diff --git a/home/.chezmoiscripts/darwin/run_once_01_install-homebrew.sh.tmpl b/home/.chezmoiscripts/darwin/run_once_01_install-homebrew.sh.tmpl
index 423a8cb..c2a7afb 100644
--- a/home/.chezmoiscripts/darwin/run_once_01_install-homebrew.sh.tmpl
+++ b/home/.chezmoiscripts/darwin/run_once_01_install-homebrew.sh.tmpl
@@ -2,5 +2,6 @@
 
 {{ if eq .chezmoi.os "darwin" }}
 {{ template "lib/common/log.sh" . }}
+{{ template "lib/common/install-prelude.sh" . }}
 {{ template "lib/install/homebrew.sh" . }}
 {{ end }}
diff --git a/home/.chezmoiscripts/darwin/run_onchange_02_install-packages.sh.tmpl b/home/.chezmoiscripts/darwin/run_onchange_02_install-packages.sh.tmpl
index fcd636c..fe2d052 100644
--- a/home/.chezmoiscripts/darwin/run_onchange_02_install-packages.sh.tmpl
+++ b/home/.chezmoiscripts/darwin/run_onchange_02_install-packages.sh.tmpl
@@ -4,6 +4,7 @@
 
 {{ if eq .chezmoi.os "darwin" }}
 {{ template "lib/common/log.sh" . }}
+{{ template "lib/common/install-prelude.sh" . }}
 HOMEBREW_BUNDLE_CONTENT=$(cat <<'BREWFILE'
 {{ range $groups }}# {{ . }}
 {{- $g := index $.packages.darwin.homebrew . }}
diff --git a/home/.chezmoiscripts/darwin/run_onchange_03_install-mise-tools.sh.tmpl b/home/.chezmoiscripts/darwin/run_onchange_03_install-mise-tools.sh.tmpl
index 63bebd4..1a169cf 100644
--- a/home/.chezmoiscripts/darwin/run_onchange_03_install-mise-tools.sh.tmpl
+++ b/home/.chezmoiscripts/darwin/run_onchange_03_install-mise-tools.sh.tmpl
@@ -4,5 +4,6 @@
 
 {{ if eq .chezmoi.os "darwin" }}
 {{ template "lib/common/log.sh" . }}
+{{ template "lib/common/install-prelude.sh" . }}
 {{ template "lib/install/mise.sh" . }}
 {{ end }}
diff --git a/home/.chezmoiscripts/darwin/run_onchange_03_install-uv-tools.sh.tmpl b/home/.chezmoiscripts/darwin/run_onchange_03_install-uv-tools.sh.tmpl
index e69c052..7760612 100644
--- a/home/.chezmoiscripts/darwin/run_onchange_03_install-uv-tools.sh.tmpl
+++ b/home/.chezmoiscripts/darwin/run_onchange_03_install-uv-tools.sh.tmpl
@@ -9,6 +9,7 @@
 
 {{ if and (eq .chezmoi.os "darwin") $hasTools }}
 {{ template "lib/common/log.sh" . }}
+{{ template "lib/common/install-prelude.sh" . }}
 UV_TOOLS=(
 {{ range $groups -}}
 {{ $tools := index $.uv.tools . -}}
diff --git a/home/.chezmoiscripts/darwin/run_onchange_04_install-vscode-extensions.sh.tmpl b/home/.chezmoiscripts/darwin/run_onchange_04_install-vscode-extensions.sh.tmpl
index 46ad3cd..92778b9 100644
--- a/home/.chezmoiscripts/darwin/run_onchange_04_install-vscode-extensions.sh.tmpl
+++ b/home/.chezmoiscripts/darwin/run_onchange_04_install-vscode-extensions.sh.tmpl
@@ -3,6 +3,7 @@
 
 {{ if eq .chezmoi.os "darwin" }}
 {{ template "lib/common/log.sh" . }}
+{{ template "lib/common/install-prelude.sh" . }}
 VSCODE_EXTENSIONS=(
   # shared
   {{ range .vscode.shared -}}
diff --git a/home/.chezmoiscripts/darwin/run_onchange_06_install-apm.sh.tmpl b/home/.chezmoiscripts/darwin/run_onchange_06_install-apm.sh.tmpl
index cd2ec78..6e6035c 100644
--- a/home/.chezmoiscripts/darwin/run_onchange_06_install-apm.sh.tmpl
+++ b/home/.chezmoiscripts/darwin/run_onchange_06_install-apm.sh.tmpl
@@ -11,5 +11,6 @@
 
