From 6786edd14a4007d6d9b5aceb91bd6fe1fe220cc3 Mon Sep 17 00:00:00 2001
From: Piotr Wilkin <piotr.wilkin@syndatis.com>
Date: Mon, 15 Jun 2026 15:00:53 +0200
Subject: [PATCH] chat: fix an "oldie but goodie" grammar generator bug that
 surfaced during last changes

---
 common/peg-parser.cpp | 24 +++++++++++++++++--
 tests/test-chat.cpp   | 55 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 77 insertions(+), 2 deletions(-)

diff --git a/common/peg-parser.cpp b/common/peg-parser.cpp
index 310bebf735..d4b491a80e 100644
--- a/common/peg-parser.cpp
+++ b/common/peg-parser.cpp
@@ -1507,6 +1507,7 @@ static std::string gbnf_excluding_pattern(const std::vector<std::string> & strin
     auto pieces = matcher.collect_prefix_and_next();
 
     std::string pattern;
+    std::string trailing;  // optional proper-prefix of a delimiter, allowed only at the very end
     for (size_t i = 0; i < pieces.size(); ++i) {
         if (i > 0) {
             pattern += " | ";
@@ -1522,13 +1523,32 @@ static std::string gbnf_excluding_pattern(const std::vector<std::string> & strin
         }
 
         if (!pre.empty()) {
-            pattern += gbnf_format_literal(common_unicode_cpts_to_utf8(pre)) + " [^" + cls + "]";
+            std::string pre_literal = gbnf_format_literal(common_unicode_cpts_to_utf8(pre));
+            pattern += pre_literal + " [^" + cls + "]";
+            // Each interior alternative consumes a delimiter-prefix plus a disambiguating
+            // char, so the repetition alone cannot match a value that *ends* on a proper
+            // prefix of a delimiter (e.g. a trailing "\n" when the delimiter is
+            // "\n</parameter>\n"). The runtime until() (greedy first-match) accepts such
+            // values, so without this the grammar would reject input the parser accepts.
+            // Allow the value to terminate on any proper prefix as an optional tail.
+            // This makes the grammar a slight superset of the runtime language (a value
+            // may end on the longest prefix, which greedy first-match would not itself
+            // produce); harmless for constrained generation, which only needs to admit
+            // every runtime-valid string.
+            if (!trailing.empty()) {
+                trailing += " | ";
+            }
+            trailing += pre_literal;
         } else {
             pattern += "[^" + cls + "]";
         }
     }
 
-    return "(" + pattern + ")*";
+    std::string result = "(" + pattern + ")*";
+    if (!trailing.empty()) {
+        result += " (" + trailing + ")?";
+    }
+    return result;
 }
 
 static std::unordered_set<std::string> collect_reachable_rules(
diff --git a/tests/test-chat.cpp b/tests/test-chat.cpp
index e65af46c63..548071c906 100644
--- a/tests/test-chat.cpp
+++ b/tests/test-chat.cpp
@@ -2024,6 +2024,61 @@ static void test_template_output_peg_parsers(bool detailed_debug) {
             })
             .run();
 
+        tst.test(
+               "<tool_call>\n"
+               "<function=edit>\n"
+               "<parameter=filename>\n"
+               "foo.c\n"
+               "</parameter>\n"
+               "<parameter=oldString>\n"
+               "#iclunde\n"
+               "</parameter>\n"
+               "<parameter=newString>\n"
+               "#include\n"
+               "</parameter>\n"
+               "</function>\n"
+               "</tool_call>")
+            .enable_thinking(false)
+            .reasoning_format(COMMON_REASONING_FORMAT_AUTO)
+            .tools({
+                edit_tool
+        })
+            .expect_tool_calls({
+                { "edit", "{\"filename\": \"foo.c\", \"oldString\": \"#iclunde\", \"newString\": \"#include\"}", {} },
+            })
+            .run();
+
+        // a parameter value that itself ends in a newline (e.g. a source file with a
+        // trailing newline). The structural delimiter is "\n</parameter>\n", so the value
+        // "#include\n" renders as "...#include\n\n</parameter>\n". The trailing newline must
+        // be preserved faithfully (no stripping), and the generated grammar must admit a
+        // value ending on a delimiter prefix. Regression test for gbnf_excluding_pattern.
+        tst.test(
+               "<tool_call>\n"
+               "<function=edit>\n"
+               "<parameter=filename>\n"
+               "foo.c\n"
+               "</parameter>\n"
+               "<parameter=oldString>\n"
+               "#iclunde\n"
+               "</parameter>\n"
+               "<parameter=newString>\n"
+               "#include\n"
+               "\n"
+               "</parameter>\n"
+               "</function>\n"
+               "</tool_call>")
+            .enable_thinking(false)
+            .reasoning_format(COMMON_REASONING_FORMAT_AUTO)
+            .tools({
+                edit_tool
+        })
+            .expect_tool_calls({
+                { "edit", "{\"filename\": \"foo.c\", \"oldString\": \"#iclunde\", \"newString\": \"#include\\n\"}", {} },
+            })
+            .run();
+
+
         // test code that starts with indent
         tst.test(
                "<tool_call>\n"