diff --git a/.ci/scripts/tests/test_link_check_diff_selection.py b/.ci/scripts/tests/test_link_check_diff_selection.py
new file mode 100644
index 00000000000..c79b9f26236
--- /dev/null
+++ b/.ci/scripts/tests/test_link_check_diff_selection.py
@@ -0,0 +1,161 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+import os
+import shutil
+import subprocess
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parents[3]
+
+# Fixture content, not real references. The linters under test would otherwise find
+# their own bait here and check it against this repo.
+BAD = "[broken](sub/missing.md)\nhttps://example.invalid/dead\n"  # @lint-ignore
+GOOD = "[fine](sub/present.md)\nhttps://example.invalid/live\n"  # @lint-ignore
+
+OVERSIZED = "x" * (1024 * 1024 + 1)
+
+CURL_STUB = """#!/bin/sh
+for arg in "$@"; do url=$arg; done
+case "$url" in *dead*) echo 404 ;; *) echo 200 ;; esac
+"""
+
+
+@pytest.mark.skipif(sys.platform == "win32", reason="The scripts under test need bash")
+class TestLinkCheckDiffSelection(unittest.TestCase):
+    """A branch cut before recent base commits keeps the base branch's old lines. Those
+    are not the branch's to answer for, so the linters must diff from the merge base.
+
+    Each fixture file pins a different part of that: base_only.md the file selection,
+    both_sides.md the per-file diff, big.bin the file size range, feature_only.md that
+    the branch's own additions are still checked at all.
+    """
+
+    def setUp(self):
+        self.repo = Path(tempfile.mkdtemp())
+        self.addCleanup(shutil.rmtree, self.repo, ignore_errors=True)
+        self.git("init", "-q")
+        self.git("symbolic-ref", "HEAD", "refs/heads/main")
+        self.git("config", "user.email", "test@example.com")
+        self.git("config", "user.name", "test")
+
+        self.write("sub/present.md", "target\n")
+        self.write("base_only.md", BAD)
+        self.write("both_sides.md", BAD)
+        self.write("big.bin", OVERSIZED)
+        self.commit("initial")
+        self.git("branch", "feature")
+
+        self.write("base_only.md", GOOD)
+        self.write("both_sides.md", GOOD)
+        self.write("big.bin", "small\n")
+        self.commit("main repairs the links and shrinks the file")
+
+        self.git("checkout", "-q", "feature")
+        self.write("both_sides.md", BAD + GOOD)
+        self.write("feature_only.md", GOOD)
+        self.commit("feature edits one file and adds another")
+
+    def env(self, **extra):
+        # A developer's git config and any inherited GIT_DIR must not reach the fixture.
+        env = {k: v for k, v in os.environ.items() if not k.startswith("GIT_")}
+        env["GIT_CONFIG_GLOBAL"] = os.devnull
+        env["GIT_CONFIG_SYSTEM"] = os.devnull
+        env.update(extra)
+        return env
+
+    def git(self, *args):
+        subprocess.run(
+            ["git", *args],
+            cwd=self.repo,
+            check=True,
+            capture_output=True,
+            env=self.env(),
+        )
+
+    def write(self, relpath, text):
+        path = self.repo / relpath
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text(text)
+
+    def commit(self, message):
+        self.git("add", "-A")
+        self.git("commit", "-q", "-m", message)
+
+    def add_on_feature(self, relpath, text):
+        self.write(relpath, text)
+        self.commit(f"feature adds {relpath}")
+
+    def lint(self, script, **env):
+        return subprocess.run(
+            ["bash", str(REPO_ROOT / "scripts" / script), "main", "feature"],
+            cwd=self.repo,
+            capture_output=True,
+            text=True,
+            env=self.env(**env),
+        )
+
+    def assert_scoped_to_feature(self, result):
+        output = result.stdout + result.stderr
+        self.assertEqual(result.returncode, 0, output)
+        self.assertIn("feature_only.md", result.stdout)
+        self.assertIn("both_sides.md", result.stdout)
+        self.assertNotIn("base_only.md", output)
+
+    def curl_stub(self):
+        stub_dir = self.repo / "stub"
+        stub_dir.mkdir()
+        stub = stub_dir / "curl"
+        stub.write_text(CURL_STUB)
+        stub.chmod(0o755)
+        return {"PATH": f"{stub_dir}{os.pathsep}{os.environ['PATH']}"}
+
+    def test_lint_urls_ignores_lines_the_branch_only_lacks(self):
+        self.assert_scoped_to_feature(self.lint("lint_urls.sh", **self.curl_stub()))
+
+    def test_lint_xrefs_ignores_lines_the_branch_only_lacks(self):
+        self.assert_scoped_to_feature(self.lint("lint_xrefs.sh"))
+
+    def test_lint_file_size_ignores_files_the_branch_only_lacks(self):
+        result = self.lint("lint_file_size.sh")
+        output = result.stdout + result.stderr
+        self.assertEqual(result.returncode, 0, output)
+        self.assertIn("feature_only.md", result.stdout)
+        self.assertNotIn("big.bin", output)
+
+    def test_lint_urls_still_catches_a_url_the_branch_adds(self):
