diff --git a/AGENTS.md b/AGENTS.md
index 3029fbb..7850599 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -53,3 +53,9 @@ zの関数は、関数名の階層とディレクトリ構成を対応させて
 - `lib/a/process.zsh` のテストは `test/lib/a/process_test.zsh`
 - `lib/a/b/process.zsh` のテストは `test/lib/a/b/process_test.zsh`
 - `mod/a/b/operator.zsh` のテストは `test/mod/a/b/operator_test.zsh`
+
+## 失敗処理
+
+`|| return 1`は、コピー失敗後の切替防止や依存関係の循環検出など、失敗時に後続の処理を止める必要がある箇所に置いてください。削除・置換では操作対象の検証と復旧処理を維持し、通常の取得・計算では同じ条件の重複検証を整理してください。
+
+引数取得と代入は、`z.arg.named ... && local ...`などの既存の書式に合わせてください。常に成功する値取得に`|| return 1`を追加する必要はありません。
diff --git a/lib/common/is/operator.zsh b/lib/common/is/operator.zsh
index 7584e46..5e061b7 100644
--- a/lib/common/is/operator.zsh
+++ b/lib/common/is/operator.zsh
@@ -26,7 +26,6 @@ z.is.truthy() {
   local value=$1
 
   z.guard; {
-    z.arg.is.empty $@ && return 1
     z.is.false $value && return 1
   }
 
@@ -68,7 +67,6 @@ z.is.falsy() {
   local value=$1
 
   z.guard; {
-    z.arg.is.empty $@ && return 0
     z.is.true $value && return 1
   }
 
diff --git a/lib/int/is/operator.zsh b/lib/int/is/operator.zsh
index 3b4dd8c..dd05430 100644
--- a/lib/int/is/operator.zsh
+++ b/lib/int/is/operator.zsh
@@ -42,8 +42,6 @@ z.int.is.eq() {
 z.int.is.zero() {
   local value=$1
 
-  z.int.is.match $value || return 1
-
   z.is.eq $value 0
 }
 
@@ -58,8 +56,6 @@ z.int.is.zero() {
 z.int.is.positive() {
   local value=$1
 
-  z.int.is.match $value || return 1
-
   z.int.is.gt $value 0
 }
 
@@ -74,8 +70,6 @@ z.int.is.positive() {
 z.int.is.negative() {
   local value=$1
 
-  z.int.is.match $value || return 1
-
   z.int.is.lt $value 0
 }
 
diff --git a/lib/update/archive/process.zsh b/lib/update/archive/process.zsh
index 0e469c1..9f17888 100644
--- a/lib/update/archive/process.zsh
+++ b/lib/update/archive/process.zsh
@@ -8,13 +8,12 @@
 # example:
 #  z.update.archive._download dir=/tmp/update branch=main
 z.update.archive._download() {
-  z.arg.named dir "$@" || { z.return; return 1; }
+  z.arg.named dir "$@"
   local dir=$REPLY
   z.dir.exists "$dir" || { z.return; return 1; }
 
   z.arg.named branch "$@" default=main
   local branch=$REPLY
-  z.is.null "$branch" && { z.return; return 1; }
 
   local file="$dir/source.tar.gz"
   z.file.exists "$file" && { z.return; return 1; }
@@ -26,8 +25,6 @@ z.update.archive._download() {
     return 1
   fi
 
-  z.file.exists "$file" || { z.return; return 1; }
-
   z.return "$file"
 }
 
@@ -41,15 +38,14 @@ z.update.archive._download() {
 # example:
 #  z.update.archive._extract file=/tmp/source.tar.gz dir=/tmp/update/source
 z.update.archive._extract() {
-  z.arg.named file "$@" || { z.return; return 1; }
+  z.arg.named file "$@"
 
   local file=$REPLY
 
-  z.arg.named dir "$@" || { z.return; return 1; }
+  z.arg.named dir "$@"
   local dir=$REPLY
   z.is.null "$dir" && { z.return; return 1; }
 
