diff --git a/lefthook.yml b/lefthook.yml
index dad9fc43f70..41acb531d48 100644
--- a/lefthook.yml
+++ b/lefthook.yml
@@ -107,3 +107,7 @@ pre-push:
         - "go.mod"
         - "go.sum"
       run: tools/githooks/.bin/githooks test {push_files}
+
+    pr-size-guard:
+      tags: [guard, diff]
+      run: tools/githooks/.bin/githooks pr-size --fail-on-large
diff --git a/tools/githooks/README.md b/tools/githooks/README.md
index 69975e6c513..2db7661789c 100644
--- a/tools/githooks/README.md
+++ b/tools/githooks/README.md
@@ -4,13 +4,14 @@
 
 ## Features
 
-- **End-of-File Normalization:** Ensures eligible text and code files (`.go`, `.py`, `.md`, `.yaml`, `.json`, etc.) end with exactly one newline (`\n`), leaving empty files 0 bytes.
+- **End-of-File Normalization:** Ensures eligible text and code files end with exactly one newline.
 - **Trailing Whitespace Fixing:** Fixes erroneous trailing whitespace across eligible text and document files (preserving Markdown 2-space hard line breaks).
 - **Module & Package Resolution:** Automatically maps changed or staged Go files to their enclosing `go.mod` module roots and specific package paths (e.g. `./core/logger`).
 - **Targeted Code Generation:** Runs `go generate` only for packages where `.proto` or generate files changed, updates config schema docs and `go.md` when relevant files change, and regenerates mocks via `mockery` when affected packages are listed in a `.mockery.yaml`.
 - **Parallel Module Tidy:** Runs `go mod tidy` in parallel across all affected modules.
 - **Targeted Linting:** Runs `golangci-lint` only against the exact changed packages within affected modules instead of scanning whole modules or the entire repository.
 - **Targeted Unit Testing:** Discovers changed test packages and executes `tools/test` with `-short` directly on those packages (aligned with CI unit test scope).
+- **PR Diff Size Guard:** Calculates branch diff against the default branch merge-base, classifies size (`small`, `medium`, `large`), warns or fails on large diffs, and prompts developers to break PRs into smaller chunks.
 - **Dependency Changes:** Automatically runs on all packages (`./...`) if a module's `go.mod` or `go.sum` is modified.
 - **Lefthook Integration:** Seamlessly works with Lefthook staged/push file filters and `stage_fixed`.
 
@@ -115,6 +116,34 @@ go -C tools/githooks run . test core/logger/logger_test.go tools/ci-testshard/ma
 go -C tools/githooks run . test --short=false
 ```
 
+### `pr-size` (aliases: `big-pr-guard`, `pr-guard`, `diff-guard`, `diff-size`)
+
+Calculates the git diff of the current branch against the default branch (the PR diff), classifies it into size categories (`small`, `medium`, `large`), and prompts the developer with recommendations to split large PRs.
+
+```bash
+# Check PR diff size with default settings
+go -C tools/githooks run . pr-size
+
+# Check with custom thresholds
+go -C tools/githooks run . pr-size --small-limit=100 --medium-limit=300
+
+# Fail the command on large PRs (useful in CI or strict pre-push mode)
+go -C tools/githooks run . pr-size --fail-on-large
+
+# Bypass large PR failure via flag or env var
+go -C tools/githooks run . pr-size --fail-on-large --allow-large-pr
+ALLOW_LARGE_PR=true go -C tools/githooks run . pr-size --fail-on-large
+
+# Select calculation strategy: per-file-max (default), sum, max, or weighted
+go -C tools/githooks run . pr-size --strategy=per-file-max
+
+# Diff against a specific base branch or ref
+go -C tools/githooks run . pr-size --base=origin/develop
+
+# Automatically ignores generated files defined in .gitattributes (linguist-generated=true)
+go -C tools/githooks run . pr-size --ignore-generated=true
+```
+
 ## Running Tests & Benchmarks
 
 ```bash
diff --git a/tools/githooks/cmd/pr_size.go b/tools/githooks/cmd/pr_size.go
new file mode 100644
index 00000000000..5b57c967ff2
--- /dev/null
+++ b/tools/githooks/cmd/pr_size.go
@@ -0,0 +1,80 @@
