diff --git a/Pantograph/Frontend/Basic.lean b/Pantograph/Frontend/Basic.lean
index 3fbc99d..ba013bb 100644
--- a/Pantograph/Frontend/Basic.lean
+++ b/Pantograph/Frontend/Basic.lean
@@ -6,8 +6,6 @@ open Lean
 namespace Lean.FileMap
 
 /-- Extract the range of a `Syntax` expressed as lines and columns. -/
--- Extracted from the private declaration `Lean.Elab.formatStxRange`,
--- in `Lean.Elab.InfoTree.Main`.
 @[export pantograph_frontend_stx_range]
 protected def stxRange (fileMap : FileMap) (stx : Syntax) : Position × Position :=
   let pos    := stx.getPos?.getD 0
@@ -19,9 +17,10 @@ namespace Lean.PersistentArray
 
 /--
 Drop the first `n` elements of a `PersistentArray`, returning the results as a `List`.
+
+We can't remove the `[Inhabited α]` hypotheses here until
+`PersistentArray`'s `GetElem` instance also does.
 -/
--- We can't remove the `[Inhabited α]` hypotheses here until
--- `PersistentArray`'s `GetElem` instance also does.
 protected def drop [Inhabited α] (t : PersistentArray α) (n : Nat) : List α :=
   List.range (t.size - n) |>.map fun i => t.get! (n + i)
 
@@ -36,8 +35,11 @@ def stxByteRange (stx : Syntax) : String.Pos × String.Pos :=
   let endPos := stx.getTailPos?.getD 0
   (pos, endPos)
 
+structure Context where
+  cancelTk? : Option IO.CancelToken := .none
 
-abbrev FrontendM := Elab.Frontend.FrontendM
+/-- This `FrontendM` comes with more options. -/
+abbrev FrontendM := ReaderT Context Elab.Frontend.FrontendM
 
 structure CompilationStep where
   scope : Elab.Command.Scope
@@ -50,10 +52,44 @@ structure CompilationStep where
   msgs : List Message
   trees : List Elab.InfoTree
 
-namespace CompilationStep
+/-- Like `Elab.Frontend.runCommandElabM`, but taking `cancelTk?` into account. -/
+@[inline] def runCommandElabM (x : Elab.Command.CommandElabM α) : FrontendM α := do
+  let config ← read
+  let ctx ← readThe Elab.Frontend.Context
+  let s ← get
+  let cmdCtx : Elab.Command.Context := {
+    cmdPos       := s.cmdPos
+    fileName     := ctx.inputCtx.fileName
+    fileMap      := ctx.inputCtx.fileMap
+    snap?        := none
+    cancelTk?    := config.cancelTk?
+  }
+  match (← liftM <| EIO.toIO' <| (x cmdCtx).run s.commandState) with
+  | Except.error e      => throw <| IO.Error.userError s!"unexpected internal error: {← e.toMessageData.toString}"
+  | Except.ok (a, sNew) => Elab.Frontend.setCommandState sNew; return a
 
-end CompilationStep
+def elabCommandAtFrontend (stx : Syntax) : FrontendM Unit := do
+  runCommandElabM do
+    let initMsgs ← modifyGet fun st => (st.messages, { st with messages := {} })
+    Elab.Command.elabCommandTopLevel stx
+    let mut msgs := (← get).messages
+    modify ({ · with messages := initMsgs ++ msgs })
 
+open Elab.Frontend in
+def processCommand : FrontendM Bool := do
+  updateCmdPos
+  let cmdState ← getCommandState
+  let ictx ← getInputContext
+  let pstate ← getParserState
+  let scope := cmdState.scopes.head!
+  let pmctx := { env := cmdState.env, options := scope.opts, currNamespace := scope.currNamespace, openDecls := scope.openDecls }
+  match profileit "parsing" scope.opts fun _ => Parser.parseCommand ictx pmctx pstate cmdState.messages with
+  | (cmd, ps, messages) =>
+    modify fun s => { s with commands := s.commands.push cmd }
+    setParserState ps
+    setMessages messages
+    elabCommandAtFrontend cmd
+    pure (Parser.isTerminalCommand cmd)
 
 /--
 Process one command, returning a `CompilationStep` and
@@ -63,17 +99,19 @@ Process one command, returning a `CompilationStep` and
 def processOneCommand: FrontendM (CompilationStep × Bool) := do
   let s := (← get).commandState
   let before := s.env
-  let done ← Elab.Frontend.processCommand
+  let done ← processCommand
   let stx := (← get).commands.back!
-  let src := (← read).inputCtx.input.toSubstring.extract (← get).cmdPos (← get).parserState.pos
+  let src := (← readThe Elab.Frontend.Context).inputCtx.input.toSubstring.extract
+    (← get).cmdPos
+    (← get).parserState.pos
   let s' := (← get).commandState
   let after := s'.env
   let msgs := s'.messages.toList.drop s.messages.toList.length
   let trees := s'.infoState.trees.drop s.infoState.trees.size
-  let ⟨_, fileName, fileMap⟩  := (← read).inputCtx
+  let ⟨_, fileName, fileMap⟩  := (← readThe Elab.Frontend.Context).inputCtx
   return ({ scope := s.scopes.head!, fileName, fileMap, src, stx, before, after, msgs, trees }, done)
 