+        self.add_on_feature("feature_bad.md", BAD)
+        result = self.lint("lint_urls.sh", **self.curl_stub())
+        output = result.stdout + result.stderr
+        self.assertEqual(result.returncode, 1, output)
+        self.assertIn("example.invalid/dead", output)
+
+    def test_lint_xrefs_still_catches_a_reference_the_branch_adds(self):
+        self.add_on_feature("feature_bad.md", BAD)
+        result = self.lint("lint_xrefs.sh")
+        output = result.stdout + result.stderr
+        self.assertEqual(result.returncode, 1, output)
+        self.assertIn("sub/missing.md", output)
+
+    def test_lint_file_size_still_catches_a_file_the_branch_adds(self):
+        self.add_on_feature("feature_big.bin", OVERSIZED)
+        result = self.lint("lint_file_size.sh")
+        output = result.stdout + result.stderr
+        self.assertEqual(result.returncode, 1, output)
+        self.assertIn("feature_big.bin", output)
+
+    def test_lint_urls_survives_a_colorizing_git_config(self):
+        config = self.repo / "colorful.gitconfig"
+        config.write_text("[color]\n\tui = always\n")
+        result = self.lint(
+            "lint_urls.sh", GIT_CONFIG_GLOBAL=str(config), **self.curl_stub()
+        )
+        self.assert_scoped_to_feature(result)
diff --git a/.github/workflows/_link_check.yml b/.github/workflows/_link_check.yml
index a713de1c0dc..97f99b9f139 100644
--- a/.github/workflows/_link_check.yml
+++ b/.github/workflows/_link_check.yml
@@ -5,7 +5,9 @@ on:
         type: string
         required: true
       base_ref:
-        description: Commit to diff against. Empty, or all zeros, means check the whole tree.
+        description: >
+          Base branch commit. Pull requests lint from its merge base with ref, pushes
+          lint from it directly. Empty, or all zeros, means check the whole tree.
         type: string
         required: false
         default: ''
@@ -21,16 +23,31 @@ jobs:
       - uses: actions/checkout@v4
         with:
           ref: ${{ inputs.ref }}
+          # Only the merge base path needs history, and it costs ~25s. Quote the
+          # branches: bare 0 is falsy, so `&& 0 || 1` would always yield 1.
+          fetch-depth: ${{ github.event_name == 'pull_request' && '0' || '1' }}
       - name: Lint URLs
         env:
           BASE_REF: ${{ inputs.base_ref }}
           HEAD_REF: ${{ inputs.ref }}
+          FROM_MERGE_BASE: ${{ github.event_name == 'pull_request' }}
         run: |
           args=()
           # A push creating a tag or branch reports an all zero SHA no remote can serve.
           if [ -n "$BASE_REF" ] && [ "$BASE_REF" != "0000000000000000000000000000000000000000" ]; then
-            git fetch --no-tags --depth=1 origin "$BASE_REF"
-            args=("$BASE_REF" "$HEAD_REF")
+            if [ "$FROM_MERGE_BASE" = "true" ]; then
+              git fetch --no-tags origin "$BASE_REF"
+              # Where the branch left the base branch, not the base branch tip, or a
+              # branch cut before recent commits is blamed for every line it merely
+              # lacks. No merge base means no usable range, so leave args empty for a
+              # whole tree scan: the scripts treat a bad range as nothing to check.
+              if merge_base=$(git merge-base "$BASE_REF" "$HEAD_REF"); then
+                args=("$merge_base" "$HEAD_REF")
+              fi
+            else
+              git fetch --no-tags --depth=1 origin "$BASE_REF"
+              args=("$BASE_REF" "$HEAD_REF")
+            fi
           fi
           ./scripts/lint_urls.sh "${args[@]}" || {
             echo
@@ -50,16 +67,31 @@ jobs:
       - uses: actions/checkout@v4
         with:
           ref: ${{ inputs.ref }}
+          # Only the merge base path needs history, and it costs ~25s. Quote the
+          # branches: bare 0 is falsy, so `&& 0 || 1` would always yield 1.
+          fetch-depth: ${{ github.event_name == 'pull_request' && '0' || '1' }}
       - name: Lint cross-references
         env:
           BASE_REF: ${{ inputs.base_ref }}
           HEAD_REF: ${{ inputs.ref }}
+          FROM_MERGE_BASE: ${{ github.event_name == 'pull_request' }}
         run: |
           args=()
           # A push creating a tag or branch reports an all zero SHA no remote can serve.
           if [ -n "$BASE_REF" ] && [ "$BASE_REF" != "0000000000000000000000000000000000000000" ]; then
-            git fetch --no-tags --depth=1 origin "$BASE_REF"
-            args=("$BASE_REF" "$HEAD_REF")
+            if [ "$FROM_MERGE_BASE" = "true" ]; then
+              git fetch --no-tags origin "$BASE_REF"
+              # Where the branch left the base branch, not the base branch tip, or a
+              # branch cut before recent commits is blamed for every line it merely
+              # lacks. No merge base means no usable range, so leave args empty for a
+              # whole tree scan: the scripts treat a bad range as nothing to check.
