diff --git a/EWC/AddResource.aplf b/EWC/AddResource.aplf
new file mode 100644
index 0000000..c5d5a01
--- /dev/null
+++ b/EWC/AddResource.aplf
@@ -0,0 +1,21 @@
+ alias AddResource folder;m;rc
+⍝ Serve a folder over HTTP as a virtual folder, e.g.
+⍝     'kendo' AddResource '/path/to/plugin/assets'
+⍝ makes that folder's contents available at /kendo/. Takes effect immediately -
+⍝ wss reads its mappings per request - so this works before or after Init.
+⍝
+⍝ An alias is a single path segment: wss matches on the first '/' of the URL.
+ 'Alias must not contain "/"'⎕SIGNAL('/'∊alias)/11
+ :If 2≠⎕NC'RESOURCES' ⋄ RESOURCES←0 2⍴⊂'' ⋄ :EndIf
+ :If 2≠⍴⍴RESOURCES ⋄ RESOURCES←1 2⍴RESOURCES ⋄ :EndIf
+
+ m←(0≠≢¨RESOURCES[;1])∧~RESOURCES[;1]∊⊂alias
+ RESOURCES←(m⌿RESOURCES)⍪alias folder
+
+ :If 9=⌊⎕NC'WSS'
+     WSS.Aliases←RESOURCES
+     rc←WSS.CheckAliases
+     :If 0≠⊃rc
+         'W'Log'AddResource ',alias,': ',2⊃rc
+     :EndIf
+ :EndIf
diff --git a/EWC/GetClass.aplf b/EWC/GetClass.aplf
index 6863d77..4bf5456 100644
--- a/EWC/GetClass.aplf
+++ b/EWC/GetClass.aplf
@@ -1,8 +1,9 @@
-r←GetClass (ns name);t;c;m
+r←GetClass (ns name);t;m
+r←⎕NULL
 :If 9=⎕NC 'ns'
 :AndIf ∨/m←0≠ns.⎕NC t←name∘,¨'.type' '.Type'
-:AndIf 9=⎕NC c←'classes.',⎕C ns⍎⊃m/t
-    r←⍎c
-:Else
+    r←findClass ns⍎⊃m/t
+:EndIf
+:If ⎕NULL≡r
     ('Not a supported GUI object: ',name) ⎕SIGNAL 6
 :EndIf
diff --git a/EWC/Init.aplf b/EWC/Init.aplf
index 64c35e0..d335742 100644
--- a/EWC/Init.aplf
+++ b/EWC/Init.aplf
@@ -74,6 +74,7 @@
  ⎕←'Using client found at ',JSClientFolder
 
  setDefaults
+ ensurePlugins ⍝ Plugins may already have registered; keep them detached from Link
 
  :If DIRECT
      'D' Log 'Direct mode enabled'
@@ -102,6 +103,7 @@
  :EndIf
  WSS.HTMLFolder←JSClientFolder
  WSS.ShowHTMLRenderer←0
+ applyPluginResources ⍝ RESOURCES may have been reassigned since a plugin registered
  WSS.Aliases←RESOURCES
  WSS.OnConnect←''
  WSS.OnWSUpgrade←(⍕⎕THIS),'.newSession'
diff --git a/EWC/RegisterPlugin.aplf b/EWC/RegisterPlugin.aplf
new file mode 100644
index 0000000..e24d7d5
--- /dev/null
+++ b/EWC/RegisterPlugin.aplf
@@ -0,0 +1,30 @@
+ {r}←RegisterPlugin plugin;name;nm;target
+⍝ Register a plugin with EWC. Safe before or after Init.
+⍝
+⍝ plugin is a namespace ref providing:
+⍝   Name      plugin id; also the key its classes are tagged with
+⍝   Classes   ref to a namespace whose sub-namespaces are EWC class definitions
+⍝   Folder    the plugin's root folder on disk
+⍝   Inject    (optional) niladic fn returning the JavaScript to evaluate in the
+⍝             client, the first time one of the plugin's classes is created
+⍝   Resources (optional) an (alias folder) pair, or a 2-column matrix of them
+⍝
+⍝ Class definitions are copied rather than referenced, so a plugin can be
+⍝ re-registered after editing its source without stale state.