+package cmd
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/spf13/cobra"
+
+	"github.com/smartcontractkit/chainlink/v2/tools/githooks/internal/prsize"
+)
+
+func newPRSizeCmd() *cobra.Command {
+	var (
+		strategy           string
+		smallLimit         int
+		mediumLimit        int
+		failOnLarge        bool
+		allowLargePR       bool
+		baseRef            string
+		ignoreLockfiles    bool
+		ignoreGenerated    bool
+		includeUncommitted bool
+	)
+
+	cmd := &cobra.Command{
+		Use:     "pr-size",
+		Aliases: []string{"big-pr-guard", "pr-guard", "diff-guard", "diff-size"},
+		Short:   "Check PR diff size against default branch and guard against oversized PRs",
+		Long: "Calculates the git diff of the current branch against the default branch (the PR diff), " +
+			"classifies the diff as small, medium, or large, and prompts developers with recommendations " +
+			"to split large PRs into smaller, reviewable chunks.",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			ctx := cmd.Context()
+
+			repoRoot, err := findRepoRoot(ctx)
+			if err != nil {
+				return fmt.Errorf("could not find repo root: %w", err)
+			}
+
+			cfg := prsize.Config{
+				RepoRoot:           repoRoot,
+				BaseRef:            baseRef,
+				Strategy:           prsize.Strategy(strategy),
+				SmallLimit:         smallLimit,
+				MediumLimit:        mediumLimit,
+				FailOnLarge:        failOnLarge,
+				AllowLargePR:       allowLargePR,
+				IgnoreLockfiles:    ignoreLockfiles,
+				IgnoreGenerated:    ignoreGenerated,
+				IncludeUncommitted: includeUncommitted,
+				Stdout:             cmd.OutOrStdout(),
+				Stderr:             cmd.ErrOrStderr(),
+				UI:                 uiForCmd(cmd),
+			}
+
+			if err := prsize.Run(ctx, cfg); err != nil {
+				if errors.Is(err, prsize.ErrLargePR) {
+					return SilentError{Err: err}
+				}
+				return err
+			}
+			return nil
+		},
+	}
+
+	cmd.Flags().StringVar(&strategy, "strategy", string(prsize.StrategyPerFileMax), "Diff size strategy: per-file-max, sum, max, weighted")
+	cmd.Flags().IntVar(&smallLimit, "small-limit", prsize.DefaultSmallLimit, "Maximum effective lines for small classification")
+	cmd.Flags().IntVar(&smallLimit, "max-small", prsize.DefaultSmallLimit, "Alias for --small-limit")
+	cmd.Flags().IntVar(&mediumLimit, "medium-limit", prsize.DefaultMediumLimit, "Maximum effective lines for medium classification")
+	cmd.Flags().IntVar(&mediumLimit, "max-medium", prsize.DefaultMediumLimit, "Alias for --medium-limit")
+	cmd.Flags().BoolVar(&failOnLarge, "fail-on-large", false, "Exit with error code if PR is classified as large")
+	cmd.Flags().BoolVar(&allowLargePR, "allow-large-pr", false, "Allow large PRs without failing (or set ALLOW_LARGE_PR=true)")
+	cmd.Flags().StringVar(&baseRef, "base", "", "Base branch or ref to diff against (default: origin default branch merge-base)")
+	cmd.Flags().StringVar(&baseRef, "rev", "", "Alias for --base")
+	cmd.Flags().BoolVar(&ignoreLockfiles, "ignore-lockfiles", true, "Ignore lockfiles (go.sum, package-lock.json, etc.) from diff line count")
+	cmd.Flags().BoolVar(&ignoreGenerated, "ignore-generated", true, "Ignore generated changes marked in .gitattributes (linguist-generated)")
+	cmd.Flags().BoolVar(&includeUncommitted, "include-uncommitted", false, "Include uncommitted staged and unstaged working-tree changes")
+
+	return cmd
+}
diff --git a/tools/githooks/cmd/pr_size_test.go b/tools/githooks/cmd/pr_size_test.go
new file mode 100644
index 00000000000..f5ae4502902
--- /dev/null
+++ b/tools/githooks/cmd/pr_size_test.go
@@ -0,0 +1,42 @@
+package cmd_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/smartcontractkit/chainlink/v2/tools/githooks/cmd"
+)
+
+//nolint:paralleltest // t.Chdir is process-global
+func TestPRSizeCmd_Run(t *testing.T) {
+	dir := t.TempDir()
+	git(t, dir, "init")
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "base.go"), []byte("package main\n"), 0o600))
+	git(t, dir, "add", ".")