 {{ if eq .chezmoi.os "darwin" }}
 {{ template "lib/common/log.sh" . }}
+{{ template "lib/common/install-prelude.sh" . }}
 {{ template "lib/install/apm.sh" . }}
 {{ end }}
diff --git a/home/.chezmoiscripts/darwin/run_onchange_07_fix-opencode-agent-tools.sh.tmpl b/home/.chezmoiscripts/darwin/run_onchange_07_fix-opencode-agent-tools.sh.tmpl
index d3a8a6d..8b7ee22 100644
--- a/home/.chezmoiscripts/darwin/run_onchange_07_fix-opencode-agent-tools.sh.tmpl
+++ b/home/.chezmoiscripts/darwin/run_onchange_07_fix-opencode-agent-tools.sh.tmpl
@@ -3,5 +3,6 @@
 
 {{ if eq .chezmoi.os "darwin" }}
 {{ template "lib/common/log.sh" . }}
+{{ template "lib/common/install-prelude.sh" . }}
 {{ template "lib/install/opencode-agent-tools.sh" . }}
 {{ end }}
diff --git a/home/.chezmoiscripts/darwin/run_onchange_08_install-graphify-skills.sh.tmpl b/home/.chezmoiscripts/darwin/run_onchange_08_install-graphify-skills.sh.tmpl
index 89a579e..1e38d42 100644
--- a/home/.chezmoiscripts/darwin/run_onchange_08_install-graphify-skills.sh.tmpl
+++ b/home/.chezmoiscripts/darwin/run_onchange_08_install-graphify-skills.sh.tmpl
@@ -10,6 +10,7 @@
 
 {{ if and (eq .chezmoi.os "darwin") (gt (len $platforms) 0) }}
 {{ template "lib/common/log.sh" . }}
+{{ template "lib/common/install-prelude.sh" . }}
 GRAPHIFY_PLATFORMS=(
 {{ range $platforms -}}
   "{{ . }}"
diff --git a/home/.chezmoitemplates/lib/common/install-prelude.sh b/home/.chezmoitemplates/lib/common/install-prelude.sh
new file mode 100644
index 0000000..692c3c0
--- /dev/null
+++ b/home/.chezmoitemplates/lib/common/install-prelude.sh
@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# @file lib/common/install-prelude.sh
+# @brief Shared prelude for install libraries under lib/install.
+# @description
+#   Provides the install-time trace toggle and the require_command guard so
+#   each install library keeps only its unique work. Depends on lib/common/log.sh
+#   for log_error. Sourceable from bats tests and injected into chezmoi run
+#   scripts via chezmoi template rendering, after lib/common/log.sh and before
+#   the install library.
+#
+#   Error-handling convention:
+#     Missing prerequisite tool: call require_command and return on failure.
+
+if [ "${DOTFILES_DEBUG:-}" ]; then
+  set -x
+fi
+
+#
+# @description Fail when a required command is absent from PATH.
+# @arg $1 Command name to look up.
+# @arg $2 Hint shown when the command is missing.
+# @exitcode 0 Command is available.
+# @exitcode 1 Command is missing.
+#
+function require_command() {
+  command -v "$1" >/dev/null 2>&1 || {
+    log_error "[$1] not found. $2"
+    return 1
+  }
+}
diff --git a/home/.chezmoitemplates/lib/install/apm.sh b/home/.chezmoitemplates/lib/install/apm.sh
index 694f0ea..fed3c3b 100644
--- a/home/.chezmoitemplates/lib/install/apm.sh
+++ b/home/.chezmoitemplates/lib/install/apm.sh
@@ -8,10 +8,6 @@
 
 set -Eeuo pipefail
 
-if [ "${DOTFILES_DEBUG:-}" ]; then
-  set -x
-fi
-
 #
 # @description Install APM dependencies from `${HOME}/.apm/apm.yml`.
 #
diff --git a/home/.chezmoitemplates/lib/install/graphify-skills.sh b/home/.chezmoitemplates/lib/install/graphify-skills.sh
index 6e1830c..9786f69 100644
--- a/home/.chezmoitemplates/lib/install/graphify-skills.sh
+++ b/home/.chezmoitemplates/lib/install/graphify-skills.sh
@@ -8,17 +8,6 @@
 
 set -Eeuo pipefail
 
-if [ "${DOTFILES_DEBUG:-}" ]; then
-  set -x
-fi
-
-#
-# @description Check if the Graphify CLI is installed.
-#
-function is_graphify_installed() {
-  command -v graphify >/dev/null 2>&1
-}
-
 #
 # @description Install Graphify skills for selected platforms.
 #
@@ -37,10 +26,7 @@ function graphify_skills_install_main() {
     return 0
   fi
 