+ ensurePlugins
+ 'Plugin must supply a Name'⎕SIGNAL(2≠plugin.⎕NC'Name')/11
+ 'Plugin must supply Classes'⎕SIGNAL(9≠plugin.⎕NC'Classes')/11
+ r←name←,plugin.Name
+
+ ⍎'PLUGINS.',name,'←plugin'
+
+ :For nm :In plugin.Classes.⎕NL ¯9
+     target←(⍕PLUGINS),'.classes.',nm
+     target ⎕NS plugin.Classes⍎nm
+     (⍎target).PluginName←name
+ :EndFor
+ ensurePlugins ⍝ the class namespaces are new, so detach those too
+
+ applyResources plugin
+
+ 'D'Log'Registered plugin "',name,'", classes: ',⍕plugin.Classes.⎕NL ¯9
diff --git a/EWC/WaitForWG.aplf b/EWC/WaitForWG.aplf
index 864c2ae..948e758 100644
--- a/EWC/WaitForWG.aplf
+++ b/EWC/WaitForWG.aplf
@@ -1,12 +1,15 @@
- R←msg WaitForWG(Props ID WGID);j;⎕TRAP;p;seqok;nl;nlc;m;t;token;wx;z;ref;ok
+ R←msg WaitForWG arg;Props;ID;WGID;timeout;j;⎕TRAP;p;seqok;nl;nlc;m;t;token;wx;z;ref;ok
+⍝ Right argument is (Props ID WGID), with an optional 4th element giving the
+⍝ seconds to wait. Defaults to WG_TIMEOUT.
+ (Props ID WGID timeout)←4↑arg,⊂WG_TIMEOUT
 
  wx←Props≡'WX' ⍝ A WX rather than a WG
 
  token←WG_TOKENBASE+TOKENSTEP×1⌈ID×MODE=2
 AGAIN:
- j←WG_TIMEOUT ⎕TGET token
+ j←timeout ⎕TGET token
  :If 0=≢j
-     'W' Log 'Client did not respond within ',(⍕WG_TIMEOUT),' seconds.'
+     'W' Log 'Client did not respond within ',(⍕timeout),' seconds.'
      'W' Log 'Request was: ',msg
      ⎕SIGNAL 6
  :EndIf
diff --git a/EWC/applyPluginResources.aplf b/EWC/applyPluginResources.aplf
new file mode 100644
index 0000000..a994546
--- /dev/null
+++ b/EWC/applyPluginResources.aplf
@@ -0,0 +1,12 @@
+ applyPluginResources;nm
+⍝ Re-mount every registered plugin's resources.
+⍝
+⍝ RESOURCES is an ordinary variable and applications assign it wholesale
+⍝ (demo/Run.aplf does), which would drop mounts made by a plugin that registered
+⍝ before Init. Init calls this once RESOURCES has settled.
+ :If 9≠⎕NC'PLUGINS' ⋄ →0 ⋄ :EndIf
+ :For nm :In PLUGINS.⎕NL ¯9
+     :If 'classes'≢nm
+         applyResources PLUGINS⍎nm
+     :EndIf
+ :EndFor
diff --git a/EWC/applyResources.aplf b/EWC/applyResources.aplf
new file mode 100644
index 0000000..0b3a8f4
--- /dev/null
+++ b/EWC/applyResources.aplf
@@ -0,0 +1,9 @@
+ applyResources plugin;res;alias;folder
+⍝ Mount whatever a plugin declared in Resources: an (alias folder) pair, or a
+⍝ 2-column matrix of them.
