Support difft install.

bin/install_tool

@@ -599,6 +599,43 @@ EOF
       go install github.com/fullstorydev/grpcui/cmd/grpcui@latest
     fi
     ;;
+  difftastic)
+    if command -v brew >/dev/null 2>&1; then
+      brew install difftastic
+    else
+      makedest_or_die
+      case "$(uname -s)" in
+        Linux)
+          target_os="unknown-linux-gnu"
+          ;;
+        Darwin)
+          target_os="apple-darwin"
+          ;;
+        *)
+          die "Unsupported OS: $(uname -s)"
+          ;;
+      esac
+      case "$(uname -m)" in
+        x86_64|amd64)
+          arch="x86_64"
+          ;;
+        aarch64|arm64)
+          arch="aarch64"
+          ;;
+        *)
+          die "Unsupported architecture: $(uname -m)"
+          ;;
+      esac
+      tar_url=$(get_latest_github_release_url "Wilfred/difftastic" "^difft-${arch}-${target_os}\\.tar\\.gz$")
+      if [ -z "${tar_url}" ]; then
+        die "Could not find difftastic release for ${arch}-${target_os}"
+      fi
+      download "${tar_url}" "${TMPDIR}/difftastic.tar.gz"
+      tar zxf "${TMPDIR}/difftastic.tar.gz" -C "${DESTDIR}"
+      chmod +x "${DESTDIR}/difft"
+      add_bin_symlink difft
+    fi
+    ;;
   *)
     echo "Unknown tool: ${TOOL}" >/dev/stderr
     list_tools

dotfiles/local/bin/git-diff-smart-pager

@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Detect installed binaries
+HAS_DELTA=$(command -v delta 2>/dev/null || true)
+HAS_DIFFT=$(command -v difft 2>/dev/null || true)
+
+# ==============================================================================
+# MODE 1: GIT_EXTERNAL_DIFF / diff.external
+# Git passes exactly 7 arguments:
+# $1: path $2: old-file $3: old-hex $4: old-mode $5: new-file $6: new-hex $7: new-mode
+# ==============================================================================
+if [ "$#" -eq 7 ]; then
+    DISPLAY_PATH="$1"
+    OLD_FILE="$2"
+    NEW_FILE="$5"
+
+    # 1. Difftastic installed -> Native AST structural file comparison
+    if [ -n "$HAS_DIFFT" ]; then
+        # Pass file metadata and display path to difft
+        exec difft --display=side-by-side-show-both "$OLD_FILE" "$NEW_FILE"
+
+    # 2. Delta installed (Difftastic missing) -> Standard diff piped into delta
+    elif [ -n "$HAS_DELTA" ]; then
+        diff -u -p --label "a/$DISPLAY_PATH" "$OLD_FILE" --label "b/$DISPLAY_PATH" "$NEW_FILE" | delta
+
+    # 3. Neither installed -> Standard unified diff
+    else
+        diff -u -p --label "a/$DISPLAY_PATH" "$OLD_FILE" --label "b/$DISPLAY_PATH" "$NEW_FILE" || true
+    fi
+
+# ==============================================================================
+# MODE 2: GIT_PAGER / core.pager
+# Invoked as a downstream filter/pager for stdout (stdin contains diff/log output)
+# ==============================================================================
+else
+    # 1. Delta installed -> Rich pager for git log, git show, git diff, etc.
+    if [ -n "$HAS_DELTA" ]; then
+        exec delta "$@"
+
+    # 2. Difftastic installed (Delta missing) -> Read unified diff from stdin
+    elif [ -n "$HAS_DIFFT" ]; then
+        exec difft "$@"
+
+    # 3. Neither installed -> Fall back to system pager
+    else
+        exec "${PAGER:-less}" ${LESS:--FRX} "$@"
+    fi
+fi