diff --git a/.github/workflows/pr-ready-review.yml b/.github/workflows/pr-ready-review.yml
index d2acde0dbe..4bfb5b4147 100644
--- a/.github/workflows/pr-ready-review.yml
+++ b/.github/workflows/pr-ready-review.yml
@@ -167,8 +167,14 @@ jobs:
         env:
           EXTRA_PROMPT: ${{ inputs.extra_prompt }}
         run: |
+          # prior-comments.md is PR comment text verbatim, and commenting needs no write access.
+          # With a fixed delimiter, a comment containing a bare `EOF` line closes the block early:
+          # the step dies, and whatever follows in that comment is read as further environment
+          # assignments for the rest of this job, which holds the review tokens. Hence a random
+          # delimiter, per GitHub's guidance for untrusted multiline values.
+          delimiter="REVIEW_PROMPT_EOF_$(openssl rand -hex 16)"
           {
-            echo 'REVIEW_PROMPT<<EOF'
+            echo "REVIEW_PROMPT<<$delimiter"
             cat REVIEW.md
             echo ''
             cat .claude/review-prompt.md
@@ -182,7 +188,7 @@ jobs:
               echo ''
               cat prior-comments.md
             fi
-            echo 'EOF'
+            echo "$delimiter"
           } >> "$GITHUB_ENV"
 
       - name: Automatic PR Review
diff --git a/.github/workflows/pr-review-commands.yml b/.github/workflows/pr-review-commands.yml
index 3c04e4e5f2..d29ffe6561 100644
--- a/.github/workflows/pr-review-commands.yml
+++ b/.github/workflows/pr-review-commands.yml
@@ -25,16 +25,20 @@ jobs:
               REMAINDER_FIRST_LINE=${FIRST_LINE#"$FIRST_WORD"}
               REMAINDER_FIRST_LINE=${REMAINDER_FIRST_LINE# }
               REST=$(printf '%s' "$BODY" | tail -n +2)
+              # The value is the comment body, which anyone can write. A fixed delimiter lets a
+              # comment close the block early and have the rest of itself read as further step
+              # outputs, so the delimiter has to be unguessable.
+              delimiter="EXTRA_EOF_$(openssl rand -hex 16)"
               {
                 echo "command=$COMMAND"
-                echo 'extra_prompt<<EXTRA_EOF'
+                echo "extra_prompt<<$delimiter"
                 if [ -n "$REMAINDER_FIRST_LINE" ]; then
                   printf '%s\n' "$REMAINDER_FIRST_LINE"
                 fi
                 if [ -n "$REST" ]; then
                   printf '%s\n' "$REST"
                 fi
-                echo 'EXTRA_EOF'
+                echo "$delimiter"
               } >> "$GITHUB_OUTPUT"
               ;;
             *)