+ :If 2≠plugin.⎕NC'Resources' ⋄ →0 ⋄ :EndIf
+ res←plugin.Resources
+ :If 2≠⍴⍴res ⋄ res←1 2⍴res ⋄ :EndIf
+ :For (alias folder) :In ↓res
+     alias AddResource folder
+ :EndFor
diff --git a/EWC/dWC.aplf b/EWC/dWC.aplf
index d71bd5d..3acbc7d 100644
--- a/EWC/dWC.aplf
+++ b/EWC/dWC.aplf
@@ -17,7 +17,8 @@
      →0⊣R←nsname caller.⎕WC Props
  :EndIf
 
- :If 9≠classes.⎕NC type
+ class←findClass type
+ :If ⎕NULL≡class
      Warning 'WC ',Name,' - unsupported class "',type,'"'
      ⎕←⎕SE.UCMD 'disp ⍪Props'
      nsname caller.⎕NS''
@@ -25,7 +26,10 @@
      →0
  :EndIf
 
- class←classes⍎type
+ :If 2=class.⎕NC'PluginName' ⍝ First use of a plugin class loads its JS
+     caller injectPlugin class.PluginName Name
+ :EndIf
+
  proplist←⎕C PropList←class.PropList
  valid←(lcp←⎕C propnames)∊allprops←⎕C AllProps←PropList,objProps,deprecatedprops
  values←valid↓¨Props
diff --git a/EWC/dWX.aplf b/EWC/dWX.aplf
index e004c53..19ad183 100644
--- a/EWC/dWX.aplf
+++ b/EWC/dWX.aplf
@@ -1,5 +1,8 @@
- {R}←dWX args;Name;Method;Info;caller;ID;conn;WX;wx;msg;z;⎕TRAP;txt
+ {R}←{timeout}dWX args;Name;Method;Info;caller;ID;conn;WX;wx;msg;z;⎕TRAP;txt
+⍝ Optional left argument: seconds to wait for the client, defaulting to
+⍝ WG_TIMEOUT. Loading a plugin's JavaScript can take longer than a property read.
  ⎕TRAP←0 'S'
+ :If 0=⎕NC'timeout' ⋄ timeout←WG_TIMEOUT ⋄ :EndIf
  (Name Method)←2↑args ⋄ Info←2↓args
 
  :If 'gettextsize'≡⎕C Method
@@ -25,7 +28,7 @@
  :EndIf
 
  :Trap 6 ⍝ If we get no response
-     R←msg WaitForWG 'WX' ID(⍕WGID)
+     R←msg WaitForWG 'WX' ID(⍕WGID)timeout
  :Else
      ⎕DMX.EM ⎕SIGNAL ⎕DMX.EN
  :EndTrap
diff --git a/EWC/ensurePlugins.aplf b/EWC/ensurePlugins.aplf
new file mode 100644
index 0000000..ee303d1
--- /dev/null
+++ b/EWC/ensurePlugins.aplf
@@ -0,0 +1,21 @@
+ {r}←ensurePlugins
+⍝ The plugin registry: PLUGINS.classes holds class definitions contributed by
+⍝ plugins, PLUGINS.<Name> the record for each registered plugin.
+⍝
+⍝ It is detached from Link. #.EWC is Link-managed, so without this a plugin's
+⍝ classes would be serialised into the EWC source tree. This function is the
+⍝ single place that decides where the registry lives; if breaking a child of a
+⍝ live link ever proves unreliable, move it out of #.EWC here and nothing else
+⍝ needs to change - everything reaches it through the PLUGINS ref.
+⍝ Registration is allowed before Init, so the globals the plugin path touches -
+⍝ RESOURCES to mount against, LOGMODES to log through - may not exist yet.
+⍝ setDefaults only fills in names that are undefined, so this cannot clobber
+⍝ anything the application has already set.
