diff --git a/CHANGELOG.md b/CHANGELOG.md
index ad32fea..ac39bb9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,12 @@ Notable changes are grouped by release. See [all releases](https://github.com/Co
 
 ## Unreleased
 
+- Building requires LLVM 20. CMake rejects an older LLVM when it configures the build, where
+  LLVM 19 passed that check and then failed to compile. On macOS, `scripts/build.sh` looks for
+  Homebrew `llvm@20` before `llvm`, a later version, and no longer for `llvm@19` or `llvm@18`.
+- The GitHub Action accepts `llvm-version: 20` only. On a runner for which apt.llvm.org
+  publishes no LLVM 20 packages, such as `ubuntu-26.04`, it stops with an error before
+  installing LLVM (#255).
 - StackBufferOverflow and MultipleStoresToStackBuffer resolve each pointer variable to its
   stack array once per function, by strongly connected components: pointer variables
   assigned from one another no longer make the analysis walk every path between them,
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8539319..516ff4c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -7,8 +7,7 @@ include(CheckLLVMVersion OPTIONAL)
 include(CheckCXXCompilerFlag)
 
 if(COMMAND check_llvm_version)
-    set(LLVM_MIN_REQUIRED_VERSION "19" CACHE STRING "Minimum required LLVM version")
-    check_llvm_version(${LLVM_MIN_REQUIRED_VERSION})
+    check_llvm_version(20)
 else()
     message(WARNING "check_llvm_version() not available, skipping LLVM version check")
 endif()
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 57d7aba..60f80e2 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -8,7 +8,7 @@ This document defines the expected workflow for code, tests, and pull requests.
 
 Prerequisites:
 - CMake `>= 3.16`
-- LLVM/Clang `>= 19` (20 recommended)
+- LLVM/Clang 20
 - A C++20 compiler
 
 Build:
diff --git a/README.md b/README.md
index 0f0d00b..420cbbd 100644
--- a/README.md
+++ b/README.md
@@ -2,6 +2,10 @@
 
 #### BUILD (macOS/Linux)
 
+The analyzer builds with LLVM and Clang 20 only. On macOS, install them with
+`brew install llvm@20`, not `llvm`, which is a later version. On Ubuntu, install
+`llvm-20-dev`, `libclang-20-dev` and `clang-20` from apt.llvm.org.
+
 ```zsh
 ./build.sh
 ```
diff --git a/scripts/build.sh b/scripts/build.sh
index 8fa04c3..fd1acfa 100755
--- a/scripts/build.sh
+++ b/scripts/build.sh
@@ -219,7 +219,7 @@ detect_from_brew() {
 
   local formula
   local prefix
-  for formula in llvm llvm@20 llvm@19 llvm@18; do
+  for formula in llvm@20 llvm; do
     prefix="$(brew --prefix "$formula" 2>/dev/null || true)"
     if [ -n "$prefix" ] && [ -d "$prefix" ]; then
       if [ -z "$LLVM_DIR" ] && [ -d "$prefix/lib/cmake/llvm" ]; then