-  if ! is_graphify_installed; then
-    log_error "[graphify] graphify not found. Ensure run_onchange_03_install-uv-tools ran successfully."
-    return 1
-  fi
+  require_command graphify "Ensure run_onchange_03_install-uv-tools ran successfully." || return 1
 
   log_info "[graphify] Installing Graphify agent skills..."
 
diff --git a/home/.chezmoitemplates/lib/install/homebrew-bundle.sh b/home/.chezmoitemplates/lib/install/homebrew-bundle.sh
index 5be19fa..bfbf39c 100644
--- a/home/.chezmoitemplates/lib/install/homebrew-bundle.sh
+++ b/home/.chezmoitemplates/lib/install/homebrew-bundle.sh
@@ -8,25 +8,11 @@
 
 set -Eeuo pipefail
 
-if [ "${DOTFILES_DEBUG:-}" ]; then
-  set -x
-fi
-
-#
-# @description Check if Homebrew is installed.
-#
-function is_homebrew_installed() {
-  command -v brew >/dev/null 2>&1
-}
-
 #
 # @description Install packages from rendered Brewfile content.
 #
 function homebrew_bundle_main() {
-  if ! is_homebrew_installed; then
-    log_error "[homebrew] Homebrew not found. Ensure run_once_01_install-homebrew ran successfully."
-    return 1
-  fi
+  require_command brew "Ensure run_once_01_install-homebrew ran successfully." || return 1
 
   log_info "[homebrew] Updating Homebrew metadata..."
   brew update
diff --git a/home/.chezmoitemplates/lib/install/homebrew.sh b/home/.chezmoitemplates/lib/install/homebrew.sh
index cb8cfa6..184180d 100644
--- a/home/.chezmoitemplates/lib/install/homebrew.sh
+++ b/home/.chezmoitemplates/lib/install/homebrew.sh
@@ -7,10 +7,6 @@
 #   rendering.
 set -Eeuo pipefail
 
-if [ "${DOTFILES_DEBUG:-}" ]; then
-  set -x
-fi
-
 #
 # @description Check if Homebrew is installed.
 #
diff --git a/home/.chezmoitemplates/lib/install/mise.sh b/home/.chezmoitemplates/lib/install/mise.sh
index 121d900..e6e9618 100644
--- a/home/.chezmoitemplates/lib/install/mise.sh
+++ b/home/.chezmoitemplates/lib/install/mise.sh
@@ -8,17 +8,6 @@
 
 set -Eeuo pipefail
 
-if [ "${DOTFILES_DEBUG:-}" ]; then
-  set -x
-fi
-
-#
-# @description Check if mise is installed.
-#
-function is_mise_installed() {
-  command -v mise >/dev/null 2>&1
-}
-
 #
 # @description Install tools declared in mise config.
 #
@@ -32,10 +21,7 @@ function mise_install_main() {
 # @description Run the mise install flow.
 #
 function main() {
-  if ! is_mise_installed; then
-    log_error "[mise] mise not found. Ensure run_onchange_02_install-packages ran successfully."
-    return 1
-  fi
+  require_command mise "Ensure run_onchange_02_install-packages ran successfully." || return 1
 
   mise_install_main
 }
diff --git a/home/.chezmoitemplates/lib/install/opencode-agent-tools.sh b/home/.chezmoitemplates/lib/install/opencode-agent-tools.sh
index 9be23a3..1a52cfa 100644
--- a/home/.chezmoitemplates/lib/install/opencode-agent-tools.sh
+++ b/home/.chezmoitemplates/lib/install/opencode-agent-tools.sh
@@ -14,10 +14,6 @@
 
 set -Eeuo pipefail
 
-if [ "${DOTFILES_DEBUG:-}" ]; then
-  set -x
-fi
-
 #
 # @description Rewrite one markdown file's frontmatter `tools:` array
 #   (if any) into opencode's expected map form. Writes back in place.
diff --git a/home/.chezmoitemplates/lib/install/uv-tools.sh b/home/.chezmoitemplates/lib/install/uv-tools.sh
index d59f1f7..262f482 100644
--- a/home/.chezmoitemplates/lib/install/uv-tools.sh
+++ b/home/.chezmoitemplates/lib/install/uv-tools.sh
@@ -8,17 +8,6 @@
 