+ setDefaults
+
+ :If 9≠⎕NC'PLUGINS'
+     'PLUGINS'⎕NS''
+     'PLUGINS.classes'⎕NS''
+ :EndIf
+ {}{0::⍬ ⋄ ⎕SE.Link.Break ⍵}'#.EWC.PLUGINS' ⍝ reports "Not linked" when already detached
+ r←PLUGINS
diff --git a/EWC/findClass.aplf b/EWC/findClass.aplf
new file mode 100644
index 0000000..02eb052
--- /dev/null
+++ b/EWC/findClass.aplf
@@ -0,0 +1,19 @@
+ r←findClass type;names;i
+⍝ Resolve an EWC class name to its definition namespace, or ⎕NULL.
+⍝
+⍝ Core classes live in #.EWC.classes under lowercase names. Plugin classes live
+⍝ in PLUGINS.classes and keep the casing their author gave them, so they are
+⍝ matched case-insensitively. See RegisterPlugin and ensurePlugins.
+ type←⎕C type
+ :If 9=classes.⎕NC type
+     →0⊣r←classes⍎type
+ :EndIf
+
+ r←⎕NULL
+ :If 9=⎕NC'PLUGINS'
+ :AndIf 9=PLUGINS.⎕NC'classes'
+     names←PLUGINS.classes.⎕NL ¯9
+     :If 0≠≢i←⍸(⎕C names)∊⊂type
+         r←PLUGINS.classes⍎(⊃i)⊃names
+     :EndIf
+ :EndIf
diff --git a/EWC/getConnection.aplf b/EWC/getConnection.aplf
index a7bf64b..bbf8492 100644
--- a/EWC/getConnection.aplf
+++ b/EWC/getConnection.aplf
@@ -1,23 +1,14 @@
- r←getConnection(caller names);name;top;n;p
- name←,⊃⊆names
- n←≢p←(⍕caller),'.'
- name←(n×p≡n↑name)↓name ⍝ drop caller prefix
- top←(¯1+name⍳'.')↑name
+ r←getConnection(caller names);ns
+ ns←caller sessionNS names
 
- :If 9=caller.⎕NC n←top,'.HR._EWC'
-     r←(caller⍎n).(ID conn)
-
- :ElseIf MODE=2 ⍝ Multi
-     :If 9=caller.⎕NC'_EWC'
-         r←caller._EWC.(ID conn)
-     :Else
-         ∘∘∘ ⍝ Unable to locate app session
-     :EndIf
-
- :Else
+ :If ⎕NULL≢ns
      :Trap 0
-         r←CODELOCATION._EWC.(ID conn)
+         r←ns.(ID conn)
      :Else
          r←0 ¯1 ⍝ no connection: ¯1 is an invalid conn (0 is a VALID Conga connection)
      :EndTrap
+ :ElseIf MODE=2 ⍝ Multi
+     ∘∘∘ ⍝ Unable to locate app session
+ :Else
+     r←0 ¯1
  :EndIf
diff --git a/EWC/getObj.aplf b/EWC/getObj.aplf
index a4f5624..ca22e45 100644
--- a/EWC/getObj.aplf
+++ b/EWC/getObj.aplf
@@ -1,11 +1,9 @@
- (type class objref)←getObj Name;t
+ (type class objref)←getObj Name
 
  :Trap 0
      objref←caller⍎Name
      type←objref.Type
-     :If 9=classes.⎕NC t←⎕C type
-         class←classes⍎t
-     :Else
+     :If ⎕NULL≡class←findClass type
          Warning'Unsupported class: ',type
          ⎕←Props
          →0
diff --git a/EWC/injectPlugin.aplf b/EWC/injectPlugin.aplf
new file mode 100644
index 0000000..b6afaf0
--- /dev/null
+++ b/EWC/injectPlugin.aplf
@@ -0,0 +1,35 @@
+ caller injectPlugin(name objname);sns;plugin;js;r;top
+⍝ Send a plugin's JavaScript to this session's client, once per session.
+⍝
+⍝ Called from ∆WC the first time one of the plugin's classes is created, before
+⍝ that object's WC message goes out. dWX blocks until the client replies, and
+⍝ the client holds every later frame until the injected code has resolved - so
+⍝ the component is registered by the time the WC is handled.
