diff --git a/.github/dependabot.yml b/.github/dependabot.yml
index ef1432591..4bf8f8782 100644
--- a/.github/dependabot.yml
+++ b/.github/dependabot.yml
@@ -24,6 +24,10 @@ updates:
       interval: daily
     cooldown:
       default-days: 7
+    ignore:
+      # astyle is deliberately pinned (see tools/astyle/requirements.txt): newer versions
+      # reformat the code, and the pinned version defines the formatting CI enforces
+      - dependency-name: astyle
 
   - package-ecosystem: pip
     directory: /tools/generate_natvis
diff --git a/tools/astyle/requirements.txt b/tools/astyle/requirements.txt
index 99458a860..cfa98d67e 100644
--- a/tools/astyle/requirements.txt
+++ b/tools/astyle/requirements.txt
@@ -1 +1,4 @@
+# astyle is deliberately pinned: newer versions reformat the code, and this version
+# defines the formatting that `make amalgamate` produces and check_amalgamation.yml enforces.
+# Dependabot is told to ignore it in .github/dependabot.yml.
 astyle==3.4.13