diff --git a/entrypoint.sh b/entrypoint.sh
index 2f4247e..a4ad5ed 100755
--- a/entrypoint.sh
+++ b/entrypoint.sh
@@ -228,6 +228,22 @@ reconcile_managed_comments() {
   fi
 }
 
+find_existing_pr_number() {
+  local pulls_json=""
+  pulls_json="$(gh api --method GET "repos/${TARGET_REPOSITORY}/pulls?state=open&base=${TARGET_BRANCH}")"
+
+  jq -r \
+    --arg repo "${TARGET_REPOSITORY}" \
+    --arg branch "${SOURCE_BRANCH}" \
+    '
+      [
+        .[]
+        | select(.head.ref == $branch)
+        | select((.head.repo.full_name // "") == $repo)
+      ][0].number // empty
+    ' <<< "${pulls_json}"
+}
+
 apply_body_limits() {
   local template_file="$1"
   local max_body_bytes="$2"
@@ -406,7 +422,7 @@ else
 fi
 
 echo -e "\nSetting template..."
-PR_NUMBER="$(gh pr list --repo "${TARGET_REPOSITORY}" --state open --base "${TARGET_BRANCH}" --head "${TARGET_OWNER}:${SOURCE_BRANCH}" --json number --jq '.[0].number // empty')"
+PR_NUMBER="$(find_existing_pr_number)"
 if [[ -z "${PR_NUMBER}" ]]; then
   if [[ -n "${INPUT_TEMPLATE}" ]]; then
     echo "Template source: input template file"
diff --git a/tests/unit/test_existing_pr_lookup.sh b/tests/unit/test_existing_pr_lookup.sh
new file mode 100755
index 0000000..87b5a2f
--- /dev/null
+++ b/tests/unit/test_existing_pr_lookup.sh
@@ -0,0 +1,178 @@
+#!/usr/bin/env bash
+
+set -Eeuo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+SCRIPT_PATH="${SCRIPT_DIR}/../../entrypoint.sh"
+TMP_DIR="$(mktemp -d)"
+trap 'rm -rf "${TMP_DIR}"' EXIT
+
+assert_contains() {
+  local file_path="$1"
+  local expected="$2"
+  if ! grep -Fq -- "${expected}" "${file_path}"; then
+    echo "Assertion failed. Expected to find: ${expected}" >&2
+    echo "----- FILE CONTENT -----" >&2
+    cat "${file_path}" >&2
+    exit 1
+  fi
+}
+
+mkdir -p "${TMP_DIR}/bin"
+mkdir -p "${TMP_DIR}/repo"
+
+cat > "${TMP_DIR}/bin/git" <<'EOF'
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+args=("$@")
+if [[ "${#args[@]}" -ge 2 && "${args[0]}" == "-C" ]]; then
+  args=("${args[@]:2}")
+fi
+
+if [[ "${#args[@]}" -ge 2 && "${args[0]}" == "config" && "${args[1]}" == "--global" ]]; then
+  exit 0
+fi
+
+if [[ "${#args[@]}" -ge 1 && "${args[0]}" == "config" ]]; then
+  exit 0
+fi
+
+if [[ "${#args[@]}" -ge 2 && "${args[0]}" == "remote" && "${args[1]}" == "set-url" ]]; then
+  exit 0
+fi
+
+if [[ "${#args[@]}" -ge 1 && "${args[0]}" == "fetch" ]]; then
+  exit 0
+fi
+
+if [[ "${#args[@]}" -ge 2 && "${args[0]}" == "rev-parse" && "${args[1]}" == "--is-inside-work-tree" ]]; then
+  echo "true"
+  exit 0
+fi
+
+if [[ "${#args[@]}" -ge 2 && "${args[0]}" == "show-ref" ]]; then
+  last_arg="${args[$((${#args[@]} - 1))]}"
+  if [[ "${last_arg}" == "refs/remotes/origin/develop" || "${last_arg}" == "refs/remotes/origin/main" ]]; then
+    exit 0
+  fi
+  exit 1
+fi
+
+if [[ "${#args[@]}" -ge 2 && "${args[0]}" == "rev-parse" ]]; then
+  last_arg="${args[$((${#args[@]} - 1))]}"
+  if [[ "${last_arg}" == "origin/develop" ]]; then
+    echo "bbb222"
+    exit 0
+  fi
+  if [[ "${last_arg}" == "origin/main" ]]; then
+    echo "aaa111"
+    exit 0
+  fi
+fi
+
+if [[ "${#args[@]}" -ge 2 && "${args[0]}" == "diff" && "${args[1]}" == "--quiet" ]]; then
+  exit 1
+fi
+
+if [[ "${#args[@]}" -ge 1 && "${args[0]}" == "diff" ]]; then
+  echo "M README.md"
+  exit 0
+fi
+
+if [[ "${#args[@]}" -ge 1 && "${args[0]}" == "log" ]]; then