 set -Eeuo pipefail
 
-if [ "${DOTFILES_DEBUG:-}" ]; then
-  set -x
-fi
-
-#
-# @description Check if uv is installed.
-#
-function is_uv_installed() {
-  command -v uv >/dev/null 2>&1
-}
-
 #
 # @description Install tools declared in UV_TOOLS.
 #
@@ -38,10 +27,7 @@ function uv_tools_install_main() {
 # @description Run the uv tool install flow.
 #
 function main() {
-  if ! is_uv_installed; then
-    log_error "[uv] uv not found. Ensure run_onchange_03_install-mise-tools ran successfully."
-    return 1
-  fi
+  require_command uv "Ensure run_onchange_03_install-mise-tools ran successfully." || return 1
 
   uv_tools_install_main
 }
diff --git a/home/.chezmoitemplates/lib/install/vscode.sh b/home/.chezmoitemplates/lib/install/vscode.sh
index de5d29c..582db2c 100644
--- a/home/.chezmoitemplates/lib/install/vscode.sh
+++ b/home/.chezmoitemplates/lib/install/vscode.sh
@@ -8,10 +8,6 @@
 
 set -Eeuo pipefail
 
-if [ "${DOTFILES_DEBUG:-}" ]; then
-  set -x
-fi
-
 #
 # @description Check if the VS Code CLI is installed.
 #
diff --git a/tests/template/darwin-install-scripts.bats b/tests/template/darwin-install-scripts.bats
index c1e2859..7a676ee 100644
--- a/tests/template/darwin-install-scripts.bats
+++ b/tests/template/darwin-install-scripts.bats
@@ -43,6 +43,18 @@ render_template_with_data() {
   assert_file_contains "$GRAPHIFY_TEMPLATE" '{{ template "lib/install/graphify-skills.sh" . }}'
 }
 
+@test "darwin install script templates inject the install prelude" {
+  local prelude='{{ template "lib/common/install-prelude.sh" . }}'
+  assert_file_contains "$DOTFILES_ROOT/home/.chezmoiscripts/darwin/run_once_01_install-homebrew.sh.tmpl" "$prelude"
+  assert_file_contains "$DOTFILES_ROOT/home/.chezmoiscripts/darwin/run_onchange_02_install-packages.sh.tmpl" "$prelude"
+  assert_file_contains "$DOTFILES_ROOT/home/.chezmoiscripts/darwin/run_onchange_03_install-mise-tools.sh.tmpl" "$prelude"
+  assert_file_contains "$DOTFILES_ROOT/home/.chezmoiscripts/darwin/run_onchange_03_install-uv-tools.sh.tmpl" "$prelude"
+  assert_file_contains "$DOTFILES_ROOT/home/.chezmoiscripts/darwin/run_onchange_04_install-vscode-extensions.sh.tmpl" "$prelude"
+  assert_file_contains "$DOTFILES_ROOT/home/.chezmoiscripts/darwin/run_onchange_06_install-apm.sh.tmpl" "$prelude"
+  assert_file_contains "$DOTFILES_ROOT/home/.chezmoiscripts/darwin/run_onchange_07_fix-opencode-agent-tools.sh.tmpl" "$prelude"
+  assert_file_contains "$GRAPHIFY_TEMPLATE" "$prelude"
+}
+
 @test "rendered darwin install scripts are syntactically valid bash" {
   for template in "$DOTFILES_ROOT"/home/.chezmoiscripts/darwin/*.tmpl; do
     rendered="$(render_template "$template")"
diff --git a/tests/unit/lib/install/graphify-skills.bats b/tests/unit/lib/install/graphify-skills.bats
index 466fdef..57877ec 100644
--- a/tests/unit/lib/install/graphify-skills.bats
+++ b/tests/unit/lib/install/graphify-skills.bats
@@ -5,6 +5,7 @@
 load '../../../test_helpers/load.bash'
 
