Release v0.3.0-alpha.26: get_correlation_matrix 0-d crash

Bug #28: when only one variable resolves (single-name correlation
request, or several names but only one in the .vmrs), the 1xN input
to numpy.corrcoef returned a 0-d scalar instead of a (1,1) matrix.
Downstream _matrix_to_optional_list then died with "iteration over
a 0-d array".

Fix: _corrcoef now promotes a 0-d numpy result to a (1,1) array
before returning. Single-variable correlation correctly serialises
as [[1.0]] in the JSON envelope.

404 tests pass (+2 covering single-row and multi-row cases as
regression sentinels).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vosesoft

CHANGELOG.md

@@ -4,6 +4,16 @@ All notable changes to ModelRisk MCP. Follows [Keep a Changelog](https://keepach
 
 ## [Unreleased]
 
+## [0.3.0-alpha.26] — 2026-05-22
+
+### Fixed
+
+- **Bug #28 — `get_correlation_matrix` crashed when only one variable resolved.** For a 1×N input matrix (one variable), `numpy.corrcoef` returns a 0-d scalar of value 1.0 (the variable's self-correlation) instead of a 2-d (1, 1) matrix. The downstream `_matrix_to_optional_list` then died with `TypeError: iteration over a 0-d array`. Surfaces in real use when `get_correlation_matrix` is called with a single name (or where several names are requested but only one resolves — the failure mode in the autonomous test pass). Fix: `_corrcoef` now promotes a 0-d numpy result to a (1, 1) array before returning. The downstream JSON envelope correctly serialises the trivial `[[1.0]]` matrix.
+
+### Tests
+
+404 unit tests pass (+2 in `test_mrservice.py::TestCorrcoefHelper` covering single-row and multi-row cases — sentinel against the 0-d regression).
+
 ## [0.3.0-alpha.25] — 2026-05-22
 
 Two bugs surfaced by the autonomous end-to-end test pass against a model with extensive text labels.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "modelrisk-mcp"
-version = "0.3.0a25"
+version = "0.3.0a26"
 description = "Open MCP server bridging Anthropic Claude (and any MCP-compatible client) with the ModelRisk Excel add-in."
 readme = "README.md"
 requires-python = ">=3.11"

server.json

@@ -3,12 +3,12 @@
   "name": "io.github.vosesoftware/modelrisk-mcp",
   "title": "ModelRisk",
   "description": "Read, build, fit, and run Monte Carlo risk models in Excel through Vose Software's ModelRisk.",
-  "version": "0.3.0a25",
+  "version": "0.3.0a26",
   "packages": [
     {
       "registryType": "pypi",
       "identifier": "modelrisk-mcp",
-      "version": "0.3.0a25",
+      "version": "0.3.0a26",
       "transport": {
         "type": "stdio"
       }

src/modelrisk_mcp/__init__.py

@@ -1 +1 @@
-__version__ = "0.3.0a25"
+__version__ = "0.3.0a26"

src/modelrisk_mcp/bridge/results.py

@@ -335,9 +335,27 @@ def _lookup_var_id(self, handle: VmrsHandle, name: str) -> int | None:
 
 
 def _corrcoef(matrix: np.ndarray) -> np.ndarray:
+    """Wrap numpy.corrcoef so the result is always a 2-d array.
+
+    Bug #28 (alpha.26): for a single-row input (1xN matrix —
+    one variable), `np.corrcoef` returns a 0-d scalar of value 1.0
+    (the correlation of one variable with itself). Downstream
+    `_matrix_to_optional_list` then crashed with "iteration over
+    a 0-d array" because it iterates over rows.
+
+    Surfaces in real use: `get_correlation_matrix` called with a
+    single name (or where only one of several names resolves) was
+    crashing instead of returning a trivial [[1.0]] matrix."""
     if matrix.size == 0 or matrix.shape[1] < 2:
         return np.full((matrix.shape[0], matrix.shape[0]), np.nan)
-    return np.asarray(np.corrcoef(matrix))
+    result = np.asarray(np.corrcoef(matrix))
+    if result.ndim == 0:
+        # Single-variable case — promote to a 1x1 matrix containing
+        # the scalar (which numpy.corrcoef computed as 1.0 by
+        # construction, but defer to the actual returned value rather
+        # than hardcoding).
+        return result.reshape(1, 1)
+    return result
 
 
 def _rank_matrix(matrix: np.ndarray) -> np.ndarray:

tests/unit/test_mrservice.py

@@ -305,6 +305,43 @@ def MRLIB_GetModelVarID(  # noqa: N802
             handle.lookup_var_id("Slow")
 
 
+class TestCorrcoefHelper:
+    """Regression for bug #28 — `_corrcoef` returned a 0-d array
+    when handed a 1×N matrix (single variable), and the downstream
+    `_matrix_to_optional_list` then crashed with "iteration over a
+    0-d array". Surfaces in real use when `get_correlation_matrix`
+    is called with one name (or only one resolves)."""
+
+    def test_single_row_returns_1x1_matrix(self) -> None:
+        import numpy as np
+
+        from modelrisk_mcp.bridge.results import _corrcoef
+
+        # One variable with 1000 samples.
+        matrix = np.arange(1000, dtype=float).reshape(1, 1000)
+        result = _corrcoef(matrix)
+        # Must be 2-d, shape (1, 1), value approximately 1.0.
+        assert result.ndim == 2
+        assert result.shape == (1, 1)
+        assert abs(float(result[0, 0]) - 1.0) < 1e-9
+
+    def test_two_row_returns_2x2_matrix(self) -> None:
+        """Sanity check: normal 2+ variable case still works."""
+        import numpy as np
+
+        from modelrisk_mcp.bridge.results import _corrcoef
+
+        a = np.arange(100, dtype=float)
+        b = a * 2.0  # perfectly correlated
+        matrix = np.stack([a, b])
+        result = _corrcoef(matrix)
+        assert result.shape == (2, 2)
+        # Self-correlation = 1, cross-correlation = 1 (perfect linear).
+        assert abs(result[0, 0] - 1.0) < 1e-9
+        assert abs(result[1, 1] - 1.0) < 1e-9
+        assert abs(result[0, 1] - 1.0) < 1e-9
+
+
 class TestVmrsDiscovery:
     def test_returns_sibling_vmrs(self, tmp_path: Path) -> None:
         workbook = tmp_path / "model.xlsx"

tests/unit/test_server_boot.py

@@ -5,7 +5,7 @@
 
 
 def test_version_is_set() -> None:
-    assert __version__ == "0.3.0a25"
+    assert __version__ == "0.3.0a26"
 
 
 def test_server_name() -> None: