diff --git a/.github/workflows/acme.yml b/.github/workflows/acme.yml
new file mode 100644
index 000000000..ff3286f44
--- /dev/null
+++ b/.github/workflows/acme.yml
@@ -0,0 +1,52 @@
+name: Check every stdlib module is compiled (acme)
+
+on:
+  pull_request:
+    paths:
+      - 'libraries/common/**'
+
+jobs:
+  check-stdlib-sync:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Generate acme and test for differences
+        id: generate-acme
+        shell: bash
+        run: |
+          # Function to convert file path to module import path
+          path_to_module() {
+            local filepath="$1"
+            # Remove libraries/common/ prefix and .effekt suffix
+            local module_path="${filepath#libraries/common/}"
+            module_path="${module_path%.effekt}"
+            echo "$module_path"
+          }
+
+          # Find all .effekt files in libraries/common, excluding acme.effekt
+          MODULES=$(find libraries/common -type f -name "*.effekt" | sort | while read -r file; do
+            path_to_module "$file"
+          done)
+
+          # Create the new acme.effekt content
+          {
+            echo "/// This module is auto-generated and checked in CI."
+            echo "/// It imports **every** stdlib module: ACME = All Common Modules in Effekt"
+            echo "module acme"
+            echo ""
+            for module in $MODULES; do
+              echo "import $module"
+            done
+            echo ""
+            echo "def main() = ()"
+          } > generated-acme.effekt
+
+          # Compare files, ignoring whitespace, blank lines, and line ending differences
+          if ! diff -Bbq examples/stdlib/acme.effekt generated-acme.effekt; then
+            echo "::error::The stdlib import file (examples/stdlib/acme.effekt) is out of sync with the modules in libraries/common."
+            echo "Differences found:"
+            diff -Bu examples/stdlib/acme.effekt generated-acme.effekt
+            exit 1
+          fi
diff --git a/effekt/jvm/src/test/scala/effekt/StdlibTests.scala b/effekt/jvm/src/test/scala/effekt/StdlibTests.scala
index 44fbe0be8..db36875d2 100644
--- a/effekt/jvm/src/test/scala/effekt/StdlibTests.scala
+++ b/effekt/jvm/src/test/scala/effekt/StdlibTests.scala
@@ -49,5 +49,9 @@ class StdlibLLVMTests extends StdlibTests {
 
     // String comparison using `<`, `<=`, `>`, `>=` is not implemented yet on LLVM
     examplesDir / "stdlib" / "string" / "compare.effekt",
+
+    // Wrong codegen for negative types, see #801
+    examplesDir / "stdlib" / "json.effekt",
+    examplesDir / "stdlib" / "buffer.effekt",
   )
 }
diff --git a/examples/stdlib/acme.check b/examples/stdlib/acme.check
new file mode 100644
index 000000000..e69de29bb
diff --git a/examples/stdlib/acme.effekt b/examples/stdlib/acme.effekt
new file mode 100644
index 000000000..b8e209be7
--- /dev/null
+++ b/examples/stdlib/acme.effekt
@@ -0,0 +1,37 @@
+/// This module is auto-generated and checked in CI.
+/// It imports **every** stdlib module: ACME = All Common Modules in Effekt
+module acme
+
+import args
+import array
+import bench
+import buffer
+import bytearray
+import char
+import dequeue
+import effekt
+import exception
+import io
+import io/console
+import io/error
+import io/filesystem
+import io/network
+import io/time
+import json
+import list
+import map
+import option
+import process
+import queue
+import ref
+import regex
+import result
+import scanner
+import seq
+import set
+import stream
+import string
+import test
+import tty
+
+def main() = ()
diff --git a/examples/stdlib/buffer.check b/examples/stdlib/buffer.check
new file mode 100644
index 000000000..5320d584f
--- /dev/null
+++ b/examples/stdlib/buffer.check
@@ -0,0 +1,8 @@
+1
+false
+Some(17)
+Some(1)
+Some(2)
+Some(3)
+Some(4)
+Some(5)
\ No newline at end of file
diff --git a/examples/stdlib/buffer.effekt b/examples/stdlib/buffer.effekt
new file mode 100644
index 000000000..4d87fdee9
--- /dev/null
+++ b/examples/stdlib/buffer.effekt
@@ -0,0 +1,3 @@
+import buffer
+
+def main() = buffer::examples::main()
\ No newline at end of file
diff --git a/examples/stdlib/json.check b/examples/stdlib/json.check
new file mode 100644
index 000000000..186686a7f
--- /dev/null
+++ b/examples/stdlib/json.check
@@ -0,0 +1,65 @@
+	a
+
+a
+
+a
+b
+f
+
+{
+"x"
+:
+"Hallo"
+,
+"y"
+:
+null
+,
+"z"
+:
+12.3783
+}
+
+{
+"a"
+:
+null
+,
+"b"
+:
+[
+true
+,
+false
+,
+false
+,
+true
+]
+,
+"f"
+:
+12.532
+}
+
+{
+"a"
+:
+null
+,
+"b"
+:
+[
+true
+,
+false
+,
+false
+,
+true
+]
+,
+"f"
+:
+12.532
+}
diff --git a/examples/stdlib/json.effekt b/examples/stdlib/json.effekt
new file mode 100644
index 000000000..7f6438d9c
--- /dev/null
+++ b/examples/stdlib/json.effekt
@@ -0,0 +1,3 @@
+import json
+
+def main() = json::test::main()
\ No newline at end of file
diff --git a/libraries/common/buffer.effekt b/libraries/common/buffer.effekt
index e958897ab..2c9abab28 100644
--- a/libraries/common/buffer.effekt
+++ b/libraries/common/buffer.effekt
@@ -34,7 +34,7 @@ def emptyBuffer[T](capacity: Int): Buffer[T] at {global} = {
 }
 def arrayBuffer[T](initialCapacity: Int): Buffer[T] at {global} = {
   // TODO allocate buffer (and array) into a region r.
-  val contents = emptyArray[T](initialCapacity)
+  val contents = array::allocate[T](initialCapacity)
   var head in global = 0
   var tail in global = 0
 
@@ -51,7 +51,7 @@ def arrayBuffer[T](initialCapacity: Int): Buffer[T] at {global} = {
     def read() = {
       if (buffer.empty?) None()
       else {
-        val result: T = contents.remove(head).getOrElse { <> };
+        val result: T = contents.unsafeGet(head);
         head = mod(head + 1, initialCapacity)
         Some(result)
       }
@@ -59,7 +59,7 @@ def arrayBuffer[T](initialCapacity: Int): Buffer[T] at {global} = {
     def write(el: T) = {
       if (buffer.full?) <> // raise(BufferOverflow())
 
-      contents.put(tail, el)
+      contents.unsafeSet(tail, el)
       tail = mod(tail + 1, initialCapacity)
     }
   }
diff --git a/libraries/common/io/network.effekt b/libraries/common/io/network.effekt
index 4adcfed53..98baa9685 100644
--- a/libraries/common/io/network.effekt
+++ b/libraries/common/io/network.effekt
@@ -57,14 +57,17 @@ def server(host: String, port: Int, handler: () => Unit / Socket at {io, async,
 
 namespace examples {
   def helloWorldApp(): Unit / Socket = {
-    val request = do receive();
+    val request = do receive().toString;
 
-    println("Received a request: " ++ request.toUTF8)
+    println("Received a request: " ++ request)
 
-    if (request.toUTF8.startsWith("GET /")) {
-      do send(fromUTF8("HTTP/1.1 200 OK\r\n\r\nHello from Effekt!"))
+    def respond(s: String): Unit / Socket =
+      do send(s.fromString)
+
+    if (request.startsWith("GET /")) {
+      respond("HTTP/1.1 200 OK\r\n\r\nHello from Effekt!")
     } else {
-      do send("HTTP/1.1 400 Bad Request\r\n\r\n".fromUTF8)
+      respond("HTTP/1.1 400 Bad Request\r\n\r\n")
     }
     do end()
   }
diff --git a/libraries/common/json.effekt b/libraries/common/json.effekt
index eeb75ae5d..eb1bd096e 100644
--- a/libraries/common/json.effekt
+++ b/libraries/common/json.effekt
@@ -62,7 +62,7 @@ def encodeJsonObject[R]{ body: => R / JsonObjectBuilder }: R / emit[String] = {
   do emit("{")
   def c(k: String) = {
     if (not(first)) { do emit(",") }
-    do emit(escape(k)); do emit(":") 
+    do emit(escape(k)); do emit(":")
     first = false
   }
   val r = encodeJson {
@@ -76,7 +76,7 @@ def encodeJsonObject[R]{ body: => R / JsonObjectBuilder }: R / emit[String] = {
 /// Main entry point for encoding json.
 /// Emits individual tokens of the resulting json.
 def encodeJson[R]{ body: => R / JsonBuilder }: R / emit[String] = {
-  try body() with JsonBuilder { 
+  try body() with JsonBuilder {
       def null() = { resume(do emit("null")) }
       def bool(v) = { resume(do emit( if(v){ "true" } else { "false" } )) }
       def number(n) = { resume(do emit(show(n))) }
@@ -112,7 +112,7 @@ def readDouble(): Double / Scan[Char] = {
     if (optionally { readIf('.') }) {
       var b = 0.1
       var r = pre.toDouble
-      while (optionally[Int] { readDigit() } is Some(d)) {
+      while (returning::optionally[Int] { readDigit() } is Some(d)) {
         r = r + b * d.toDouble
         b = b * 0.1
       }
@@ -126,14 +126,14 @@ def readDouble(): Double / Scan[Char] = {
 
