fix(rlm): honor odd truncation limits

dspy/primitives/repl_types.py

@@ -58,8 +58,9 @@ def from_value(
             value_str = str(jsonable)
         is_truncated = len(value_str) > preview_chars
         if is_truncated:
-            half = preview_chars // 2
-            preview = value_str[:half] + "..." + value_str[-half:]
+            head_chars = preview_chars // 2
+            tail_chars = preview_chars - head_chars
+            preview = value_str[:head_chars] + "..." + value_str[-tail_chars:]
         else:
             preview = value_str
 
@@ -113,9 +114,10 @@ def format_output(output: str, max_output_chars: int = 10_000) -> str:
         """Format output with head+tail truncation, preserving true length in header."""
         raw_len = len(output)
         if raw_len > max_output_chars:
-            half = max_output_chars // 2
+            head_chars = max_output_chars // 2
+            tail_chars = max_output_chars - head_chars
             omitted = raw_len - max_output_chars
-            output = output[:half] + f"\n\n... ({omitted:,} characters omitted) ...\n\n" + output[-half:]
+            output = output[:head_chars] + f"\n\n... ({omitted:,} characters omitted) ...\n\n" + output[-tail_chars:]
         return f"Output ({raw_len:,} chars):\n{output}"
 
     def format(self, index: int, max_output_chars: int = 10_000) -> str:

tests/predict/test_rlm.py

@@ -426,6 +426,15 @@ def test_repl_entry_format_truncation(self):
         # True original length shown in header
         assert "200 chars" in formatted
 
+    @pytest.mark.parametrize(
+        ("limit", "head", "tail"),
+        [(1, "", "f"), (5, "ab", "def")],
+    )
+    def test_repl_entry_truncation_preserves_exact_odd_limit(self, limit, head, tail):
+        formatted = REPLEntry.format_output("abcdef", max_output_chars=limit)
+
+        assert formatted == f"Output (6 chars):\n{head}\n\n... ({6 - limit} characters omitted) ...\n\n{tail}"
+
     def test_repl_entry_format_no_truncation(self):
         """Test REPLEntry.format() passes short output through without truncation."""
         output = "a" * 50
@@ -458,6 +467,15 @@ def test_repl_variable_truncation(self):
         assert var.preview.endswith("b" * 25)
         assert "..." in var.preview
 
+    @pytest.mark.parametrize(
+        ("limit", "expected"),
+        [(1, "...f"), (5, "ab...def")],
+    )
+    def test_repl_variable_truncation_preserves_exact_odd_limit(self, limit, expected):
+        var = REPLVariable.from_value("value", "abcdef", preview_chars=limit)
+
+        assert var.preview == expected
+
     def test_repl_variable_with_field_info(self):
         """Test REPLVariable includes desc and constraints from field_info."""
         import dspy