refactor(toolcall): remove lower param from skipXMLIgnoredSection

The lower parameter was a footgun: callers had to keep it in sync with the
loop bound over text. Instead, skipXMLIgnoredSection now accepts only text
and constructs strings.ToLower(tail) internally for its prefix checks.

This eliminates the entire class of len(text) vs len(lower) boundary bugs
along with the min() workaround.

Also changes:
- findToolCDATAEnd: drop lower param, use text directly for closeMarker
  search (]]> is ASCII, ToLower is a no-op for it)
- cdataEndLooksStructural: drop lower param, use raw text byte comparison
- All external callers: loop bound reverts to plain len(text)

The inner tag-matching functions (findXMLStartTagOutsideCDATA,
findMatchingXMLEndTagOutsideCDATA) retain their own local lower for
HasPrefix comparisons against the target tag name, keeping concerns
properly separated.

Fixes #435.

internal/toolcall/toolcalls_dsml.go

@@ -17,11 +17,10 @@ func rewriteDSMLToolMarkupOutsideIgnored(text string) string {
 	if text == "" {
 		return ""
 	}
-	lower := strings.ToLower(text)
 	var b strings.Builder
 	b.Grow(len(text))
 	for i := 0; i < len(text); {
-		next, advanced, blocked := skipXMLIgnoredSection(text, lower, i)
+		next, advanced, blocked := skipXMLIgnoredSection(text, i)
 		if blocked {
 			b.WriteString(text[i:])
 			break

internal/toolcall/toolcalls_parse_markup.go

@@ -144,7 +144,7 @@ func findXMLStartTagOutsideCDATA(text, tag string, from int) (start, bodyStart i
 	lower := strings.ToLower(text)
 	target := "<" + strings.ToLower(tag)
 	for i := maxInt(from, 0); i < len(text); {
-		next, advanced, blocked := skipXMLIgnoredSection(text, lower, i)
+		next, advanced, blocked := skipXMLIgnoredSection(text, i)
 		if blocked {
 			return -1, -1, "", false
 		}
@@ -170,7 +170,7 @@ func findMatchingXMLEndTagOutsideCDATA(text, tag string, from int) (closeStart,
 	closeTarget := "</" + strings.ToLower(tag)
 	depth := 1
 	for i := maxInt(from, 0); i < len(text); {
-		next, advanced, blocked := skipXMLIgnoredSection(text, lower, i)
+		next, advanced, blocked := skipXMLIgnoredSection(text, i)
 		if blocked {
 			return -1, -1, false
 		}
@@ -206,16 +206,20 @@ func findMatchingXMLEndTagOutsideCDATA(text, tag string, from int) (closeStart,
 	return -1, -1, false
 }
 
-func skipXMLIgnoredSection(text, lower string, i int) (next int, advanced bool, blocked bool) {
+func skipXMLIgnoredSection(text string, i int) (next int, advanced bool, blocked bool) {
+	if i < 0 || i >= len(text) {
+		return i, false, false
+	}
+	tail := strings.ToLower(text[i:])
 	switch {
-	case strings.HasPrefix(lower[i:], "<![cdata["):
-		end := findToolCDATAEnd(text, lower, i+len("<![cdata["))
+	case strings.HasPrefix(tail, "<![cdata["):
+		end := findToolCDATAEnd(text, i+len("<![cdata["))
 		if end < 0 {
 			return 0, false, true
 		}
 		return end + len("]]>"), true, false
-	case strings.HasPrefix(lower[i:], "<!--"):
-		end := strings.Index(lower[i+len("<!--"):], "-->")
+	case strings.HasPrefix(tail, "<!--"):
+		end := strings.Index(tail[len("<!--"):], "-->")
 		if end < 0 {
 			return 0, false, true
 		}
@@ -225,14 +229,14 @@ func skipXMLIgnoredSection(text, lower string, i int) (next int, advanced bool,
 	}
 }
 