-  z.file.exists "$file" || { z.return; return 1; }
   z.dir.exists "$dir" && { z.return; return 1; }
 
   z.dir.make path="$dir" || return 1
diff --git a/lib/update/core/process.zsh b/lib/update/core/process.zsh
index f549555..6c6a4ab 100644
--- a/lib/update/core/process.zsh
+++ b/lib/update/core/process.zsh
@@ -8,24 +8,17 @@
 # example:
 #  z.update.core._copy source=/tmp/source dir=/tmp/update/next
 z.update.core._copy() {
-  z.arg.named source "$@" || return 1
+  z.arg.named source "$@"
   local source_dir=$REPLY
+  z.dir.exists "$source_dir" || return 1
 
-  z.arg.named dir "$@" || return 1
+  z.arg.named dir "$@"
   local dir=$REPLY
   z.is.null "$dir" && return 1
   z.dir.exists "$dir" && return 1
 
   local core_files=(main.zsh install.zsh uninstall.zsh version.zsh help.zsh README.md)
 
-  for directory in lib test; do
-    z.dir.exists "$source_dir/$directory" || return 1
-  done
-
-  for core_file in "${core_files[@]}"; do
-    z.file.exists "$source_dir/$core_file" || return 1
-  done
-
   z.dir.make path="$dir" || return 1
 
   for entry in lib test "${core_files[@]}"; do
diff --git a/lib/update/installation/is/operator.zsh b/lib/update/installation/is/operator.zsh
index 9e20f99..f7abcb2 100644
--- a/lib/update/installation/is/operator.zsh
+++ b/lib/update/installation/is/operator.zsh
@@ -7,7 +7,7 @@
 # example:
 #  z.update.installation.is._valid dir=~/.z
 z.update.installation.is._valid() {
-  z.arg.named dir "$@" || return 1
+  z.arg.named dir "$@"
   local dir=$REPLY
 
   z.is.null "$dir" && return 1
diff --git a/lib/update/installation/process.zsh b/lib/update/installation/process.zsh
index 36dcec4..5d667f0 100644
--- a/lib/update/installation/process.zsh
+++ b/lib/update/installation/process.zsh
@@ -9,13 +9,13 @@
 # example:
 #  z.update.installation._replace dir=~/.z prepared=/tmp/next backup=/tmp/previous
 z.update.installation._replace() {
-  z.arg.named dir "$@" || return 1
+  z.arg.named dir "$@"
   local dir=$REPLY
 
-  z.arg.named prepared "$@" || return 1
+  z.arg.named prepared "$@"
   local prepared=$REPLY
 
-  z.arg.named backup "$@" || return 1
+  z.arg.named backup "$@"
   local backup=$REPLY
 
   z.is.null "$backup" && return 1
@@ -45,7 +45,7 @@ z.update.installation._replace() {
 # example:
 #  z.update.installation._reload dir=~/.z previous=$PWD
 z.update.installation._reload() {
-  z.arg.named dir "$@" || return 1
+  z.arg.named dir "$@"
   local dir=$REPLY
   z.file.exists "$dir/main.zsh" || return 1
 
@@ -53,7 +53,7 @@ z.update.installation._reload() {
   local previous_dir=$REPLY
 
   if (( ${+functions[z.completion.cache._cancel]} )); then
-    z.completion.cache._cancel || return 1
+    z.completion.cache._cancel
   fi
 
   if z.is.eq "$previous_dir" "$dir" || z.str.start_with "$previous_dir" "$dir/"; then
diff --git a/lib/update/mod/analysis.zsh b/lib/update/mod/analysis.zsh
index bea922c..e49eeff 100644
--- a/lib/update/mod/analysis.zsh
+++ b/lib/update/mod/analysis.zsh
@@ -8,7 +8,7 @@
 # example:
 #  z.update.mod._resolve source=/tmp/source completion
 z.update.mod._resolve() {
-  z.arg.named source "$@" || { z.return; return 1; }
+  z.arg.named source "$@"
   local source_dir=$REPLY
   z.dir.exists "$source_dir" || { z.return; return 1; }
 
diff --git a/lib/update/mod/process.zsh b/lib/update/mod/process.zsh
index fd80967..d7695c4 100644
--- a/lib/update/mod/process.zsh
+++ b/lib/update/mod/process.zsh
@@ -10,13 +10,13 @@
 # example:
 #  z.update.mod._copy source=/tmp/source current=~/.z dir=/tmp/update/next job
 z.update.mod._copy() {
-  z.arg.named source "$@" || return 1
+  z.arg.named source "$@"
   local source_dir=$REPLY
 
