@@ -131,8 +131,19 @@ export function classify(stdout, stderr = "") {
       diffs: [],
     };
   const diffs = Array.isArray(result.diffs) ? result.diffs : [];
-  if (result.error)
-    return { status: "error", error: String(result.error), diffs };
+  if (result.error) {
+    // `error` is the bare exception; the entry in `diffs` says which build
+    // failed ("Base server probe failed: …" / "Target probe failed: …").
+    const context = diffs.find((d) => d.endpoint === "error")?.diff;
+    return { status: "error", error: String(context ?? result.error), diffs };
+  }
+  // A report without a verdict is not a pass.
+  if (typeof result.hasDifferences !== "boolean")
+    return {
+      status: "error",
+      error: "mcp-server-diff's report has no hasDifferences verdict",
+      diffs,
+    };
   return {
     status: result.hasDifferences ? "changed" : "unchanged",
     diffs,
 
@@ -77,19 +77,21 @@ test("a probe failure is an error, though the CLI reports it as a difference", (
   const v = classify(
     cliJson({
       hasDifferences: true,
-      error: "Target probe failed: boom",
+      error: "boom",
       diffs: [{ endpoint: "error", diff: "Target probe failed: boom" }],
     }),
   );
   assert.equal(v.status, "error");
-  assert.match(v.error, /boom/);
+  // The context (which build failed) comes from the diffs entry.
+  assert.equal(v.error, "Target probe failed: boom");
 });
 
 test("output that is not the JSON report is an error, never a pass", () => {
   assert.equal(classify("").status, "error");
   assert.equal(classify("Fatal error: x").status, "error");
   assert.equal(classify("{}").status, "error");
   assert.equal(classify(JSON.stringify({ results: [] })).status, "error");
+  assert.equal(classify(JSON.stringify({ results: [{}] })).status, "error");
 });
 
 test("a CLI crash before the report carries its stderr as the reason", () => {