100644
--- /dev/null
+++ b/src/Pure/General/json_api.scala
@@ -0,0 +1,76 @@
+/*  Title:      Pure/General/json_api.scala
+    Author:     Makarius
+
+Support for JSON:API: https://jsonapi.org/format
+*/
+
+package isabelle
+
+
+object JSON_API
+{
+  val mime_type = "application/vnd.api+json"
+
+  def api_error(msg: String): Nothing = error("JSON API error: " + msg)
+  def api_errors(msgs: List[String]): Nothing = error(("JSON API errors:" :: msgs).mkString("\n  "))
+
+  sealed case class Root(json: JSON.T)
+  {
+    def get_links: Option[Links] = JSON.value(json, "links").map(Links)
+    def get_included: List[Resource] = JSON.list(json, "included", Resource.some).getOrElse(Nil)
+    def get_data: Option[Data] = JSON.value(json, "data").map(Data(_, get_included))
+    def get_errors: Option[List[JSON.T]] = JSON.list(json, "errors", Some(_))
+
+    def ok: Boolean = get_errors.isEmpty
+    def check: Root =
+      get_errors match {
+        case None => this
+        case Some(List(err)) => api_error(JSON.Format(err))
+        case Some(errs) => api_errors(errs.map(JSON.Format.apply))
+      }
+    def check_data: Data = check.get_data getOrElse api_error("missing data")
+
+    override def toString: String = "Root(" + (if (ok) "ok" else "errors") + ")"
+  }
+
+  sealed case class Links(json: JSON.T)
+  {
+    def get_next: Option[Root] =
+      for (next <- JSON.value(json, "next") if next != null) yield Root(next)
+
+    override def toString: String =
+      if (get_next.isEmpty) "Links()" else "Links(next)"
+  }
+
+  sealed case class Data(data: JSON.T, included: List[Resource])
+  {
+    def is_multi: Boolean = JSON.Value.List.unapply(data, Some(_)).nonEmpty
+
+    def resource: Resource =
+      if (is_multi) api_error("singleton resource expected")
+      else Resource.some(data).get
+
+    def resources: List[Resource] =
+      JSON.Value.List.unapply(data, Resource.some)
+        .getOrElse(api_error("multiple resources expected"))
+
+    override def toString: String =
+      if (is_multi) "Data(resources)" else "Data(resource)"
+  }
+
+  object Resource
+  {
+    def some(json: JSON.T): Some[Resource] = Some(Resource(json))
+  }
+
+  sealed case class Resource(json: JSON.T)
+  {
+    val id: JSON.T = JSON.value(json, "id") getOrElse api_error("missing id")
+    val type_ : JSON.T = JSON.value(json, "type") getOrElse api_error("missing type")
+    val fields: JSON.Object.T =
+      JSON.value(json, "attributes", JSON.Object.unapply).getOrElse(JSON.Object.empty) ++
+      JSON.value(json, "relationships", JSON.Object.unapply).getOrElse(JSON.Object.empty)
+
+    override def toString: String = JSON.Format(json)
+  }
+}
diff --git a/src/Pure/Tools/flarum.scala b/src/Pure/Tools/flarum.scala
--- a/src/Pure/Tools/flarum.scala
+++ b/src/Pure/Tools/flarum.scala
@@ -1,29 +1,32 @@
 /*  Title:      Pure/General/flarum.scala
     Author:     Makarius
 
 Support for Flarum forum server: https://flarum.org
 */
 
 package isabelle
 
 
 import java.net.URL
 
 
 object Flarum
 {
   // see RFC3339 in https://www.php.net/manual/en/class.datetimeinterface.php
   val Date_Format = Date.Format("uuuu-MM-dd'T'HH:mm:ssxxx")
 
   def server(url: URL): Server = new Server(url)
 
   class Server private[Flarum](url: URL)
   {
     override def toString: String = url.toString
 
     def get(route: String): HTTP.Content =
-      HTTP.Client.get(new URL(url, route))
+      HTTP.Client.get(if (route.isEmpty) url else new URL(url, route))
 
-    val api: JSON.T = get("api").json
+    def get_api(route: String = ""): JSON_API.Root =
+      JSON_API.Root(get("api" + (if (route.isEmpty) "" else "/" + route)).json)
+
+    val api: JSON_API.Root = get_api()
   }
 }