+              if merge_base=$(git merge-base "$BASE_REF" "$HEAD_REF"); then
+                args=("$merge_base" "$HEAD_REF")
+              fi
+            else
+              git fetch --no-tags --depth=1 origin "$BASE_REF"
+              args=("$BASE_REF" "$HEAD_REF")
+            fi
           fi
           ./scripts/lint_xrefs.sh "${args[@]}" || {
             echo
@@ -79,16 +111,31 @@ jobs:
       - uses: actions/checkout@v4
         with:
           ref: ${{ inputs.ref }}
+          # Only the merge base path needs history, and it costs ~25s. Quote the
+          # branches: bare 0 is falsy, so `&& 0 || 1` would always yield 1.
+          fetch-depth: ${{ github.event_name == 'pull_request' && '0' || '1' }}
       - name: Lint file sizes
         env:
           BASE_REF: ${{ inputs.base_ref }}
           HEAD_REF: ${{ inputs.ref }}
+          FROM_MERGE_BASE: ${{ github.event_name == 'pull_request' }}
         run: |
           args=()
           # A push creating a tag or branch reports an all zero SHA no remote can serve.
           if [ -n "$BASE_REF" ] && [ "$BASE_REF" != "0000000000000000000000000000000000000000" ]; then
-            git fetch --no-tags --depth=1 origin "$BASE_REF"
-            args=("$BASE_REF" "$HEAD_REF")
+            if [ "$FROM_MERGE_BASE" = "true" ]; then
+              git fetch --no-tags origin "$BASE_REF"
+              # Where the branch left the base branch, not the base branch tip, or a
+              # branch cut before recent commits is blamed for every line it merely
+              # lacks. No merge base means no usable range, so leave args empty for a
+              # whole tree scan: the scripts treat a bad range as nothing to check.
+              if merge_base=$(git merge-base "$BASE_REF" "$HEAD_REF"); then
+                args=("$merge_base" "$HEAD_REF")
+              fi
+            else
+              git fetch --no-tags --depth=1 origin "$BASE_REF"
+              args=("$BASE_REF" "$HEAD_REF")
+            fi
           fi
           chmod +x ./scripts/lint_file_size.sh
           ./scripts/lint_file_size.sh "${args[@]}" || {
diff --git a/scripts/lint_file_size.sh b/scripts/lint_file_size.sh
index ec2256453fa..a579254c936 100644
--- a/scripts/lint_file_size.sh
+++ b/scripts/lint_file_size.sh
@@ -40,8 +40,8 @@ is_exception() {
 if [ $# -eq 2 ]; then
   base=$1
   head=$2
-  echo "Checking changed files between $base..$head"
-  files=$(git diff --name-only "$base..$head")
+  echo "Checking changed files between $base...$head"
+  files=$(git diff --no-color --name-only "$base...$head")
 else
   echo "Checking all files in repository"
   files=$(git ls-files)
diff --git a/scripts/lint_urls.sh b/scripts/lint_urls.sh
index 92b57171bc1..a49a52b011a 100755
--- a/scripts/lint_urls.sh
+++ b/scripts/lint_urls.sh
@@ -70,8 +70,10 @@ done < <(
     ':(exclude,glob)**/third_party/**'
   )
   if [ $# -eq 2 ]; then
-    for filename in $(git diff --name-only --unified=0 "$1..$2"); do
-      git diff --unified=0 "$1..$2" -- "$filename" "${excludes[@]}" \
+    # Three dots, not two. Against the base branch tip, a branch cut before recent
+    # commits looks like it is adding back every line those commits touched.
+    for filename in $(git diff --no-color --name-only --unified=0 "$1...$2"); do
+      git diff --no-color --unified=0 "$1...$2" -- "$filename" "${excludes[@]}" \
         | grep -E '^\+' \
         | grep -Ev '^\+\+\+' \
         | perl -nle 'print for m#'"$pattern"'#g' \
diff --git a/scripts/lint_xrefs.sh b/scripts/lint_xrefs.sh
index 54917b26d8e..4845da0d27f 100755
--- a/scripts/lint_xrefs.sh
+++ b/scripts/lint_xrefs.sh
@@ -35,8 +35,10 @@ done < <(
     ':(exclude,glob)**/third_party/**'
   )
   if [ $# -eq 2 ]; then
-    for filename in $(git diff --name-only --unified=0 "$1..$2"); do
-      git diff --unified=0 "$1..$2" -- "$filename" "${excludes[@]}" \
+    # Three dots, not two. Against the base branch tip, a branch cut before recent
+    # commits looks like it is adding back every line those commits touched.
+    for filename in $(git diff --no-color --name-only --unified=0 "$1...$2"); do
+      git diff --no-color --unified=0 "$1...$2" -- "$filename" "${excludes[@]}" \
         | grep -E '^\+' \
         | grep -Ev '^\+\+\+' \
         | perl -nle 'print for m#'"$pattern"'#g' \