+	git(t, dir, "commit", "-m", "init")
+	baseSHA := git(t, dir, "rev-parse", "HEAD")
+	git(t, dir, "update-ref", "refs/remotes/origin/develop", baseSHA)
+	git(t, dir, "symbolic-ref", "refs/remotes/origin/HEAD", "refs/remotes/origin/develop")
+
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "feature.go"), []byte("package main\n// new\n"), 0o600))
+	git(t, dir, "add", ".")
+	git(t, dir, "commit", "-m", "feature")
+
+	t.Chdir(dir)
+
+	root := cmd.NewRootCmd()
+	buf := new(bytes.Buffer)
+	root.SetOut(buf)
+	root.SetErr(buf)
+	root.SetArgs([]string{"pr-size", "--no-color"})
+
+	err := root.Execute()
+	require.NoError(t, err)
+	assert.Contains(t, buf.String(), "PR Diff Size")
+	assert.Contains(t, buf.String(), "Classification: [ SMALL ] [ OK ]")
+}
diff --git a/tools/githooks/cmd/root.go b/tools/githooks/cmd/root.go
index b889e49d37e..4f87469e825 100644
--- a/tools/githooks/cmd/root.go
+++ b/tools/githooks/cmd/root.go
@@ -30,6 +30,8 @@ func NewRootCmd() *cobra.Command {
 	rootCmd.AddCommand(newGenerateCmd())
 	rootCmd.AddCommand(newEOFCmd())
 	rootCmd.AddCommand(newWhitespaceCmd())
+	rootCmd.AddCommand(newPRSizeCmd())
+	rootCmd.AddCommand(newSampleOutputsCmd())
 	return rootCmd
 }
 
diff --git a/tools/githooks/cmd/sample_outputs.go b/tools/githooks/cmd/sample_outputs.go
new file mode 100644
index 00000000000..214e57d67e6
--- /dev/null
+++ b/tools/githooks/cmd/sample_outputs.go
@@ -0,0 +1,89 @@
+package cmd
+
+import (
+	"fmt"
+	"io"
+
+	"github.com/spf13/cobra"
+
+	"github.com/smartcontractkit/chainlink/v2/tools/githooks/internal/ui"
+)
+
+func newSampleOutputsCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:    "sample-outputs",
+		Short:  "Showcase all terminal UI output styles (hidden developer tool)",
+		Hidden: true,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			u := uiForCmd(cmd)
+			out := cmd.OutOrStdout()
+
+			renderSampleOutputs(out, u)
+			return nil
+		},
+	}
+
+	return cmd
+}
+
+func renderSampleOutputs(out io.Writer, u *ui.UI) {
+	// 1. Headers & Section
+	fmt.Fprintln(out, u.Bold("=== [githooks UI Output Showcase] ==="))
+	fmt.Fprintln(out)
+
+	// 2. Badges & Tags
+	fmt.Fprintln(out, u.Bold("--- Status Badges & Tags ---"))
+	fmt.Fprintf(out, "Success:     %s\n", u.BadgeSuccess("OK"))
+	fmt.Fprintf(out, "Warning:     %s\n", u.BadgeWarning("MEDIUM"))
+	fmt.Fprintf(out, "Danger:      %s\n", u.BadgeDanger("LARGE"))
+	fmt.Fprintf(out, "Info:        %s\n", u.BadgeInfo("SMALL"))
+	fmt.Fprintf(out, "Tags:        %s %s %s %s\n", u.BadgeTag("LINT"), u.BadgeTag("TEST"), u.BadgeTag("FIXED"), u.BadgeTag("CHECK FAIL"))
+	fmt.Fprintln(out)
+
+	// 3. PR Size Guard Samples
+	fmt.Fprintln(out, u.Bold("--- PR Size Guard (pr-size) ---"))
+	// Small
+	fmt.Fprintf(out, "PR Diff Size: %s -> Classification: %s %s\n",