-func findToolCDATAEnd(text, lower string, from int) int {
-	if from < 0 || from > len(text) {
+func findToolCDATAEnd(text string, from int) int {
+	if from < 0 || from >= len(text) {
 		return -1
 	}
 	const closeMarker = "]]>"
 	firstNonFenceEnd := -1
 	for searchFrom := from; searchFrom < len(text); {
-		rel := strings.Index(lower[searchFrom:], closeMarker)
+		rel := strings.Index(text[searchFrom:], closeMarker)
 		if rel < 0 {
 			break
 		}
@@ -241,27 +245,28 @@ func findToolCDATAEnd(text, lower string, from int) int {
 		if cdataOffsetIsInsideMarkdownFence(text[from:end]) {
 			continue
 		}
+		if cdataEndLooksStructural(text, searchFrom) {
+			return end
+		}
 		if firstNonFenceEnd < 0 {
 			firstNonFenceEnd = end
 		}
-		if cdataEndLooksStructural(lower, searchFrom) {
-			return end
-		}
 	}
 	return firstNonFenceEnd
 }
 
-func cdataEndLooksStructural(lower string, after int) bool {
-	for after < len(lower) {
-		switch lower[after] {
-		case ' ', '\t', '\r', '\n':
+func cdataEndLooksStructural(text string, after int) bool {
+	for after < len(text) {
+		switch {
+		case text[after] == ' ' || text[after] == '\t' || text[after] == '\r' || text[after] == '\n':
 			after++
-			continue
+		case after+1 < len(text) && text[after] == '<' && text[after+1] == '/':
+			return true
 		default:
+			return false
 		}
-		break
 	}
-	return strings.HasPrefix(lower[after:], "</")
+	return false
 }
 
 func cdataOffsetIsInsideMarkdownFence(fragment string) bool {

internal/toolcall/toolcalls_scan.go

@@ -28,9 +28,8 @@ type ToolMarkupTag struct {
 }
 
 func ContainsToolMarkupSyntaxOutsideIgnored(text string) (hasDSML, hasCanonical bool) {
-	lower := strings.ToLower(text)
 	for i := 0; i < len(text); {
-		next, advanced, blocked := skipXMLIgnoredSection(text, lower, i)
+		next, advanced, blocked := skipXMLIgnoredSection(text, i)
 		if blocked {
 			return hasDSML, hasCanonical
 		}
@@ -56,9 +55,8 @@ func ContainsToolMarkupSyntaxOutsideIgnored(text string) (hasDSML, hasCanonical
 }
 
 func ContainsToolCallWrapperSyntaxOutsideIgnored(text string) (hasDSML, hasCanonical bool) {
-	lower := strings.ToLower(text)
 	for i := 0; i < len(text); {
-		next, advanced, blocked := skipXMLIgnoredSection(text, lower, i)
+		next, advanced, blocked := skipXMLIgnoredSection(text, i)
 		if blocked {
 			return hasDSML, hasCanonical
 		}
@@ -88,9 +86,8 @@ func ContainsToolCallWrapperSyntaxOutsideIgnored(text string) (hasDSML, hasCanon
 }
 
 func FindToolMarkupTagOutsideIgnored(text string, start int) (ToolMarkupTag, bool) {
-	lower := strings.ToLower(text)
 	for i := maxInt(start, 0); i < len(text); {
-		next, advanced, blocked := skipXMLIgnoredSection(text, lower, i)
+		next, advanced, blocked := skipXMLIgnoredSection(text, i)
 		if blocked {
 			return ToolMarkupTag{}, false
 		}
@@ -107,7 +104,7 @@ func FindToolMarkupTagOutsideIgnored(text string, start int) (ToolMarkupTag, boo
 }
 
 func FindMatchingToolMarkupClose(text string, open ToolMarkupTag) (ToolMarkupTag, bool) {
-	if text == "" || open.Name == "" || open.Closing {
+	if text == "" || open.Name == "" || open.Closing || open.End >= len(text) {
 		return ToolMarkupTag{}, false
 	}
 	depth := 1

