fix(registry): bump docstring truncation cap to 4000 chars (#335)

src/sktime_mcp/registry/interface.py

@@ -12,6 +12,37 @@
 
 logger = logging.getLogger(__name__)
 
+# Maximum characters of docstring to expose to LLMs in to_dict() output.
+# Earlier versions truncated at 500 chars, but sktime's numpydoc-style
+# docstrings put the "Parameters" section, the most useful information
+# for an agent reasoning about hyperparameters, past the first 500
+# chars in many estimators (issue #335). Bumping the cap to 4000 keeps
+# the response compact enough for an MCP tool result while preserving
+# the Parameters section for the vast majority of sktime estimators.
+_DOCSTRING_MAX_CHARS = 4000
+
+
+def _truncate_docstring(docstring: str | None) -> str | None:
+    """Return *docstring* truncated to a length that preserves the
+    Parameters section when possible.
+
+    The function:
+    - returns ``None`` for a missing docstring;
+    - returns the full docstring if it is already short enough;
+    - otherwise returns the first ``_DOCSTRING_MAX_CHARS`` characters,
+      with a trailing ellipsis when content was dropped.
+
+    Per #335 the cap is large enough to capture the numpydoc
+    ``Parameters`` block of every estimator we have looked at without
+    embedding the full docstring (which can run several pages on
+    composite forecasters).
+    """
+    if docstring is None:
+        return None
+    if len(docstring) <= _DOCSTRING_MAX_CHARS:
+        return docstring
+    return docstring[:_DOCSTRING_MAX_CHARS].rstrip() + "..."
+
 
 @dataclass
 class EstimatorNode:
@@ -47,9 +78,7 @@ def to_dict(self) -> dict[str, Any]:
             "module": self.module,
             "tags": self.tags,
             "hyperparameters": self.hyperparameters,
-            "docstring": (
-                self.docstring[:500] if self.docstring else None
-            ),  # L-1: Truncate docstring to 500 characters, we can also try summarization
+            "docstring": _truncate_docstring(self.docstring),
         }
 
     def to_summary(self) -> dict[str, Any]:

tests/test_core.py

@@ -250,3 +250,48 @@ def fake_save_model(**kwargs):
 
 if __name__ == "__main__":
     pytest.main([__file__, "-v"])
+
+
+class TestDocstringTruncation:
+    """Regression test for issue #335, bump the docstring truncation
+    cap so the numpydoc ``Parameters`` section, which appears past
+    the first 500 chars in many sktime estimators, survives the
+    EstimatorNode.to_dict serialisation."""
+
+    def test_short_docstring_unchanged(self):
+        from sktime_mcp.registry.interface import _truncate_docstring
+
+        assert _truncate_docstring("hi") == "hi"
+
+    def test_none_passes_through(self):
+        from sktime_mcp.registry.interface import _truncate_docstring
+
+        assert _truncate_docstring(None) is None
+
+    def test_long_docstring_truncated_with_ellipsis(self):
+        from sktime_mcp.registry.interface import (
+            _DOCSTRING_MAX_CHARS,
+            _truncate_docstring,
+        )
+
+        long_doc = "x" * (_DOCSTRING_MAX_CHARS + 50)
+        out = _truncate_docstring(long_doc)
+        assert out is not None
+        assert out.endswith("...")
+        # Plain content (without ellipsis) is at most the cap.
+        assert len(out) - 3 <= _DOCSTRING_MAX_CHARS
+
+    def test_cap_preserves_parameters_section_past_500_chars(self):
+        """The whole point of #335: Parameters section appears past 500
+        chars in many sktime docstrings, and the new cap must keep it."""
+        from sktime_mcp.registry.interface import _truncate_docstring
+
+        intro = "x" * 600
+        doc = (
+            intro
+            + "\n\nParameters\n----------\nn_neighbors : int, default=5\n"
+        )
+        out = _truncate_docstring(doc)
+        assert out is not None
+        assert "Parameters" in out
+        assert "n_neighbors" in out