diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0e76bf2e4c7..e88b6cbf50d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -37,8 +37,16 @@ jobs:
     - name: Calculate the fetch depth
       run: |
         if [ "$GITHUB_EVENT_NAME" == "pull_request" ]; then
-          echo "GIT_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+          if [ "${{ github.event.pull_request.head.repo.full_name }}" != "${{ github.event.pull_request.base.repo.full_name }}" ]; then
+            echo "IS_FORK_PR=true" >> "${GITHUB_ENV}"
+            # Fork PRs need full history to find the merge base
+            echo "GIT_FETCH_DEPTH=0" >> "${GITHUB_ENV}"
+          else
+            echo "IS_FORK_PR=false" >> "${GITHUB_ENV}"
+            echo "GIT_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+          fi
         else
+          echo "IS_FORK_PR=false" >> "${GITHUB_ENV}"
           echo "GIT_FETCH_DEPTH=2" >> "${GITHUB_ENV}"
         fi
     - name: Check out the revision with minimal required history
@@ -75,7 +83,12 @@ jobs:
     - name: Calculate base ref
       run: |
         if [ "$GITHUB_EVENT_NAME" == "pull_request" ]; then
-          git fetch --depth=1 origin "${{ github.event.pull_request.base.sha }}"
+          if [ "$IS_FORK_PR" == "true" ]; then
+            # Fork PR: full history already fetched, just ensure the base SHA is available
+            git fetch origin "${{ github.event.pull_request.base.sha }}"
+          else
+            git fetch --depth=1 origin "${{ github.event.pull_request.base.sha }}"
+          fi
           BASE_REF="${{ github.event.pull_request.base.sha }}"
         else
           BASE_REF="HEAD~1"
@@ -282,8 +295,16 @@ jobs:
     - name: Calculate the fetch depth
       run: |
         if [ "$GITHUB_EVENT_NAME" == "pull_request" ]; then
-          echo "GIT_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+          if [ "${{ github.event.pull_request.head.repo.full_name }}" != "${{ github.event.pull_request.base.repo.full_name }}" ]; then
+            echo "IS_FORK_PR=true" >> "${GITHUB_ENV}"
+            # Fork PRs need full history to find the merge base
+            echo "GIT_FETCH_DEPTH=0" >> "${GITHUB_ENV}"
+          else
+            echo "IS_FORK_PR=false" >> "${GITHUB_ENV}"
+            echo "GIT_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+          fi
         else
+          echo "IS_FORK_PR=false" >> "${GITHUB_ENV}"
           echo "GIT_FETCH_DEPTH=2" >> "${GITHUB_ENV}"
         fi
     - name: Check out the revision with minimal required history
@@ -327,7 +348,12 @@ jobs:
             tests/component/:: -- -v
         else
           if [ "$GITHUB_EVENT_NAME" == "pull_request" ]; then
-            git fetch --depth=1 origin "${{ github.event.pull_request.base.sha }}"
+            if [ "$IS_FORK_PR" == "true" ]; then
+              # Fork PR: full history already fetched, just ensure the base SHA is available
+              git fetch origin "${{ github.event.pull_request.base.sha }}"
+            else
+              git fetch --depth=1 origin "${{ github.event.pull_request.base.sha }}"
+            fi
             BASE_REF="${{ github.event.pull_request.base.sha }}"
           else
             BASE_REF="HEAD~1"
diff --git a/changes/9617.misc.md b/changes/9617.misc.md
new file mode 100644
index 00000000000..f5a5bcc2b82
--- /dev/null
+++ b/changes/9617.misc.md
@@ -0,0 +1 @@
+Use full git clone for fork PRs to fix merge-base computation failure in CI.