-  z.arg.named current "$@" || return 1
+  z.arg.named current "$@"
   local current_dir=$REPLY
 
-  z.arg.named dir "$@" || return 1
+  z.arg.named dir "$@"
   local dir=$REPLY
 
   z.dir.exists "$source_dir" || return 1
@@ -29,10 +29,6 @@ z.update.mod._copy() {
   z.arg.named.shift dir "${(@)REPLY}"
   local selected=(${(@)REPLY})
 
-  for mod_name in "${selected[@]}"; do
-    z.dir.exists "$source_dir/mod/$mod_name" || return 1
-  done
-
   if z.dir.exists "$current_dir/mod"; then
     z.dir.make path="$dir/mod" || return 1
 
diff --git a/lib/update/process.zsh b/lib/update/process.zsh
index fcff6fb..ee40e2f 100644
--- a/lib/update/process.zsh
+++ b/lib/update/process.zsh
@@ -69,7 +69,7 @@ z.update() {
 # example:
 #  z.update._confirm dir=~/.z
 z.update._confirm() {
-  z.arg.named dir "$@" || return 1
+  z.arg.named dir "$@"
   local dir=$REPLY
   z.is.null "$dir" && return 1
 
diff --git a/test/lib/arg/process_test.zsh b/test/lib/arg/process_test.zsh
index f67bdcb..ddca315 100644
--- a/test/lib/arg/process_test.zsh
+++ b/test/lib/arg/process_test.zsh
@@ -75,6 +75,17 @@ z.t.describe "z.arg.named"; {
     }
   }
 
+  z.t.context "指定した名前の引数がない場合"; {
+    z.t.it "成功して空の値を返す"; {
+      z.arg.named name other=fuga
+      local result_code=$?
+      local result_reply=$REPLY
+
+      z.t.expect "$result_code" 0
+      z.t.expect "$result_reply" ""
+    }
+  }
+
   z.t.context "defaultが指定されていて、値がない場合"; {
     z.t.it "デフォルト値を返す"; {
       z.arg.named name name="" other=fuga default=default_value
diff --git a/test/lib/update/archive/process_test.zsh b/test/lib/update/archive/process_test.zsh
index 5d497ee..cad59be 100644
--- a/test/lib/update/archive/process_test.zsh
+++ b/test/lib/update/archive/process_test.zsh
@@ -165,8 +165,8 @@ z.t.describe "z.update.archive._extract"; {
   }
 
   z.t.context "file引数がない場合"; {
-    z.t.it "展開先を作らない"; {
-      z.update.archive._extract dir=/tmp/z_t/update_extract_absent
+    z.t.it "失敗して展開先を残さない"; {
+      z.update.archive._extract dir=/tmp/z_t/update_extract_absent 2>/dev/null
       local extract_status=$?
       local result=$REPLY
 
diff --git a/test/lib/update/core/process_test.zsh b/test/lib/update/core/process_test.zsh
index 108f006..94e0f6f 100644
--- a/test/lib/update/core/process_test.zsh
+++ b/test/lib/update/core/process_test.zsh
@@ -60,37 +60,31 @@ z.t.describe "z.update.core._copy"; {
   }
 
   z.t.context "配布元のtest directoryがない場合"; {
-    z.t.it "コピーを開始しない"; {
+    z.t.it "コピーの失敗を返す"; {
       local dir=/tmp/z_t/update_core/missing_directory
       z.dir.make path="$dir/source/lib"
 
-      z.update.core._copy source="$dir/source" dir="$dir/prepared"
+      z.update.core._copy source="$dir/source" dir="$dir/prepared" 2>/dev/null
       local copy_status=$?
 
