diff --git a/experiments/shell/local_shell_diagnostics.hpp b/experiments/shell/local_shell_diagnostics.hpp
index b88fdb22..f6d07f9b 100644
--- a/experiments/shell/local_shell_diagnostics.hpp
+++ b/experiments/shell/local_shell_diagnostics.hpp
@@ -45,4 +45,72 @@ inline void local_shell_size_readiness(const std::filesystem::path& directory, s
             <<",\"observed_output_tail_bytes\":"<<observed_bytes<<"}\n";
     progress.close(); require(bool(progress), "local shell size readiness write failed");
 }
+// Diagnostic-only copy of the transport oracle's line rules. It never changes a
+// pass or fail: the shells scenario calls it on its timeout path so a stage five
+// failure can report which transport condition was unmet.
+inline void local_shell_transport_probe(const std::filesystem::path& directory, std::string_view role,
+                                        std::string_view raw, std::string_view marker) {
+    bool output_seen = false, prompt_seen = false, literal = true, malformed = false;
+    std::string line;
+    const auto prompt = [](std::string_view text) { return text == "TF>" || text == "TF> "; };
+    const auto end_line = [&] {
+        if (literal && line == marker) output_seen = true;
+        else if (literal && output_seen && prompt(line)) prompt_seen = true;
+        line.clear();
+        literal = true;
+    };
+    for (std::size_t i = 0; i < raw.size();) {
+        const auto byte = static_cast<unsigned char>(raw[i++]);
+        if (byte == 0x1b) {
+            if (i == raw.size()) { malformed = true; break; }
+            const char kind = raw[i++];
+            if (kind == ']') {
+                bool terminated = false;
+                while (i < raw.size()) {
+                    const char next = raw[i++];
+                    if (next == '\a') { terminated = true; break; }
+                    if (next == '\x1b') {
+                        if (i == raw.size() || raw[i++] != '\\') { malformed = true; break; }
+                        terminated = true;
+                        break;
+                    }
+                }
+                if (!terminated) { malformed = true; break; }
+            } else if (kind == '[') {
+                while (i < raw.size() && ((raw[i] >= '0' && raw[i] <= '9') || raw[i] == ';' || raw[i] == '?')) ++i;
+                if (i == raw.size()) { malformed = true; break; }
+                ++i;
+            } else { malformed = true; break; }
+        } else if (byte == '\r') {
+        } else if (byte == '\n') {
+            end_line();
+        } else if (byte == '\b') {
+            if (output_seen) { malformed = true; break; }
+            literal = false;
+        } else {
+            if (byte < 0x20 || byte == 0x7f) { malformed = true; break; }
+            line.push_back(static_cast<char>(byte));
+        }
+    }
+    std::string escaped;
+    for (const char ch : line) {
+        if (escaped.size() > 160) { escaped += "..."; break; }
+        const auto byte = static_cast<unsigned char>(ch);
+        if (byte >= 0x20 && byte < 0x7f && ch != '"' && ch != '\\') escaped.push_back(ch);
+        else {
+            char buffer[8]{};
+            std::snprintf(buffer, sizeof(buffer), "\\x%02x", byte);
+            escaped += buffer;
+        }
+    }
+    std::ofstream progress(directory/"shells-transport-probe.jsonl", std::ios::binary | std::ios::app);
+    progress << "{\"schema\":\"termforge-local-shell-transport-probe-v1\",\"role\":\"" << role
+             << "\",\"marker\":\"" << marker
+             << "\",\"output_seen\":" << (output_seen ? "true" : "false")
+             << ",\"prompt_seen\":" << (prompt_seen ? "true" : "false")
+             << ",\"malformed\":" << (malformed ? "true" : "false")
+             << ",\"last_line\":\"" << escaped << "\"}\n";
+    progress.close();
+}
+
 } // namespace tf_shell_test
diff --git a/experiments/shell/workspace_shells_scenario.hpp b/experiments/shell/workspace_shells_scenario.hpp
index 7fd41847..91a03892 100644
--- a/experiments/shell/workspace_shells_scenario.hpp
+++ b/experiments/shell/workspace_shells_scenario.hpp
@@ -19,10 +19,12 @@ inline void Workspace::scenario_shells() {
             const std::string role(tab.launch->kind.begin(), tab.launch->kind.end());
             // WSL also has to show a fresh prompt after its marker; record that
             // condition explicitly instead of leaving it to inference.
+            const std::string size_marker = "TF_SIZE_" + std::to_string(columns) + "x" + std::to_string(rows);
             const bool transport_completed = tab.launch->kind != L"wsl" ||
-                tf_shell_test::transport_output_completed(
-                    tab.shell->observed_output(),
-                    "TF_SIZE_" + std::to_string(columns) + "x" + std::to_string(rows));
+                tf_shell_test::transport_output_completed(tab.shell->observed_output(), size_marker);
+            if (tab.launch->kind == L"wsl")
+                tf_shell_test::local_shell_transport_probe(diagnostic_directory, role,
+                                                           tab.shell->observed_output(), size_marker);
             tf_shell_test::local_shell_size_readiness(
                 diagnostic_directory, role, static_cast<unsigned long>(columns), static_cast<unsigned long>(rows),
                 tab.shell->settled(),