diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 3bd323237..45199b268 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -47,7 +47,7 @@ jobs:
     - name: Bootstrap
       run: |
         sudo rm /etc/apt/sources.list.d/* && sudo apt-get update
-        sudo apt-get --no-install-recommends install -y clang-format-10 clang-tidy-10 shellcheck
+        sudo apt-get --no-install-recommends install -y clang-format-9 clang-tidy-9 shellcheck
         python3 -m pip install yapf==0.29.0
         sudo snap install shfmt
     - name: Check
@@ -99,7 +99,7 @@ jobs:
     - name: Bootstrap
       run: |
         sudo rm /etc/apt/sources.list.d/* && sudo apt-get update
-        sudo apt-get --no-install-recommends install -y clang-tools-10
+        sudo apt-get --no-install-recommends install -y clang-tools-9
     - name: Run
       run: |
         script/check-scan-build
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 5fca49804..b043d6a28 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -109,7 +109,7 @@ This will open up a text editor where you can specify which commits to squash.
 
 #### Coding Conventions and Style
 
-OpenThread uses and enforces the [OpenThread Coding Conventions and Style](STYLE_GUIDE.md) on all code, except for code located in [third_party](third_party). Use `script/make-pretty` and `script/make-pretty check` to automatically reformat code and check for code-style compliance, respectively. OpenThread currently requires [clang-format v10.0.0](http://releases.llvm.org/download.html#10.0.0) for C/C++ and [yapf v0.29.0](https://github.com/google/yapf) for Python.
+OpenThread uses and enforces the [OpenThread Coding Conventions and Style](STYLE_GUIDE.md) on all code, except for code located in [third_party](third_party). Use `script/make-pretty` and `script/make-pretty check` to automatically reformat code and check for code-style compliance, respectively. OpenThread currently requires [clang-format v9.0.0](https://releases.llvm.org/download.html#9.0.0) for C/C++ and [yapf v0.29.0](https://github.com/google/yapf) for Python.
 
 As part of the cleanup process, you should also run `script/make-pretty check` to ensure that your code passes the baseline code style checks.
 
diff --git a/STYLE_GUIDE.md b/STYLE_GUIDE.md
index 74e9c698c..04f96aba6 100644
--- a/STYLE_GUIDE.md
+++ b/STYLE_GUIDE.md
@@ -116,7 +116,7 @@
 
 - OpenThread uses `script/make-pretty` to reformat code and enforce code format and style. `script/make-pretty check` build target is included in OpenThread's continuous integration and must pass before a pull request is merged.
 
-- `script/make-pretty` requires [clang-format v10.0.0](http://releases.llvm.org/download.html#10.0.0) for C/C++ and [yapf v0.29.0](https://github.com/google/yapf) for Python.
+- `script/make-pretty` requires [clang-format v9.0.0](https://releases.llvm.org/download.html#9.0.0) for C/C++ and [yapf v0.29.0](https://github.com/google/yapf) for Python.
 
 ### File Names
 
@@ -188,4 +188,4 @@
 
 ## Format and Style
 
-- All code should adhere to [Google Python Style Guide](http://google.github.io/styleguide/pyguide.html) except maximum line length being 119.
+- All code should adhere to [Google Python Style Guide](https://google.github.io/styleguide/pyguide.html) except maximum line length being 119.
diff --git a/script/bootstrap b/script/bootstrap
index e84eda6ef..b2ea32ba6 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -59,7 +59,7 @@ install_packages_apt()
     echo 'Installing pretty tools useful for code contributions...'
 
     # add clang-format and clang-tidy for pretty
-    sudo apt-get --no-install-recommends install -y clang-format-10 clang-tidy-10
+    sudo apt-get --no-install-recommends install -y clang-format-9 clang-tidy-9
 
     # add yapf for pretty
     python3 -m pip install yapf==0.29.0 || echo 'WARNING: could not install yapf, which is useful if you plan to contribute python code to the OpenThread project.'
@@ -104,9 +104,9 @@ install_packages_brew()
 
     # add clang-format for pretty
     CLANG_FORMAT_VERSION="clang-format version 10"
-    command -v clang-format-10 || (command -v clang-format && (clang-format --version | grep -q "${CLANG_FORMAT_VERSION}")) || {
-        brew install llvm@10
-        sudo ln -s "$(brew --prefix llvm@10)/bin/clang-format" /usr/local/bin/clang-format-10
+    command -v clang-format-9 || (command -v clang-format && (clang-format --version | grep -q "${CLANG_FORMAT_VERSION}")) || {
+        brew install llvm@9
+        sudo ln -s "$(brew --prefix llvm@9)/bin/clang-format" /usr/local/bin/clang-format-9
     }
 