       z.t.expect "$copy_status" 1
 
-      z.dir.not.exists "$dir/prepared"
-      z.t.expect.status.is.true
-
       z.dir.remove path=/tmp/z_t/update_core
     }
   }
 
   z.t.context "必須fileがない場合"; {
-    z.t.it "コピーを開始しない"; {
+    z.t.it "コピーの失敗を返す"; {
       local dir=/tmp/z_t/update_core/missing_file
       z.dir.make path="$dir/source/lib"
       z.dir.make path="$dir/source/test"
       z.file.write path="$dir/source/main.zsh" content=main
 
-      z.update.core._copy source="$dir/source" dir="$dir/prepared"
+      z.update.core._copy source="$dir/source" dir="$dir/prepared" 2>/dev/null
       local copy_status=$?
 
       z.t.expect "$copy_status" 1
 
-      z.dir.not.exists "$dir/prepared"
-      z.t.expect.status.is.true
-
       z.dir.remove path=/tmp/z_t/update_core
     }
   }
diff --git a/test/lib/update/installation/process_test.zsh b/test/lib/update/installation/process_test.zsh
index 4fee652..96976f0 100644
--- a/test/lib/update/installation/process_test.zsh
+++ b/test/lib/update/installation/process_test.zsh
@@ -254,20 +254,28 @@ z.t.describe "z.update.installation._reload"; {
     }
   }
 
-  z.t.context "cancelが失敗した場合"; {
-    z.t.it "sourceを実行しない"; {
-      local dir=/tmp/z_t/update_reload/cancel_failure
+  z.t.context "補完jobが回収済みの場合"; {
+    z.t.it "IDを破棄して本体の再読込を続ける"; {
+      local dir=/tmp/z_t/update_reload/collected_job
+      local z_completion_cache_job_id=collected-job
       z.dir.make path="$dir"
       z.file.write path="$dir/main.zsh" content="touch \"$dir/loaded\""
 
-      z.t.mock name="z.completion.cache._cancel" behavior="return 1"
+      z.t.mock name="z.job.status" behavior="return 1"
+      z.t.mock name="z.completion.enable" behavior="return 0"
 
-      z.update.installation._reload dir="$dir"
+      () {
+        local Z_ROOT=$z_root
+
+        z.update.installation._reload dir="$dir"
+      }
       local reload_status=$?
+      local remaining_id=$z_completion_cache_job_id
 
-      z.t.expect "$reload_status" 1
+      z.t.expect "$reload_status" 0
+      z.t.expect.is.null "$remaining_id"
 
-      z.file.not.exists "$dir/loaded"
+      z.file.exists "$dir/loaded"
       z.t.expect.status.is.true
 
       z.dir.remove path=/tmp/z_t/update_reload
diff --git a/test/lib/update/mod/process_test.zsh b/test/lib/update/mod/process_test.zsh
index abccbbf..98a6a1c 100644
--- a/test/lib/update/mod/process_test.zsh
+++ b/test/lib/update/mod/process_test.zsh
@@ -112,14 +112,14 @@ z.t.describe "z.update.mod._copy"; {
   }
 
   z.t.context "配布元に選択modがない場合"; {
-    z.t.it "既存と準備先を変更しない"; {
+    z.t.it "コピーの失敗を返して現行modを保持する"; {
       local dir=/tmp/z_t/update_mod_copy/missing
       z.dir.make path="$dir/current/mod/app"
       z.dir.make path="$dir/source"
       z.dir.make path="$dir/prepared"
       z.file.write path="$dir/current/mod/app/value" content=original
 
-      z.update.mod._copy source="$dir/source" current="$dir/current" dir="$dir/prepared" missing
+      z.update.mod._copy source="$dir/source" current="$dir/current" dir="$dir/prepared" missing 2>/dev/null
       local copy_status=$?
 
       z.t.expect "$copy_status" 1
@@ -127,9 +127,6 @@ z.t.describe "z.update.mod._copy"; {
       z.file.read path="$dir/current/mod/app/value"
       z.t.expect.reply original
 
-      z.dir.not.exists "$dir/prepared/mod"
-      z.t.expect.status.is.true
-
       z.dir.remove path=/tmp/z_t/update_mod_copy
     }
   }