-partial def mapCompilationSteps { α } (f: CompilationStep → IO α) : FrontendM (List α) := do
+partial def mapCompilationSteps { α } (f: CompilationStep → FrontendM α) : FrontendM (List α) := do
   let (cmd, done) ← processOneCommand
   if done then
     if cmd.src.isEmpty then
@@ -106,10 +144,11 @@ def createContextStateFromFile
   --let file ← IO.FS.readFile (← findSourcePath module)
   let inputCtx := Parser.mkInputContext file fileName
 
+  let (header, parserState, messages) ← Parser.parseHeader inputCtx
   let (env, parserState, messages) ← match env? with
     | .some env => pure (env, {}, .empty)
     | .none =>
-      let (header, parserState, messages) ← Parser.parseHeader inputCtx
+      -- Only process the header if we don't have an environment.
       let (env, messages) ← Elab.processHeader header opts messages inputCtx
       pure (env, parserState, messages)
   let commandState := Elab.Command.mkState env messages opts
diff --git a/Pantograph/Library.lean b/Pantograph/Library.lean
index f361046..a2095c3 100644
--- a/Pantograph/Library.lean
+++ b/Pantograph/Library.lean
@@ -2,7 +2,6 @@ import Pantograph.Environment
 import Pantograph.Goal
 import Pantograph.Protocol
 import Pantograph.Delate
-import Pantograph.Version
 
 import Lean
 
@@ -188,4 +187,9 @@ def runCancelTokenWithTimeout (cancelToken : IO.CancelToken) (timeout : UInt32)
     cancelToken.set
   return ()
 
+def spawnCancelToken (timeout : UInt32) : IO IO.CancelToken := do
+  let token ← IO.CancelToken.new
+  runCancelTokenWithTimeout token timeout
+  return token
+
 end Pantograph
diff --git a/Repl.lean b/Repl.lean
index ef91a5e..ae116f1 100644
--- a/Repl.lean
+++ b/Repl.lean
@@ -200,7 +200,7 @@ def frontend_process (args: Protocol.FrontendProcess): EMainM Protocol.FrontendP
     else do
       .some <$> getEnv
   let (context, state) ← do Frontend.createContextStateFromFile file fileName env? {}
-  let frontendM: Elab.Frontend.FrontendM (List CompilationUnit) :=
+  let frontendM: Frontend.FrontendM (List CompilationUnit) :=
     Frontend.mapCompilationSteps λ step => do
     let boundary := (step.src.startPos.byteIdx, step.src.stopPos.byteIdx)
     let invocations: Option (List Protocol.InvokedTactic) ← if args.invocations?.isSome then
@@ -224,7 +224,10 @@ def frontend_process (args: Protocol.FrontendProcess): EMainM Protocol.FrontendP
       messages,
       newConstants
     }
-  let (li, state') ← frontendM.run context |>.run state
+  let cancelTk? ← match (← get).options.timeout with
+    | 0 => pure .none
+    | timeout => .some <$> spawnCancelToken (timeout := .ofBitVec timeout)
+  let (li, state') ← frontendM.run { cancelTk? } |>.run context |>.run state
   if args.inheritEnv then
     setEnv state'.commandState.env
     if let .some scope := state'.commandState.scopes.head? then
diff --git a/Test/Frontend.lean b/Test/Frontend.lean
index 4062fb7..fec2e01 100644
--- a/Test/Frontend.lean
+++ b/Test/Frontend.lean
@@ -7,11 +7,16 @@ import LSpec
 open Lean Pantograph
 namespace Pantograph.Test.Frontend
 
-def runFrontend { α } (source: String) (f : Frontend.CompilationStep → IO α) : MetaM (List α) := do
+open Frontend
+
+def runFrontend { α } (source: String) (f : CompilationStep → FrontendM α) (timeout : UInt32 := 0): MetaM (List α) := do
   let filename := "<anonymous>"
-  let (context, state) ← do Frontend.createContextStateFromFile source filename (← getEnv) {}
-  let m := Frontend.mapCompilationSteps f
-  m.run context |>.run' state
+  let (context, state) ← do createContextStateFromFile source filename (← getEnv) {}
+  let m := mapCompilationSteps f
+  let cancelTk? ← match timeout with
+    | 0 => pure .none
+    | timeout => .some <$> spawnCancelToken timeout
+  m.run { cancelTk? } |>.run context |>.run' state
 
 def test_open : TestT MetaM Unit := do
   let sketch := "
@@ -29,7 +34,7 @@ def collectSorrysFromSource (source: String) (options : Frontend.GoalCollectionO
   let (context, state) ← do Frontend.createContextStateFromFile source filename (← getEnv) {}
   let m := Frontend.mapCompilationSteps λ step => do
     return (step.before, ← Frontend.collectSorrys step options)
-  let li ← m.run context |>.run' state
+  let li ← m.run {} |>.run context |>.run' state
   let goalStates ← li.filterMapM λ (env, sorrys) => withEnv env do
     if sorrys.isEmpty then
       return .none
@@ -227,7 +232,7 @@ def collectNewConstants (source: String) : MetaM (List (List Name)) := do
   let (context, state) ← do Frontend.createContextStateFromFile source filename (← getEnv) {}
   let m := Frontend.mapCompilationSteps λ step => do
     Frontend.collectNewDefinedConstants step
-  m.run context |>.run' state
+  m.run {} |>.run context |>.run' state
 
 def test_collect_one_constant : TestT MetaM Unit := do
   let input := "