diff --git a/PR.md b/PR.md
new file mode 100644
index 0000000..1dfcdc4
--- /dev/null
+++ b/PR.md
@@ -0,0 +1,64 @@
+# fix(strutil): prevent Substr panic on out-of-range negative length
+
+## Problem
+
+`strutil.Substr` panics with a slice-bounds error when a negative length trims
+more characters than remain after `pos`:
+
+```go
+strutil.Substr("hello", 4, -3)      // panic: slice bounds out of range [4:2]
+strutil.Substr("hello", 3, -5)      // panic
+strutil.Substr("abcDEF123", 8, -3)  // panic
+```
+
+A negative `length` means "stop that many runes before the end of the string"
+(this is the documented, tested behaviour, e.g. `Substr("abcDEF123", 2, -3) ==
+"cDEF"`). When the resulting stop index falls before `pos`, the final slice
+`runes[pos:stopIdx]` has `low > high` and panics.
+
+## Root cause
+
+`strutil/split.go`, `Substr`:
+
+```go
+stopIdx := pos + length
+if length == 0 || stopIdx > strLn {
+	stopIdx = strLn
+} else if length < 0 {
+	stopIdx = strLn + length   // can become < pos
+}
+return string(runes[pos:stopIdx])
+```
+
+For example `Substr("hello", 4, -3)`: `strLn == 5`, `pos == 4`, so
+`stopIdx = 5 + (-3) = 2`, and `runes[4:2]` panics.
+
+## Fix
+
+Clamp `stopIdx` to `pos` so an over-long negative trim yields an empty string
+instead of panicking:
+
+```go
+if stopIdx < pos {
+	stopIdx = pos
+}
+```
+
+This is consistent with the existing out-of-range guard `if pos >= strLn` which
+also returns an empty string.
+
+## Test
+
+Extended `TestSubstr` in `strutil/split_test.go` with the three panicking cases,
+each asserting an empty-string result.
+
+- Gate: `go test ./strutil/ -run 'TestSubstr$'` -> ok (PASS)
+- `go vet ./strutil/` -> exit 0
+- `go build ./...` -> exit 0
+
+Counterfactual: reverting the clamp makes `TestSubstr` panic with
+`slice bounds out of range [4:2]`.
+
+Note: `TestDateSNv3/dateLen4_base62` in `strutil/gensn_test.go` is flaky and
+already fails on the unmodified base commit (it asserts uniqueness of randomly
+generated serial numbers); it is unrelated to this change.
diff --git a/strutil/split.go b/strutil/split.go
index 4961b9b..d41dc60 100644
--- a/strutil/split.go
+++ b/strutil/split.go
@@ -166,6 +166,12 @@ func Substr(s string, pos, length int) string {
 		stopIdx = strLn + length
 	}
 
+	// A negative length can move stopIdx before pos when it trims more than
+	// the remaining substring; clamp to pos to avoid an out-of-range slice.
+	if stopIdx < pos {
+		stopIdx = pos
+	}
+
 	return string(runes[pos:stopIdx])
 }
 
diff --git a/strutil/split_test.go b/strutil/split_test.go
index 2a97caf..4e57ee7 100644
--- a/strutil/split_test.go
+++ b/strutil/split_test.go
@@ -120,6 +120,12 @@ func TestSubstr(t *testing.T) {
 	assert.Eq(t, "", strutil.Substr("abcDEF", 23, 5))
 	assert.Eq(t, "cDEF12", strutil.Substr("abcDEF123", 2, -1))
 	assert.Eq(t, "cDEF", strutil.Substr("abcDEF123", 2, -3))
+
+	// negative length trimming more than the remaining substring must not
+	// panic; it should yield an empty string.
+	assert.Eq(t, "", strutil.Substr("hello", 4, -3))
+	assert.Eq(t, "", strutil.Substr("hello", 3, -5))
+	assert.Eq(t, "", strutil.Substr("abcDEF123", 8, -3))
 }
 
 func TestSplitInlineComment(t *testing.T) {