+  echo "stub log"
+  exit 0
+fi
+
+if [[ "${#args[@]}" -ge 2 && "${args[0]}" == "symbolic-ref" ]]; then
+  echo "develop"
+  exit 0
+fi
+
+echo "Unsupported git call: $*" >&2
+exit 1
+EOF
+
+cat > "${TMP_DIR}/bin/gh" <<'EOF'
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+cmd="$*"
+
+if [[ "$#" -ge 1 && "$1" == "api" ]]; then
+  if [[ "${cmd}" == *"repos/owner/repo/pulls?state=open&base=main"* ]]; then
+    printf '%s\n' '[{"number":88,"head":{"ref":"other-branch","repo":{"full_name":"owner/repo"}}},{"number":123,"head":{"ref":"develop","repo":{"full_name":"owner/repo"}}},{"number":321,"head":{"ref":"develop","repo":{"full_name":"fork/repo"}}}]'
+    exit 0
+  fi
+  if [[ "${cmd}" == *"repos/owner/repo/pulls/123"* && "${cmd}" == *"--method GET"* ]]; then
+    echo "OLD BODY"
+    exit 0
+  fi
+  if [[ "${cmd}" == *"repos/owner/repo/pulls/123"* && "${cmd}" == *"--method PATCH"* ]]; then
+    echo "https://example.test/pr/123"
+    exit 0
+  fi
+  if [[ "${cmd}" == *"repos/owner/repo/issues/123/comments"* ]]; then
+    echo "[]"
+    exit 0
+  fi
+fi
+
+if [[ "$#" -ge 2 && "$1" == "pr" && "$2" == "create" ]]; then
+  echo "gh pr create should not be called when PR already exists" >&2
+  exit 1
+fi
+
+echo "Unsupported gh call: $*" >&2
+exit 1
+EOF
+
+cat > "${TMP_DIR}/template.md" <<'EOF'
+## Template body from file
+EOF
+
+chmod +x "${TMP_DIR}/bin/git" "${TMP_DIR}/bin/gh"
+
+LOG_FILE="${TMP_DIR}/run.log"
+set +e
+PATH="${TMP_DIR}/bin:${PATH}" \
+GITHUB_ACTOR="ci-user" \
+GITHUB_TOKEN="token" \
+GITHUB_REPOSITORY="owner/repo" \
+GITHUB_WORKSPACE="${TMP_DIR}" \
+GITHUB_OUTPUT="${TMP_DIR}/output.txt" \
+INPUT_GITHUB_TOKEN="token" \
+INPUT_REPOSITORY_PATH="repo" \
+INPUT_SOURCE_BRANCH="develop" \
+INPUT_TARGET_BRANCH="main" \
+INPUT_TITLE="" \
+INPUT_TEMPLATE="${TMP_DIR}/template.md" \
+INPUT_BODY="" \
+INPUT_REVIEWER="" \
+INPUT_ASSIGNEE="" \
+INPUT_LABEL="" \
+INPUT_MILESTONE="" \
+INPUT_DRAFT="false" \
+INPUT_GET_DIFF="false" \
+INPUT_OLD_STRING="" \
+INPUT_NEW_STRING="" \
+INPUT_IGNORE_USERS="dependabot" \
+INPUT_ALLOW_NO_DIFF="false" \
+INPUT_MAX_BODY_BYTES="65000" \
+INPUT_MAX_DIFF_LINES="0" \
+bash "${SCRIPT_PATH}" >"${LOG_FILE}" 2>&1
+STATUS="$?"
+set -e
+
+if [[ "${STATUS}" != "0" ]]; then
+  echo "Expected successful execution in update mode" >&2
+  cat "${LOG_FILE}" >&2
+  exit 1
+fi
+
+assert_contains "${LOG_FILE}" "Updating pull request"
+assert_contains "${TMP_DIR}/output.txt" "url=https://example.test/pr/123"
+assert_contains "${TMP_DIR}/output.txt" "pr_number=123"
+
+echo "Existing PR lookup test passed."
diff --git a/tests/unit/test_template_source_selection.sh b/tests/unit/test_template_source_selection.sh
index 1b91628..1841d98 100755
--- a/tests/unit/test_template_source_selection.sh
+++ b/tests/unit/test_template_source_selection.sh
@@ -98,13 +98,12 @@ cat > "${TMP_DIR}/bin/gh" <<'EOF'
 #!/usr/bin/env bash
 set -Eeuo pipefail
 
-if [[ "$#" -ge 2 && "$1" == "pr" && "$2" == "list" ]]; then
-  echo "123"
-  exit 0
-fi
-
 if [[ "$#" -ge 1 && "$1" == "api" ]]; then
   cmd="$*"
+  if [[ "${cmd}" == *"repos/owner/repo/pulls?state=open&base=release/MAPL-v3"* ]]; then
+    printf '%s\n' '[{"number":123,"head":{"ref":"develop","repo":{"full_name":"owner/repo"}}}]'
+    exit 0
+  fi
   if [[ "${cmd}" == *"repos/owner/repo/issues/123/comments"* ]]; then
     echo "[]"
     exit 0