diff --git a/experiments/core/text_selection.hpp b/experiments/core/text_selection.hpp
index 36c72885..068c2cfa 100644
--- a/experiments/core/text_selection.hpp
+++ b/experiments/core/text_selection.hpp
@@ -21,6 +21,9 @@ class TextSelection {
         size_t row = 0;
         size_t saved_bytes = 0, current_bytes = 0;
         size_t saved_columns = 0, current_columns = 0;
+        // #194: the bytes that differed, so a partial repaint can be told apart
+        // from a real content change without re-running the scenario.
+        std::string saved_text, current_text;
     };
     using DiscardObserver = void (*)(const DiscardReport&);
     static void set_discard_observer(DiscardObserver observer) { discard_observer_ = observer; }
@@ -106,6 +109,8 @@ class TextSelection {
                 if (!same_text(saved, current, compare_boundary)) {
                     DiscardReport report; report.reason = "row-text-changed"; report.row = i;
                     report.saved_bytes = saved.projection.text.size(); report.current_bytes = current.projection.text.size();
+        report.saved_text = saved.projection.text.substr(0, 48);
+        report.current_text = current.projection.text.substr(0, 48);
                     report.saved_columns = saved.columns; report.current_columns = current.columns;
                     return invalidate(report);
                 }
diff --git a/experiments/shell/run.py b/experiments/shell/run.py
index 90ac222f..70fe82b6 100644
--- a/experiments/shell/run.py
+++ b/experiments/shell/run.py
@@ -17,6 +17,7 @@
 ROOT = Path(__file__).resolve().parents[2]
 sys.path.insert(0, str(ROOT))
 from experiments.build_paths import binary
+from experiments.shell import summarize_failure
 from experiments.conpty.run import OwnedJob
 from experiments.shell.history_checks import validate_history, validate_native_scroll
 from experiments.shell.paste_checks import validate_paste_workflow
@@ -504,7 +505,15 @@ def execute(exe, mode, out, timeout=40, settings_source=None, *, resources=False
         if not record["desktop_gone"]:
             raise ValueError("owned desktop remained")
         if mode != "watchdog" and (record["timed_out"] or record["exit_code"]):
-            raise ValueError("shell probe failed: " + record["stderr"].strip())
+            # Read the dump back with the shared parser so the raised message always
+            # carries the same verdict instead of a raw multi-line stderr (#73).
+            failures = summarize_failure.parse(record["stderr"])
+            summary = ""
+            if failures:
+                verdicts = summarize_failure.describe(failures[0])
+                if verdicts:
+                    summary = " | " + "; ".join(verdicts)
+            raise ValueError("shell probe failed: " + record["stderr"].strip() + summary)
         result = json.loads(record["stdout"])
         if mode == "watchdog":
             if not record["timed_out"] or result != {"watchdog_ready": True} or record["job_at_timeout"]["active"] < 2:
diff --git a/experiments/shell/shell.cpp b/experiments/shell/shell.cpp
index 90126d04..8b33b4c6 100644
--- a/experiments/shell/shell.cpp
+++ b/experiments/shell/shell.cpp
@@ -41,6 +41,17 @@ std::string env_path(const char* name) {
 #endif
 }
 std::string selection_diag_path() { return env_path("TERMFORGE_SELECTION_DIAG"); }
+// JSON-safe copy of a short diagnostics string; control bytes become hex escapes.
+std::string json_escape(const std::string& text) {
+    static constexpr char digits[] = "0123456789abcdef";
+    std::string escaped;
+    for (const char ch : text) {
+        const auto byte = static_cast<unsigned char>(ch);
+        if (byte >= 0x20 && byte < 0x7f && ch != '"' && ch != 0x5c) escaped.push_back(ch);
+        else { escaped += 0x5c; escaped += 'x'; escaped += digits[byte >> 4]; escaped += digits[byte & 15]; }
+    }
+    return escaped;
+}
 void selection_discard_diagnostic(const termforge::TextSelection::DiscardReport& report) {
     static const std::string path = selection_diag_path();
     if (path.empty()) return;
@@ -49,7 +60,10 @@ void selection_discard_diagnostic(const termforge::TextSelection::DiscardReport&
     stream << "{\"schema\":\"termforge-selection-discard-v1\",\"reason\":\"" << report.reason
            << "\",\"row\":" << report.row << ",\"saved_bytes\":" << report.saved_bytes
            << ",\"current_bytes\":" << report.current_bytes << ",\"saved_columns\":" << report.saved_columns
-           << ",\"current_columns\":" << report.current_columns << ",\"t_ms\":" << GetTickCount64() << "}\n";
+           << ",\"current_columns\":" << report.current_columns
+           << ",\"saved_text\":\"" << json_escape(report.saved_text)
+           << "\",\"current_text\":\"" << json_escape(report.current_text)
+           << "\",\"t_ms\":" << GetTickCount64() << "}\n";
 }
 // Each window owns its trace stream; the hook resolves it by queue pointer so a
 // window that is being destroyed can never leave a dangling global stream.