diff --git a/action.yml b/action.yml
index 1f1b9475..160846a6 100644
--- a/action.yml
+++ b/action.yml
@@ -78,7 +78,7 @@ inputs:
     required: false
     default: "false"
   fetch_depth:
-    description: "Limit depth of the branch history fetched. **NOTE**: This can be adjusted to resolve errors with insufficient history."
+    description: "Initial depth of the branch history fetched. **NOTE**: This can be adjusted to resolve errors with insufficient history."
     required: false
     default: "40"
   max_fetch_depth:
diff --git a/diff-sha.sh b/diff-sha.sh
index 3cdd64ef..2c583f28 100644
--- a/diff-sha.sh
+++ b/diff-sha.sh
@@ -5,6 +5,8 @@ set -euo pipefail
 INITIAL_COMMIT="false"
 GITHUB_OUTPUT=${GITHUB_OUTPUT:-""}
 EXTRA_ARGS="--no-tags"
+PREVIOUS_SHA=""
+CURRENT_SHA=""
 
 if [[ "$GITHUB_REF" == "refs/tags/"* ]]; then
   EXTRA_ARGS=""
@@ -145,6 +147,10 @@ else
   echo "Running on a pull request event..."
   TARGET_BRANCH=$GITHUB_BASE_REF
   CURRENT_BRANCH=$GITHUB_HEAD_REF
+  
+  if [[ "$INPUT_SINCE_LAST_REMOTE_COMMIT" == "true" ]]; then
+    TARGET_BRANCH=$CURRENT_BRANCH
+  fi
 
   echo "Fetching remote refs..."
 
@@ -188,10 +194,14 @@ else
 
       if ! git rev-parse --quiet --verify "$PREVIOUS_SHA^{commit}" 1>/dev/null 2>&1; then
         # shellcheck disable=SC2046
-        PREVIOUS_SHA=$(git rev-parse $(git branch -r --sort=-committerdate | head -1) 2>&1) && exit_status=$? || exit_status=$?
+        PREVIOUS_SHA=$(git rev-parse origin/"$CURRENT_BRANCH" 2>&1) && exit_status=$? || exit_status=$?
       fi
     else
-      PREVIOUS_SHA=$(git rev-list -n 1 "$TARGET_BRANCH" 2>&1) && exit_status=$? || exit_status=$?
+      PREVIOUS_SHA=$GITHUB_EVENT_PULL_REQUEST_BASE_SHA && exit_status=$? || exit_status=$?
+      
+      if ! git diff --name-only --ignore-submodules=all "$PREVIOUS_SHA"..."$CURRENT_SHA" 1>/dev/null 2>&1; then
+        PREVIOUS_SHA=$(git rev-parse origin/"$TARGET_BRANCH" 2>&1) && exit_status=$? || exit_status=$?
+      fi
     fi
 
     if [[ -z "$PREVIOUS_SHA" || "$PREVIOUS_SHA" == "$CURRENT_SHA" ]]; then
@@ -215,7 +225,7 @@ else
         echo "Fetching $depth commits..."
 
         # shellcheck disable=SC2086
-        git fetch $EXTRA_ARGS --deepen="$depth" origin "$TARGET_BRANCH" "$CURRENT_SHA";
+        git fetch $EXTRA_ARGS -u --progress --deepen="$depth" origin +"$GITHUB_REF":refs/remotes/origin/"$CURRENT_BRANCH"
 
         if [[ $depth -gt $max_depth ]]; then
           echo "::error::Unable to locate a common ancestor between $TARGET_BRANCH and $CURRENT_SHA"