+⍝
+⍝ Load state lives in the session's own _EWC namespace: per clone in Multi mode,
+⍝ per HTMLRenderer in Desktop, one in CODELOCATION otherwise.
+ sns←caller sessionNS objname
+ :If ⎕NULL≡sns ⋄ →0 ⋄ :EndIf ⍝ no client yet
+ :If 0=sns.⎕NC'PLUGINS_LOADED' ⋄ sns.PLUGINS_LOADED←⍬ ⋄ :EndIf
+
+⍝ Keyed by connection, not just name: outside Multi mode _EWC outlives any one
+⍝ browser, and a reload gives the client a fresh, empty registry. Entries for
+⍝ connections that have gone are dropped rather than accumulating.
+ sns.PLUGINS_LOADED←(sns.conn=⊃¨sns.PLUGINS_LOADED)/sns.PLUGINS_LOADED
+ :If (⊂sns.conn name)∊sns.PLUGINS_LOADED ⋄ →0 ⋄ :EndIf
+ :If 9≠PLUGINS.⎕NC name ⋄ →0 ⋄ :EndIf
+
+ plugin←PLUGINS⍎name
+ sns.PLUGINS_LOADED,←⊂sns.conn name ⍝ marked before injecting: a failure must not retry per object
+ :If ~(⌊plugin.⎕NC'Inject')∊2 3 ⋄ →0 ⋄ :EndIf
+
+ js←,⊆plugin.Inject ⍝ a niladic function, or the JavaScript itself
+ top←(¯1+objname⍳'.')↑objname
+ :Trap 6 85
+     r←PLUGIN_TIMEOUT dWX(top 'EvalJS'),js
+     :If ∨/¯1=⊃¨,⊆r
+         'E'Log'Plugin "',name,'" injection failed: ',⍕r
+     :EndIf
+ :Else
+     'E'Log'Plugin "',name,'" injection got no response from the client'
+ :EndTrap
diff --git a/EWC/sessionNS.aplf b/EWC/sessionNS.aplf
new file mode 100644
index 0000000..f0c0b19
--- /dev/null
+++ b/EWC/sessionNS.aplf
@@ -0,0 +1,28 @@
+ r←caller sessionNS names;name;top;n;p
+⍝ The per-session _EWC namespace for an object name, or ⎕NULL if there is none.
+⍝
+⍝ Desktop gives each Form's HTMLRenderer its own copy; Multi clones the whole
+⍝ application namespace per connection; otherwise there is a single one, in
+⍝ CODELOCATION. getConnection reads (ID conn) out of whatever this returns.
+ name←,⊃⊆names
+ n←≢p←(⍕caller),'.'
+ name←(n×p≡n↑name)↓name ⍝ drop caller prefix
+ top←(¯1+name⍳'.')↑name
+
+ :If 9=caller.⎕NC n←top,'.HR._EWC'
+     r←caller⍎n
+
+ :ElseIf MODE=2 ⍝ Multi
+     :If 9=caller.⎕NC'_EWC'
+         r←caller._EWC
+     :Else
+         r←⎕NULL
+     :EndIf
+
+ :Else
+     :Trap 0
+         r←CODELOCATION._EWC
+     :Else
+         r←⎕NULL
+     :EndTrap
+ :EndIf
diff --git a/EWC/setDefaults.aplf b/EWC/setDefaults.aplf
index 63ef638..f356bb5 100644
--- a/EWC/setDefaults.aplf
+++ b/EWC/setDefaults.aplf
@@ -10,3 +10,4 @@ setDefaults
  'BREAK'        default ⍬ ⍬ ⍝ IDs Events
  'SHOWDEVTOOLS' default 0
  'FONTMAP'      default FontMap
+ 'PLUGIN_TIMEOUT' default 30 ⍝ Seconds to wait while a plugin's JS loads
diff --git a/demo/DemoPluginRegistry.aplf b/demo/DemoPluginRegistry.aplf
new file mode 100644
index 0000000..c2c925a
--- /dev/null
+++ b/demo/DemoPluginRegistry.aplf
@@ -0,0 +1,57 @@
+ DemoPluginRegistry connected;plugin;cls;js;r
+⍝ The plugin mechanism end to end, with no external dependency: an EWC class
+⍝ registered at runtime, its React component injected as JavaScript the first
+⍝ time the class is used, and an event travelling from that component to APL.
