2 files changed, 21 insertions, 2 deletions

diff --git a/llm-tools-hl-test.el b/llm-tools-hl-test.el
index 879a4b4..82117b7 100644
--- a/llm-tools-hl-test.el
+++ b/llm-tools-hl-test.el
@@ -680,3 +680,22 @@ tried to parse a line like \"~payload\" as an operation."
         (should-not (member "some payload" file-anchors))
         (should-not (member "another line" file-anchors))
         (should-not (member "replacement" file-anchors))))))
+
+(ert-deftest llm-tools--hl-apply-replace-delete-offset-test ()
+  "Replace same-count lines then delete a later line.
+The replace's offset delta should be 0 (2 payload lines replacing 2 lines).
+With the buggy formula (- b a 1) this computed to +2 instead, causing the
+delete to target past the end of the buffer."
+  (let* ((content "A\nB\nC\nD\nE\n")
+         (file (make-temp-file nil nil nil content))
+         (a2 (llm-tools--hl-file-line-hash file 2))
+         (a3 (llm-tools--hl-file-line-hash file 3))
+         (a5 (llm-tools--hl-file-line-hash file 5))
+         (patch (format "@@ %s\n= %s..%s\n~X\n~Y\n- %s..%s"
+                        file a2 a3 a5 a5)))
+    (unwind-protect
+        (progn
+          (llm-tools--hl-apply patch)
+          (let ((lines (llm-tools--hl-file-lines file)))
+            (should (equal lines '("A" "X" "Y" "D")))))
+      (delete-file file))))
diff --git a/llm-tools-hl.el b/llm-tools-hl.el
index daf9edd..513b3e8 100644
--- a/llm-tools-hl.el
+++ b/llm-tools-hl.el
@@ -442,7 +442,7 @@ FILENAME."
         (b (+ (llm-tools--hl-anchor-to-line-num (cadr range)) offset)))
     (cons (append (cl-subseq content 0 (1- a))
                   (cl-subseq content b))
-          (- offset (- b a 1)))))
+          (- offset (1+ (- b a))))))
 
 (defun llm-tools--hl-apply-replace (range payload content offset)
   "Replace lines from (car RANGE) to (cadr RANGE) with PAYLOAD (nil to '(\"\"))"
@@ -452,7 +452,7 @@ FILENAME."
     (cons (append (cl-subseq content 0 (1- a))
                   payload
                   (cl-subseq content b))
-          (+ offset (- (length payload) (- b a 1))))))
+          (+ offset (- (length payload) (1+ (- b a)))))))
 
 (defun llm-tools--hl-apply-one-op (op content offset)
   "Apply a single OP to CONTENT (list of strings) at the given OFFSET.