     # add yapf for pretty
diff --git a/script/check-scan-build b/script/check-scan-build
index fd327f78f..38323717e 100755
--- a/script/check-scan-build
+++ b/script/check-scan-build
@@ -96,12 +96,12 @@ do_scan_build()
     )
 
     export CPPFLAGS="${options[*]} -DOPENTHREAD_CONFIG_NCP_SPI_ENABLE=1"
-    scan-build-10 ./configure "${configure_options[@]}"
-    scan-build-10 --status-bugs -analyze-headers -v make -j"${OT_BUILD_JOBS}"
+    scan-build-9 ./configure "${configure_options[@]}"
+    scan-build-9 --status-bugs -analyze-headers -v make -j"${OT_BUILD_JOBS}"
 
     export CPPFLAGS="${options[*]} -DOPENTHREAD_CONFIG_NCP_UART_ENABLE=1"
-    scan-build-10 ./configure "${configure_options[@]}"
-    scan-build-10 --status-bugs -analyze-headers -v make -j"${OT_BUILD_JOBS}"
+    scan-build-9 ./configure "${configure_options[@]}"
+    scan-build-9 --status-bugs -analyze-headers -v make -j"${OT_BUILD_JOBS}"
 }
 
 main()
diff --git a/script/clang-format b/script/clang-format
index addb7fa47..01b9535e0 100755
--- a/script/clang-format
+++ b/script/clang-format
@@ -27,7 +27,7 @@
 #  POSSIBILITY OF SUCH DAMAGE.
 #
 
-CLANG_FORMAT_VERSION="clang-format version 10.0"
+CLANG_FORMAT_VERSION="clang-format version 9.0"
 
 die()
 {
@@ -39,18 +39,18 @@ die()
 # expand_aliases shell option is set using shopt.
 shopt -s expand_aliases
 
-if command -v clang-format-10 >/dev/null; then
-    alias clang-format=clang-format-10
+if command -v clang-format-9 >/dev/null; then
+    alias clang-format=clang-format-9
 elif command -v clang-format >/dev/null; then
     case "$(clang-format --version)" in
         "$CLANG_FORMAT_VERSION"*) ;;
 
         *)
-            die "$(clang-format --version); clang-format 10.0 required"
+            die "$(clang-format --version); clang-format 9.0 required"
             ;;
     esac
 else
-    die "clang-format 10.0 required"
+    die "clang-format 9.0 required"
 fi
 
 clang-format "$@" || die
diff --git a/script/clang-tidy b/script/clang-tidy
index 48f03db25..b2dfb4d4d 100755
--- a/script/clang-tidy
+++ b/script/clang-tidy
@@ -27,8 +27,8 @@
 #  POSSIBILITY OF SUCH DAMAGE.
 #
 
-CLANG_TIDY_VERSION="LLVM version 10.0"
-CLANG_APPLY_REPLACEMENTS_VERSION="clang-apply-replacements version 10.0"
+CLANG_TIDY_VERSION="LLVM version 9.0"
+CLANG_APPLY_REPLACEMENTS_VERSION="clang-apply-replacements version 9.0"
 
 die()
 {
@@ -36,9 +36,9 @@ die()
     exit 1
 }
 
-# Search for clang-tidy-10
-if command -v clang-tidy-10 >/dev/null; then
-    clang_tidy=$(command -v clang-tidy-10)
+# Search for clang-tidy-9
+if command -v clang-tidy-9 >/dev/null; then
+    clang_tidy=$(command -v clang-tidy-9)
 elif command -v clang-tidy >/dev/null; then
     clang_tidy=$(command -v clang-tidy)
     case "$($clang_tidy --version)" in
@@ -49,12 +49,12 @@ elif command -v clang-tidy >/dev/null; then
             ;;
     esac
 else
-    die "clang-tidy 10.0 required"
+    die "clang-tidy 9.0 required"
 fi
 
-# Search for clang-apply-replacements-10
-if command -v clang-apply-replacements-10 >/dev/null; then
-    clang_apply_replacements=$(command -v clang-apply-replacements-10)
+# Search for clang-apply-replacements-9
+if command -v clang-apply-replacements-9 >/dev/null; then
+    clang_apply_replacements=$(command -v clang-apply-replacements-9)
 elif command -v clang-apply-replacements >/dev/null; then
     clang_apply_replacements=$(command -v clang-apply-replacements)
     case "$($clang_apply_replacements --version)" in
@@ -65,20 +65,20 @@ elif command -v clang-apply-replacements >/dev/null; then
             ;;
     esac
 else
-    die "clang-apply-replacements 10.0 required"
+    die "clang-apply-replacements 9.0 required"
 fi
 
-# Search for run-clang-tidy-10.py
-if command -v run-clang-tidy-10.py >/dev/null; then
-    run_clang_tidy=$(command -v run-clang-tidy-10.py)
-elif command -v run-clang-tidy-10 >/dev/null; then
-    run_clang_tidy=$(command -v run-clang-tidy-10)
+# Search for run-clang-tidy-9.py
+if command -v run-clang-tidy-9.py >/dev/null; then
+    run_clang_tidy=$(command -v run-clang-tidy-9.py)
+elif command -v run-clang-tidy-9 >/dev/null; then
+    run_clang_tidy=$(command -v run-clang-tidy-9)
 elif command -v run-clang-tidy.py >/dev/null; then
     run_clang_tidy=$(command -v run-clang-tidy.py)
 elif command -v run-clang-tidy >/dev/null; then
     run_clang_tidy=$(command -v run-clang-tidy)
 else
-    die "run-clang-tidy.py 10.0 required"
+    die "run-clang-tidy.py 9.0 required"
 fi
 
 $run_clang_tidy -clang-tidy-binary "$clang_tidy" -clang-apply-replacements-binary "$clang_apply_replacements" "$@" || die