+ EWC.CONNECTED←connected
+
+ CBPluginBox←{'F1.STATUS'eWS'Caption'('Select from ',(⊃⍵),' at ',⍕3↑3↓⎕TS)}
+
+ 'F1'eWC'Form' 'Plugin registry'(50 50)(220 460)('Coord' 'Pixel')
+ 'F1.L1'eWC'Label' 'The box below is drawn by injected JavaScript.'('Posn' 10 10)('Size' 20 440)
+ 'F1.STATUS'eWC'Label' 'no event yet'('Posn' 170 10)('Size' 20 440)
+
+ plugin←⎕NS''
+ plugin.Name←'DemoBox'
+ plugin.Folder←EWC.FOLDER
+
+ plugin.Classes←⎕NS''
+ 'PluginBox'plugin.Classes.⎕NS''
+ cls←plugin.Classes.PluginBox
+ cls.ClassName←'PluginBox'
+ cls.PropList←'Type' 'Posn' 'Size' 'Caption' 'Event' 'MethodList' 'ChildList' 'EventList' 'PropList'
+ cls.Defaults←('PluginBox')(10 10)(50 100)('')(⍬)(⍬)(⍬)(⍬)(cls.PropList)
+ cls.Supported←cls.PropList
+ cls.Dynamic←⍬
+ cls.methodlist←⍬
+ cls.SupportedEvents←,⊂'Select'
+
+⍝ The delay is deliberate: it proves the client waits for an asynchronous
+⍝ injection before handling the WC that follows.
+ js←'(async () => {'
+ js,←'  await new Promise(r => setTimeout(r, 250));'
+ js,←'  const R = window.EWC.React;'
+ js,←'  window.EWC.registerComponent("PluginBox", ({ data, ewc }) => {'
+ js,←'    const P = data.Properties || {};'
+ js,←'    const style = Object.assign({}, ewc.utils.setStyle(P), {'
+ js,←'      background: "#cfe8ff", border: "2px solid #0066cc", borderRadius: "6px",'
+ js,←'      display: "flex", alignItems: "center", justifyContent: "center",'
+ js,←'      cursor: "pointer", userSelect: "none" });'
+ js,←'    const fire = () => ewc.socket.send(JSON.stringify('
+ js,←'      { Event: { EventName: "Select", ID: data.ID } }));'
+ js,←'    return R.createElement("div", { id: data.ID, style, onClick: fire }, P.Caption || "");'
+ js,←'  });'
+ js,←'  return "PluginBox registered";'
+ js,←'})()'
+ plugin.Inject←js
+
+ EWC.RegisterPlugin plugin
+
+ 'F1.BOX'eWC'PluginBox'('Posn' 60 10)('Size' 55 440)('Caption' 'Click me - I am a plugin')('Event' 'Select' 'CBPluginBox')
+
+⍝ A second object of the same class must reuse the loaded plugin: the log
+⍝ should show one EvalJS, not two.
+ 'F1.BOX2'eWC'PluginBox'('Posn' 120 10)('Size' 40 440)('Caption' 'and so am I')('Event' 'Select' 'CBPluginBox')
+
+⍝ An injection that rejects must come back as ¯1, not hang until the timeout.
+ r←2 eNQ'F1' 'EvalJS' 'import("/no-such-plugin-bundle.js")'
+ 'F1.REJECT'eWC'Label'('Caption'('rejected import rc=',⍕⊃⊃,⊆r))('Posn' 195 10)('Size' 20 440)