 def expectString(string: String): Unit / { Scan[Char], Exception[WrongFormat] } =
   for[Char] { string.each } { char =>
-    expect[Unit]("Expected " ++ string) { readIf(char) }
+    expect("Expected " ++ string) { readIf(char) }
   }
 
 /// Read and unescape a string in ""
 def readQuotedString(): Unit / { Scan[Char], emit[Char], Exception[WrongFormat] } = {
   try {
     skipWhitespace()
-    expect[Unit]("Expected \"") { readIf('"') }
+    expect("Expected \"") { readIf('"') }
     while(read[Char]() is c and c != '"') {
       c match {
         case '\\' => read[Char]() match {
@@ -191,7 +191,7 @@ def decodeJsonObject(): Unit / {Scan[Char], JsonObjectBuilder, Exception[WrongFo
   }
   do skip[Char]()
 }
-def decodeJsonList(): Unit / {Scan[Char], JsonBuilder, Exception[WrongFormat]} = { 
+def decodeJsonList(): Unit / {Scan[Char], JsonBuilder, Exception[WrongFormat]} = {
   var first = true
   expectString("[")
   with boundary
@@ -278,7 +278,7 @@ def build[R](){ body: => R / JsonBuilder }: (R, JsonValue) = {
   }
   (x, r)
 }
-def buildList[R](){ body: => R / JsonBuilder }: (R, List[JsonValue]) = collectList[JsonValue, R] {
+def buildList[R](){ body: => R / JsonBuilder }: (R, List[JsonValue]) = returning::collectList[JsonValue, R] {
   try body() with JsonBuilder {
     def number(n) = { do emit(Number(n)); resume(()) }
     def bool(b) = { do emit(Bool(b)); resume(()) }
@@ -296,7 +296,7 @@ def buildList[R](){ body: => R / JsonBuilder }: (R, List[JsonValue]) = collectLi
     }
   }
 }
-def buildDict[R](){ body: => R / JsonObjectBuilder }: (R, List[(String, JsonValue)]) = collectList[(String, JsonValue), R] {
+def buildDict[R](){ body: => R / JsonObjectBuilder }: (R, List[(String, JsonValue)]) = returning::collectList[(String, JsonValue), R] {
   try body() with JsonObjectBuilder {
     def field(k) = resume { {v} =>
       val x = build{v}
@@ -315,10 +315,12 @@ namespace test {
 
     // Read quoted string
     feed("\"\ta\n\ra\"") {
-      with scanner[Char, Unit]
+      with scanner[Char]
       println(collectString { readQuotedString() })
     }
 
+    println("")
+
     // Parse example
     feed("""{ "a": null, "b": [true,false,false,true], "f": 12.532   }"""){
       with scanner[Char]
@@ -336,8 +338,10 @@ namespace test {
       }
       handleJsonBuilder{d}{ decodeJson() }
     }
-    try{
-  
+
+    println("")
+
+    try {
       // Encode example
       encodeJson {
         do dict{
@@ -352,13 +356,15 @@ namespace test {
           }
         }
       }
-  
+
+      println("")
+
       // format with intermediate value
-      val j = feed("""{ 
-                  "a": null, 
-                  "b": [true, false, false, true], "f": 12.532   
+      val j = feed[JsonValue]("""{
+                  "a": null,
+                  "b": [true, false, false, true], "f": 12.532
                 } """){
-        with scanner[Char, JsonValue]
+        with returning::scanner[Char, JsonValue]
         build {
           decodeJson()
         }.second
@@ -366,18 +372,20 @@ namespace test {
       encodeJson{
         unbuild(j)
       }
-      
+
+      println("")
+
       // format (minify) example
       encodeJson{
-        feed("""{ 
-                  "a": null, 
-                  "b": [true, false, false, true], "f": 12.532   
+        feed("""{
+                  "a": null,
+                  "b": [true, false, false, true], "f": 12.532
                 } """){
-          with scanner[Char, Unit]
+          with scanner[Char]
           decodeJson()
         }
       }
-  
+
     } with emit[String] { e =>
       resume(println(e))
     }