 LOG_LIB="$DOTFILES_ROOT/home/.chezmoitemplates/lib/common/log.sh"
+PRELUDE="$DOTFILES_ROOT/home/.chezmoitemplates/lib/common/install-prelude.sh"
 LIB="$DOTFILES_ROOT/home/.chezmoitemplates/lib/install/graphify-skills.sh"
 
 setup() {
@@ -21,7 +22,7 @@ GRAPHIFY
 }
 
 @test "main: installs each Graphify platform" {
-  run bash -c "source '$LOG_LIB' && source '$LIB' && GRAPHIFY_PLATFORMS=('agents' 'claude') && main"
+  run bash -c "source '$LOG_LIB' && source '$PRELUDE' && source '$LIB' && GRAPHIFY_PLATFORMS=('agents' 'claude') && main"
 
   assert_success
   assert_line "[graphify] Installing Graphify agent skills..."
@@ -30,7 +31,7 @@ GRAPHIFY
 }
 
 @test "main: skips empty platform entries" {
-  run bash -c "source '$LOG_LIB' && source '$LIB' && GRAPHIFY_PLATFORMS=('agents' '' 'claude') && main"
+  run bash -c "source '$LOG_LIB' && source '$PRELUDE' && source '$LIB' && GRAPHIFY_PLATFORMS=('agents' '' 'claude') && main"
 
   assert_success
   [ "$(<"$GRAPHIFY_ARGS_FILE")" = $'install --platform agents\ninstall --platform claude' ]
@@ -39,7 +40,7 @@ GRAPHIFY
 @test "main: exits cleanly with no Graphify platforms" {
   rm -f "$BATS_TEST_TMPDIR/bin/graphify"
 
-  run bash -c "source '$LOG_LIB' && source '$LIB' && GRAPHIFY_PLATFORMS=() && main"
+  run bash -c "source '$LOG_LIB' && source '$PRELUDE' && source '$LIB' && GRAPHIFY_PLATFORMS=() && main"
 
   assert_success
   assert_line "[graphify] No Graphify platforms to install."
@@ -49,8 +50,8 @@ GRAPHIFY
 @test "main: fails when graphify is missing" {
   rm -f "$BATS_TEST_TMPDIR/bin/graphify"
 
-  run bash -c "source '$LOG_LIB' && source '$LIB' && GRAPHIFY_PLATFORMS=('agents') && main"
+  run bash -c "source '$LOG_LIB' && source '$PRELUDE' && source '$LIB' && GRAPHIFY_PLATFORMS=('agents') && main"
 
   assert_failure 1
-  assert_line "error: [graphify] graphify not found. Ensure run_onchange_03_install-uv-tools ran successfully."
+  assert_line "error: [graphify] not found. Ensure run_onchange_03_install-uv-tools ran successfully."
 }
diff --git a/tests/unit/lib/install/homebrew-bundle.bats b/tests/unit/lib/install/homebrew-bundle.bats
index 396d249..4023b74 100644
--- a/tests/unit/lib/install/homebrew-bundle.bats
+++ b/tests/unit/lib/install/homebrew-bundle.bats
@@ -5,6 +5,7 @@
 load '../../../test_helpers/load.bash'
 
 LOG_LIB="$DOTFILES_ROOT/home/.chezmoitemplates/lib/common/log.sh"
+PRELUDE="$DOTFILES_ROOT/home/.chezmoitemplates/lib/common/install-prelude.sh"
 LIB="$DOTFILES_ROOT/home/.chezmoitemplates/lib/install/homebrew-bundle.sh"
 
 setup() {
@@ -35,7 +36,7 @@ BREW
 @test "homebrew_bundle_main: runs brew bundle with rendered Brewfile content" {
   export HOMEBREW_BUNDLE_CONTENT=$'tap "homebrew/core"\nbrew "git"'
 
-  run bash -c "source '$LOG_LIB' && source '$LIB' && homebrew_bundle_main"
+  run bash -c "source '$LOG_LIB' && source '$PRELUDE' && source '$LIB' && homebrew_bundle_main"
 
   assert_success
   assert_line "[homebrew] Updating Homebrew metadata..."
@@ -49,7 +50,7 @@ BREW
   export HOMEBREW_BUNDLE_CONTENT='brew "git"'
   export BREW_BUNDLE_EXIT_CODE=7
 
-  run bash -c "source '$LOG_LIB' && source '$LIB' && homebrew_bundle_main"
+  run bash -c "source '$LOG_LIB' && source '$PRELUDE' && source '$LIB' && homebrew_bundle_main"
 
   assert_failure 7
   assert_line "[homebrew] Running brew bundle..."