diff --git a/lib/src/commonMain/kotlin/world/phantasmal/lib/asm/Assembly.kt b/lib/src/commonMain/kotlin/world/phantasmal/lib/asm/Assembly.kt
index eccbf69d..b0a60280 100644
--- a/lib/src/commonMain/kotlin/world/phantasmal/lib/asm/Assembly.kt
+++ b/lib/src/commonMain/kotlin/world/phantasmal/lib/asm/Assembly.kt
@@ -15,7 +15,7 @@ class AssemblyProblem(
     cause: Throwable? = null,
     val lineNo: Int,
     val col: Int,
-    val length: Int,
+    val len: Int,
 ) : Problem(severity, uiMessage, message, cause)
 
 fun assemble(
@@ -214,7 +214,7 @@ private class Assembler(private val asm: List<String>, private val inlineStackAr
         }
     }
 
-    private fun addError(col: Int, length: Int, uiMessage: String, message: String? = null) {
+    private fun addError(col: Int, len: Int, uiMessage: String, message: String? = null) {
         result.addProblem(
             AssemblyProblem(
                 Severity.Error,
@@ -222,7 +222,7 @@ private class Assembler(private val asm: List<String>, private val inlineStackAr
                 message ?: "$uiMessage At $lineNo:$col.",
                 lineNo = lineNo,
                 col = col,
-                length = length
+                len = len
             )
         )
     }
@@ -246,7 +246,7 @@ private class Assembler(private val asm: List<String>, private val inlineStackAr
                 uiMessage,
                 lineNo = lineNo,
                 col = token.col,
-                length = token.len,
+                len = token.len,
             )
         )
     }
@@ -375,7 +375,9 @@ private class Assembler(private val asm: List<String>, private val inlineStackAr
                 addError(
                     identToken.col,
                     errorLength,
-                    "Expected $paramCount argument ${if (paramCount == 1) "" else "s"}, got $argCount.",
+                    "Expected $paramCount argument${
+                        if (paramCount == 1) "" else "s"
+                    }, got $argCount.",
                 )
 
                 return
@@ -385,7 +387,9 @@ private class Assembler(private val asm: List<String>, private val inlineStackAr
                 addError(
                     identToken.col,
                     errorLength,
-                    "Expected at least $paramCount argument${if (paramCount == 1) "" else "s"}, got $argCount.",
+                    "Expected at least $paramCount argument${
+                        if (paramCount == 1) "" else "s"
+                    }, got $argCount.",
                 )
 
                 return
diff --git a/web/assembly-worker/src/main/kotlin/world/phantasmal/web/assemblyWorker/AssemblyWorker.kt b/web/assembly-worker/src/main/kotlin/world/phantasmal/web/assemblyWorker/AssemblyWorker.kt
index 0d02afcc..8921a8b9 100644
--- a/web/assembly-worker/src/main/kotlin/world/phantasmal/web/assemblyWorker/AssemblyWorker.kt
+++ b/web/assembly-worker/src/main/kotlin/world/phantasmal/web/assemblyWorker/AssemblyWorker.kt
@@ -158,7 +158,7 @@ class AssemblyWorker(private val sendMessage: (ServerMessage) -> Unit) {
 
         @Suppress("UNCHECKED_CAST")
         val problems = (assemblyResult.problems as List<AssemblerAssemblyProblem>).map {
-            AssemblyProblem(it.severity, it.uiMessage)
+            AssemblyProblem(it.severity, it.uiMessage, it.lineNo, it.col, it.len)
         }
 
         if (problems != this.problems) {
diff --git a/web/shared/src/main/kotlin/world/phantasmal/web/shared/Messages.kt b/web/shared/src/main/kotlin/world/phantasmal/web/shared/Messages.kt
index 2d90c540..8bd857be 100644
--- a/web/shared/src/main/kotlin/world/phantasmal/web/shared/Messages.kt
+++ b/web/shared/src/main/kotlin/world/phantasmal/web/shared/Messages.kt
@@ -157,4 +157,7 @@ class AsmChange(
 class AssemblyProblem(
     val severity: Severity,
     val message: String,
+    val lineNo: Int,
+    val col: Int,
+    val len: Int,
 )
diff --git a/web/src/main/kotlin/world/phantasmal/web/externals/monacoEditor/editor.kt b/web/src/main/kotlin/world/phantasmal/web/externals/monacoEditor/editor.kt
index 685c8959..4e305733 100644
--- a/web/src/main/kotlin/world/phantasmal/web/externals/monacoEditor/editor.kt
+++ b/web/src/main/kotlin/world/phantasmal/web/externals/monacoEditor/editor.kt
@@ -22,6 +22,11 @@ external fun createModel(
 
 external fun defineTheme(themeName: String, themeData: IStandaloneThemeData)
 
+/**
+ * Set the markers for a model.
+ */
+external fun setModelMarkers(model: ITextModel, owner: String, markers: Array<IMarkerData>)
+
 external interface IStandaloneThemeData {
     var base: String /* 'vs' | 'vs-dark' | 'hc-black' */
     var inherit: Boolean
diff --git a/web/src/main/kotlin/world/phantasmal/web/questEditor/stores/AsmStore.kt b/web/src/main/kotlin/world/phantasmal/web/questEditor/stores/AsmStore.kt
index 1e374310..5a64bd0b 100644
--- a/web/src/main/kotlin/world/phantasmal/web/questEditor/stores/AsmStore.kt
+++ b/web/src/main/kotlin/world/phantasmal/web/questEditor/stores/AsmStore.kt
@@ -1,6 +1,7 @@
 package world.phantasmal.web.questEditor.stores
 
 import kotlinx.browser.window
+import world.phantasmal.core.Severity
 import world.phantasmal.core.disposable.Disposer
 import world.phantasmal.core.disposable.disposable
 import world.phantasmal.lib.asm.assemble
@@ -78,6 +79,35 @@ class AsmStore(
         observe(asmAnalyser.mapDesignations) {
             questEditorStore.currentQuest.value?.setMapDesignations(it)
         }
+
+        observe(problems) { problems ->
+            textModel.value?.let { model ->
+                val markers = Array<IMarkerData>(problems.size) {
+                    val problem = problems[it]
+                    obj {
+                        severity = when (problem.severity) {
+                            Severity.Trace, Severity.Debug -> MarkerSeverity.Hint
+                            Severity.Info -> MarkerSeverity.Info
+                            Severity.Warning -> MarkerSeverity.Warning
+                            Severity.Error -> MarkerSeverity.Error
+                        }
+                        message = problem.message
+                        startLineNumber = problem.lineNo
+                        startColumn = problem.col
+                        endLineNumber = problem.lineNo
+                        endColumn = problem.col + problem.len
+
+                        // Hack: because only one warning is generated at the moment, "Unnecessary
+                        // section marker.", we can simply add the Unnecessary tag here.
+                        if (problem.severity == Severity.Warning) {
+                            tags = arrayOf(MarkerTag.Unnecessary)
+                        }
+                    }
+                }
+                // Not sure what the "owner" parameter is for.
+                setModelMarkers(model, owner = ASM_LANG_ID, markers)
+            }
+        }
     }
 
     fun makeUndoCurrent() {