Say a comparison is unavailable when the pair cannot be compared

CHANGELOG.md

@@ -13,6 +13,9 @@ project adheres to [Semantic Versioning](https://semver.org/).
 - Playwright setup note in the Quick start section of `README.md`.
 
 ### Fixed
+- The workbench comparison panel now says the comparison is unavailable when
+  the pair is not comparable. It previously said no compared field differs,
+  which disagreed with the Markdown download for the same result.
 - Comparison and case summaries now refuse a `runs` directory that is a
   symlink, matching list, export, and prune. The comparison path used to
   follow the link and read the target tree. The GUI compare route returns

bin/aas-gui.mjs

@@ -225,7 +225,9 @@ export function compareModel(result) {
     : "";
   const differences = Array.isArray(body.differences) ? body.differences : [];
   const rows = differences.length === 0
-    ? "<p>No compared field differs.</p>"
+    ? (body.classification === "not-comparable"
+      ? "<p>Comparison unavailable.</p>"
+      : "<p>No compared field differs.</p>")
     : `<ul>${differences.map((entry) => `<li>${escapeHtml(entry.field)}: ${escapeHtml(JSON.stringify(entry.left))} vs ${escapeHtml(JSON.stringify(entry.right))}</li>`).join("")}</ul>`;
   const notes = Array.isArray(body.notes) && body.notes.length > 0
     ? body.notes

test/gui.test.mjs

@@ -551,6 +551,17 @@ test("history and comparison models escape untrusted values and state limits", (
 
   const empty = compareModel({ classification: "identical", left: { run_id: "a" }, right: { run_id: "b" } });
   assert.match(empty, /No compared field differs/);
+
+  const unavailable = compareModel({
+    classification: "not-comparable",
+    left: null,
+    right: null,
+    errors: ["left (missing): Cannot read run missing."],
+    differences: [],
+  });
+  assert.match(unavailable, /Comparison unavailable/);
+  assert.doesNotMatch(unavailable, /No compared field differs/);
+  assert.match(unavailable, /Cannot read run missing/);
 });