diff --git a/docs/install.md b/docs/install.md
index aac6c95e69fe..6a7494797d8c 100644
--- a/docs/install.md
+++ b/docs/install.md
@@ -32,6 +32,9 @@ cargo install --locked cargo-nextest
 # Install a local codex binary with reuse of previous build artifacts.
 ./install.sh
 
+# If your main volume is tight on space, keep Cargo build artifacts on another disk.
+CARGO_TARGET_DIR=/path/on/larger/disk/codex-target ./install.sh
+
 # Or build Codex manually from the Rust workspace.
 cd codex-rs
 cargo build
diff --git a/install.sh b/install.sh
index 435295102a4a..b0581ac3f2b5 100755
--- a/install.sh
+++ b/install.sh
@@ -18,6 +18,7 @@ Environment knobs:
   CODEX_SOURCE_BIN          Prebuilt binary path to install (skips build unless CODEX_REBUILD=always)
   CODEX_INSTALL_LOCK_DIR    Lock dir to prevent concurrent installers
   CARGO_TARGET_DIR          Build cache/output directory (default: codex-rs/target)
+                           Set this to a path on another disk when local space is tight.
 EOF
 }
 
@@ -68,6 +69,23 @@ case "$target_dir" in
 esac
 build_stamp_file="$target_dir/release/.codex-install-build-stamp"
 
+existing_dir_for_path() {
+    path=$1
+    while [ ! -e "$path" ]; do
+        parent=$(dirname -- "$path")
+        if [ "$parent" = "$path" ]; then
+            break
+        fi
+        path=$parent
+    done
+
+    if [ -d "$path" ]; then
+        printf '%s\n' "$path"
+    else
+        dirname -- "$path"
+    fi
+}
+
 collect_source_files() {
     find "$ROOT_DIR/codex-rs" \
         -path "$ROOT_DIR/codex-rs/target" -prune -o \
@@ -271,16 +289,21 @@ fi
 
 if [ "$should_build" -eq 1 ]; then
     required_kb=${CODEX_MIN_FREE_KB:-3145728}
-    available_kb=$(df -Pk "$ROOT_DIR" | awk 'NR==2 {print $4}')
+    space_check_dir=$(existing_dir_for_path "$target_dir")
+    available_kb=$(df -Pk "$space_check_dir" | awk 'NR==2 {print $4}')
     if [ "$available_kb" -lt "$required_kb" ]; then
-        echo "Low free disk space detected (${available_kb} KB). Running cargo clean..." >&2
-        cargo clean --manifest-path "$ROOT_DIR/codex-rs/Cargo.toml" >/dev/null 2>&1 || true
-        available_kb=$(df -Pk "$ROOT_DIR" | awk 'NR==2 {print $4}')
+        echo "Low free disk space detected for build target (${available_kb} KB at $space_check_dir)." >&2
+        echo "Running cargo clean for CARGO_TARGET_DIR=$target_dir..." >&2
+        CARGO_TARGET_DIR="$target_dir" cargo clean --manifest-path "$ROOT_DIR/codex-rs/Cargo.toml" >/dev/null 2>&1 || true
+        available_kb=$(df -Pk "$space_check_dir" | awk 'NR==2 {print $4}')
     fi
     if [ "$available_kb" -lt "$required_kb" ]; then
         echo "error: not enough free disk space to build codex release" >&2
         echo "available: ${available_kb} KB, required: ${required_kb} KB" >&2
-        echo "free space and retry (or lower threshold with CODEX_MIN_FREE_KB)." >&2
+        echo "target dir: $target_dir" >&2
+        echo "free space and retry, or point CARGO_TARGET_DIR at a larger disk, for example:" >&2
+        echo "  CARGO_TARGET_DIR=/path/on/larger/disk/codex-target ./install.sh" >&2
+        echo "You can also lower the threshold with CODEX_MIN_FREE_KB." >&2
         exit 1
     fi
 
@@ -297,9 +320,9 @@ if [ "$should_build" -eq 1 ]; then
     fi
 
     if [ -n "$build_reason" ]; then
-        echo "Building codex release binary (mode=$build_mode jobs=$build_jobs incremental=$resolved_build_incremental reason=$build_reason)..."
+        echo "Building codex release binary (mode=$build_mode jobs=$build_jobs incremental=$resolved_build_incremental target=$target_dir reason=$build_reason)..."
     else
-        echo "Building codex release binary (mode=$build_mode jobs=$build_jobs incremental=$resolved_build_incremental)..."
+        echo "Building codex release binary (mode=$build_mode jobs=$build_jobs incremental=$resolved_build_incremental target=$target_dir)..."
     fi
     if [ -n "$profile_overrides" ]; then
         env $profile_overrides CARGO_PROFILE_RELEASE_DEBUG=0 CARGO_INCREMENTAL="$resolved_build_incremental" \
diff --git a/scripts/install/test-source-build-install.sh b/scripts/install/test-source-build-install.sh
index 42f1dbf50b8c..d45b9597597a 100755
--- a/scripts/install/test-source-build-install.sh
+++ b/scripts/install/test-source-build-install.sh
@@ -23,6 +23,13 @@ assert_file_exists() {
     fi
 }
 
+assert_contains() {
+    haystack=$1
+    needle=$2
+    context=$3
+    printf '%s' "$haystack" | grep -F -- "$needle" >/dev/null || fail "$context: missing '$needle'"
+}
+
 make_fake_workspace() {
     workspace=$1
 
@@ -41,11 +48,15 @@ EOF
 fn main() {}
 EOF
 
-    cat >"$workspace/fake-bin/cargo" <<'EOF'
+cat >"$workspace/fake-bin/cargo" <<'EOF'
 #!/bin/sh
 set -eu
 
 test_log=${TEST_LOG:?}
+command=${1:-}
+if [ -n "$command" ]; then
+    shift
+fi
 manifest_path=""
 while [ "$#" -gt 0 ]; do
     case "$1" in
@@ -66,6 +77,14 @@ fi
 
 manifest_dir=$(dirname -- "$manifest_path")
 target_dir=${CARGO_TARGET_DIR:-"$manifest_dir/target"}
+if [ "$command" = "clean" ]; then
+    rm -rf "$target_dir"
+    exit 0
+fi
+if [ "$command" != "build" ]; then
+    echo "unexpected cargo invocation: $command" >&2
+    exit 1
+fi
 mkdir -p "$target_dir/release"
 printf 'incremental=%s\n' "${CARGO_INCREMENTAL:-unset}" >>"$test_log"
 cat >"$target_dir/release/codex" <<'BIN'
@@ -76,6 +95,34 @@ chmod +x "$target_dir/release/codex"
 EOF
     chmod +x "$workspace/fake-bin/cargo"
 
+    cat >"$workspace/fake-bin/df" <<'EOF'
+#!/bin/sh
+set -eu
+
+path=""
+for arg in "$@"; do
+    path=$arg
+done
+
+default_kb=${TEST_DF_DEFAULT_KB:-10485760}
+low_path=${TEST_DF_LOW_PATH:-}
+low_kb=${TEST_DF_LOW_KB:-1}
+high_path=${TEST_DF_HIGH_PATH:-}
+high_kb=${TEST_DF_HIGH_KB:-10485760}
+
+available_kb=$default_kb
+if [ -n "$high_path" ] && [ "$path" = "$high_path" ]; then
+    available_kb=$high_kb
+fi
+if [ -n "$low_path" ] && [ "$path" = "$low_path" ]; then
+    available_kb=$low_kb
+fi
+
+printf 'Filesystem 1024-blocks Used Available Capacity Mounted on\n'
+printf 'testfs 9999999 0 %s 0%% %s\n' "$available_kb" "$path"
+EOF
+    chmod +x "$workspace/fake-bin/df"
+
     cat >"$workspace/fake-bin/git" <<'EOF'
 #!/bin/sh
 set -eu
@@ -123,6 +170,7 @@ assert_help_mentions_new_knobs() {
     help_output=$(sh "$ROOT_DIR/install.sh" --help)
     printf '%s\n' "$help_output" | grep "CODEX_BUILD_INCREMENTAL" >/dev/null || fail "--help should mention CODEX_BUILD_INCREMENTAL"
     printf '%s\n' "$help_output" | grep "CARGO_TARGET_DIR" >/dev/null || fail "--help should mention CARGO_TARGET_DIR"
+    printf '%s\n' "$help_output" | grep "another disk" >/dev/null || fail "--help should explain when to move CARGO_TARGET_DIR"
 }
 
 test_skips_rebuild_when_source_content_is_unchanged() {
@@ -174,6 +222,64 @@ test_honors_custom_target_dir() {
     trap - EXIT INT TERM
 }
 
+test_uses_custom_target_dir_filesystem_for_space_check() {
+    workspace=$(mktemp -d)
+    test_log="$workspace/cargo.log"
+    target_dir="$workspace/shared-target"
+    trap 'rm -rf "$workspace"' EXIT INT TERM
+
+    make_fake_workspace "$workspace"
+    mkdir -p "$target_dir"
+
+    run_install "$workspace" "$test_log" \
+        CARGO_TARGET_DIR="$target_dir" \
+        CODEX_MIN_FREE_KB=100 \
+        TEST_DF_DEFAULT_KB=10 \
+        TEST_DF_LOW_PATH="$workspace" \
+        TEST_DF_LOW_KB=10 \
+        TEST_DF_HIGH_PATH="$target_dir" \
+        TEST_DF_HIGH_KB=1000
+    assert_eq "1" "$(count_builds "$test_log")" "custom target dir filesystem should satisfy the space check"
+    assert_file_exists "$target_dir/release/codex"
+
+    rm -rf "$workspace"
+    trap - EXIT INT TERM
+}
+
+test_low_space_error_mentions_external_target_dir() {
+    workspace=$(mktemp -d)
+    test_log="$workspace/cargo.log"
+    trap 'rm -rf "$workspace"' EXIT INT TERM
+
+    make_fake_workspace "$workspace"
+
+    set +e
+    output=$(
+        cd "$workspace" && env PATH="$workspace/fake-bin:$PATH" \
+            TEST_LOG="$test_log" \
+            CODEX_INSTALL_DIR="$workspace/bin" \
+            CODEX_MIN_FREE_KB=100 \
+            TEST_DF_DEFAULT_KB=10 \
+            TEST_DF_LOW_PATH="$workspace/codex-rs/target" \
+            TEST_DF_LOW_KB=10 \
+            ./install.sh 2>&1
+    )
+    status=$?
+    set -e
+
+    if [ "$status" -eq 0 ]; then
+        fail "install should fail when the build target filesystem remains full"
+    fi
+
+    assert_contains "$output" "target dir: $workspace/codex-rs/target" "low-space error should show the target dir"
+    assert_contains "$output" "CARGO_TARGET_DIR=/path/on/larger/disk/codex-target ./install.sh" "low-space error should suggest using another disk"
+
+    rm -rf "$workspace"
+    trap - EXIT INT TERM
+}
+
 assert_help_mentions_new_knobs
 test_skips_rebuild_when_source_content_is_unchanged
 test_honors_custom_target_dir
+test_uses_custom_target_dir_filesystem_for_space_check
+test_low_space_error_mentions_external_target_dir