internal/toolcall/toolcalls_test.go

@@ -892,3 +892,107 @@ func TestParseToolCallsSkipsProseMentionOfSameWrapperVariant(t *testing.T) {
 		t.Fatalf("expected command to parse, got %q", got)
 	}
 }
+
+func TestTurkishILowercaseMapping(t *testing.T) {
+	tests := []struct {
+		name     string
+		text     string
+		start    int
+		wantOk   bool
+		wantName string
+	}{
+		{"turkish_i_at_name_start", "İ<tool>", 0, false, ""},
+		{"turkish_i_at_name_end", "<toolİ>", 0, false, ""},
+		{"turkish_i_before_tag", "İ<tool>", 0, false, ""},
+		{"normal_tool_calls", "<tool_calls>", 0, true, "tool_calls"},
+		{"normal_invoke", "<invoke name=\"test\">", 0, true, "invoke"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, ok := FindToolMarkupTagOutsideIgnored(tt.text, tt.start)
+			if ok != tt.wantOk {
+				t.Errorf("FindToolMarkupTagOutsideIgnored(%q, %d) ok = %v, want %v", tt.text, tt.start, ok, tt.wantOk)
+				return
+			}
+			if ok && got.Name != tt.wantName {
+				t.Errorf("FindToolMarkupTagOutsideIgnored(%q, %d) name = %q, want %q", tt.text, tt.start, got.Name, tt.wantName)
+			}
+		})
+	}
+}
+
+func TestSkipXMLIgnoredSectionBoundaryConditions(t *testing.T) {
+	text := "hello"
+
+	tests := []struct {
+		name     string
+		i        int
+		wantNext int
+		wantAdv  bool
+		wantBlk  bool
+	}{
+		{"valid_index", 2, 2, false, false},
+		{"at_end_equal_len", 5, 5, false, false},
+		{"beyond_end", 6, 6, false, false},
+		{"negative", -1, -1, false, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			next, adv, blk := skipXMLIgnoredSection(text, tt.i)
+			if next != tt.wantNext || adv != tt.wantAdv || blk != tt.wantBlk {
+				t.Errorf("skipXMLIgnoredSection(%q, %d) = (%d, %v, %v), want (%d, %v, %v)",
+					text, tt.i, next, adv, blk, tt.wantNext, tt.wantAdv, tt.wantBlk)
+			}
+		})
+	}
+}
+
+func TestFindToolCDATAEndBoundaryConditions(t *testing.T) {
+	text := "<![CDATA[hello]]>"
+
+	tests := []struct {
+		name       string
+		from       int
+		wantResult int
+	}{
+		{"valid", 12, 14},
+		{"at_end", 17, -1},
+		{"beyond_end", 18, -1},
+		{"negative", -1, -1},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := findToolCDATAEnd(text, tt.from)
+			if got != tt.wantResult {
+				t.Errorf("findToolCDATAEnd(%q, %d) = %d, want %d",
+					text, tt.from, got, tt.wantResult)
+			}
+		})
+	}
+}
+
+func TestFindMatchingToolMarkupCloseBoundaryConditions(t *testing.T) {
+	tests := []struct {
+		name   string
+		text   string
+		open   ToolMarkupTag
+		wantOk bool
+	}{
+		{"empty_text", "", ToolMarkupTag{Name: "tool_calls", End: 0}, false},
+		{"open_end_beyond_text", "hello", ToolMarkupTag{Name: "tool_calls", End: 100}, false},
+		{"open_end_equals_len", "hello", ToolMarkupTag{Name: "tool_calls", End: 5}, false},
+		{"valid_simple", "<tool_calls></tool_calls>", ToolMarkupTag{Name: "tool_calls", End: 11}, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, ok := FindMatchingToolMarkupClose(tt.text, tt.open)
+			if ok != tt.wantOk {
+				t.Errorf("FindMatchingToolMarkupClose(%q, %+v) ok = %v, want %v", tt.text, tt.open, ok, tt.wantOk)
+			}
+		})
+	}
+}