+		u.FormatDiff(45, 45, 0, 2, "per-file-max"),
+		u.BadgeInfo("SMALL"),
+		u.BadgeSuccess("OK"),
+	)
+	// Medium
+	fmt.Fprintf(out, "PR Diff Size: %s -> Classification: %s %s\n",
+		u.FormatDiff(320, 280, 40, 6, "per-file-max"),
+		u.BadgeWarning("MEDIUM"),
+		u.BadgeSuccess("OK"),
+	)
+	fmt.Fprintf(out, "  %s\n", u.Dim("(Excluded 3 lock/ignored files)"))
+	fmt.Fprintln(out)
+
+	// Large PR Warning Card (Variation 1A)
+	title := fmt.Sprintf("%s %s", u.BadgeDangerPill("LARGE PR"), u.Danger("(1250 lines > 500 limit)"))
+	diffLine := fmt.Sprintf("%s      %s / %s in 14 files (per-file-max) • %s",
+		u.Bold("Diff:"),
+		u.Additions(1100),
+		u.Deletions(150),
+		u.Dim("Excluded: 2 lockfiles"),
+	)
+	bypassLine := fmt.Sprintf("%s    %s", u.Bold("Bypass:"), u.Dim("ALLOW_LARGE_PR=true git push"))
+	promptLine := u.Bold("AI Prompt:") + " Execute the skill @tools/githooks/skills/split-pr/SKILL.md to break feature-branch branch up."
+
+	fmt.Fprintln(out, u.CompactWarningCard(title, []string{diffLine, bypassLine, promptLine}))
+	fmt.Fprintln(out)
+
+	// 4. Runner Step Headers
+	fmt.Fprintln(out, u.Bold("--- Runners (lint & test) ---"))
+	fmt.Fprintln(out, u.RunnerHeader("LINT", "core", "4 packages"))
+	fmt.Fprintln(out, u.RunnerHeader("LINT", "deployment", "./environment"))
+	fmt.Fprintln(out, u.RunnerHeader("TEST", "plugins", "2 packages"))
+	fmt.Fprintln(out, u.RunnerHeader("TEST", ".", "./core/logger ./core/services"))
+	fmt.Fprintln(out)
+
+	// 5. Fixer Status Items
+	fmt.Fprintln(out, u.Bold("--- Fixers (whitespace & eof) ---"))
+	fmt.Fprintln(out, u.StatusItem("FIXED", "core/services/app.go", ""))
+	fmt.Fprintln(out, u.StatusItem("FIXED", "deployment/environment.go", ""))
+	fmt.Fprintln(out, u.StatusItem("CHECK FAIL", "core/config/config.go", "erroneous trailing whitespace"))
+	fmt.Fprintln(out, u.StatusItem("CHECK FAIL", "README.md", "missing trailing newline"))
+	fmt.Fprintln(out)
+}
diff --git a/tools/githooks/cmd/sample_outputs_test.go b/tools/githooks/cmd/sample_outputs_test.go
new file mode 100644
index 00000000000..79176f8062f
--- /dev/null
+++ b/tools/githooks/cmd/sample_outputs_test.go
@@ -0,0 +1,31 @@
+package cmd_test
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/smartcontractkit/chainlink/v2/tools/githooks/cmd"
+)
+
+func TestSampleOutputsCmd_ExecutesSuccessfully(t *testing.T) {
+	t.Parallel()
+
+	root := cmd.NewRootCmd()
+	buf := new(bytes.Buffer)
+	root.SetOut(buf)
+	root.SetErr(buf)
+	root.SetArgs([]string{"sample-outputs", "--no-color"})
+
+	err := root.Execute()
+	require.NoError(t, err)
+
+	out := buf.String()
+	assert.Contains(t, out, "PR Diff Size")
+	assert.Contains(t, out, "LARGE PR")
+	assert.Contains(t, out, "LINT")
+	assert.Contains(t, out, "TEST")
+	assert.Contains(t, out, "FIXED")
+}