diff --git a/apps/qsl-client/main.cpp b/apps/qsl-client/main.cpp
index 04c4efe..7cddd4f 100644
--- a/apps/qsl-client/main.cpp
+++ b/apps/qsl-client/main.cpp
@@ -2,18 +2,43 @@
 
 #include <arpa/inet.h>
 #include <array>
+#include <charconv>
 #include <cstddef>
 #include <cstdint>
 #include <iostream>
+#include <limits>
 #include <netinet/in.h>
+#include <optional>
 #include <span>
 #include <string>
+#include <string_view>
 #include <sys/socket.h>
 #include <unistd.h>
 #include <vector>
 
 namespace {
 
+// Parse a whole token as a TCP port (0-65535). Rejects junk, trailing characters, and out-of-range
+// values instead of std::stoul's throw-or-silently-truncate behavior.
+std::optional<std::uint16_t> parse_port(std::string_view s) {
+    std::uint32_t value = 0;
+    const char *begin = s.data();
+    const char *end = begin + s.size();
+    const auto [ptr, ec] = std::from_chars(begin, end, value);
+    if (ec != std::errc{} || ptr != end || value > std::numeric_limits<std::uint16_t>::max()) {
+        return std::nullopt;
+    }
+    return static_cast<std::uint16_t>(value);
+}
+
+// The optional port arg defaults to 9009; nullopt means it was present but malformed.
+std::optional<std::uint16_t> port_from_args(int argc, char **argv) {
+    if (argc < 2) {
+        return std::uint16_t{9009};
+    }
+    return parse_port(argv[1]);
+}
+
 bool write_all(int fd, const std::vector<std::byte> &data) {
     std::size_t offset = 0;
     while (offset < data.size()) {
@@ -73,7 +98,12 @@ void print_responses(std::span<const std::byte> bytes) {
 
 // qsl-client [port]  -> connect to 127.0.0.1:port, send a NewOrder and a Heartbeat, print replies.
 int main(int argc, char **argv) {
-    const std::uint16_t port = (argc >= 2) ? static_cast<std::uint16_t>(std::stoul(argv[1])) : 9009;
+    const auto port_opt = port_from_args(argc, argv);
+    if (!port_opt) {
+        std::cerr << "usage: qsl-client [port]   (one optional port, 0-65535)\n";
+        return 2;
+    }
+    const std::uint16_t port = *port_opt;
 
     const int fd = ::socket(AF_INET, SOCK_STREAM, 0);
     if (fd < 0) {
diff --git a/apps/qsl-export-fixture/main.cpp b/apps/qsl-export-fixture/main.cpp
index 47ca8fd..cb3cb13 100644
--- a/apps/qsl-export-fixture/main.cpp
+++ b/apps/qsl-export-fixture/main.cpp
@@ -4,10 +4,13 @@
 #include "qsl/replay/dispatch.hpp"
 #include "qsl/replay/recovery.hpp"
 
+#include <charconv>
 #include <cstddef>
 #include <cstdint>
 #include <iostream>
+#include <optional>
 #include <ostream>
+#include <string_view>
 #include <type_traits>
 #include <variant>
 
@@ -15,6 +18,34 @@ using namespace qsl;
 
 namespace {
 
+// Parse a whole token as an unsigned integer; rejects junk/trailing chars/overflow rather than
+// std::stoull's throw-on-bad-input (which would call std::terminate from main).
+std::optional<std::uint64_t> parse_u64(std::string_view s) {
+    std::uint64_t value = 0;
+    const char *begin = s.data();
+    const char *end = begin + s.size();
+    const auto [ptr, ec] = std::from_chars(begin, end, value);
+    if (ec != std::errc{} || ptr != end) {
+        return std::nullopt;
+    }
+    return value;
+}
+
+struct FixtureArgs {
+    std::uint64_t seed;
+    std::size_t orders;
+};
+
+// Optional [seed] [orders] args (defaults 42 / 200); nullopt if either is present but malformed.
+std::optional<FixtureArgs> parse_args(int argc, char **argv) {
+    const auto seed = (argc >= 2) ? parse_u64(argv[1]) : std::optional<std::uint64_t>{42};
+    const auto orders = (argc >= 3) ? parse_u64(argv[2]) : std::optional<std::uint64_t>{200};
+    if (!seed || !orders) {
+        return std::nullopt;
+    }
+    return FixtureArgs{*seed, static_cast<std::size_t>(*orders)};
+}
+
 // Emit one normalized line per engine event (in emission order).
 void emit_events(std::ostream &os, const std::vector<engine::EngineEvent> &events) {
     for (const auto &event : events) {
@@ -45,8 +76,13 @@ void emit_events(std::ostream &os, const std::vector<engine::EngineEvent> &event
 // max_order_quantity makes some new orders reject, so the fixture exercises the
 // "rejected order never rests" invariant. Engine-neutral: no engine code is modified.
 int main(int argc, char **argv) {
-    const std::uint64_t seed = (argc >= 2) ? std::stoull(argv[1]) : 42;
-    const std::size_t orders = (argc >= 3) ? std::stoull(argv[2]) : 200;
+    const auto args = parse_args(argc, argv);
+    if (!args) {
+        std::cerr << "usage: qsl-export-fixture [seed] [orders]\n";
+        return 2;
+    }
+    const std::uint64_t seed = args->seed;
+    const std::size_t orders = args->orders;
     const core::SymbolId symbols = 4;
     const core::Quantity max_qty = 8; // generate_flow can emit qty > 8 -> some orders reject
 
diff --git a/apps/qsl-mdfeed/main.cpp b/apps/qsl-mdfeed/main.cpp
index 79c17a2..a0a21a0 100644
--- a/apps/qsl-mdfeed/main.cpp
+++ b/apps/qsl-mdfeed/main.cpp
@@ -4,14 +4,57 @@
 #include "qsl/feed/udp_feed.hpp"
 #include "qsl/replay/recovery.hpp"
 
+#include <charconv>
 #include <cstddef>
 #include <cstdint>
 #include <iostream>
+#include <limits>
+#include <optional>
 #include <string>
+#include <string_view>
 #include <variant>
 
 namespace {
 
+// Parse a whole token as an unsigned integer, rejecting junk/trailing chars/overflow (unlike
+// std::stoul/std::stoull which throw on bad input and std::stoi which can wrap a port silently).
+std::optional<std::uint64_t> parse_u64(std::string_view s) {
+    std::uint64_t value = 0;
+    const char *begin = s.data();
+    const char *end = begin + s.size();
+    const auto [ptr, ec] = std::from_chars(begin, end, value);
+    if (ec != std::errc{} || ptr != end) {
+        return std::nullopt;
+    }
+    return value;
+}
+
+std::optional<std::uint16_t> parse_port(std::string_view s) {
+    const auto value = parse_u64(s);
+    if (!value || *value > std::numeric_limits<std::uint16_t>::max()) {
+        return std::nullopt;
+    }
+    return static_cast<std::uint16_t>(*value);
+}
+
+std::optional<int> parse_rcvbuf(std::string_view s) {
+    const auto value = parse_u64(s);
+    if (!value || *value > std::numeric_limits<int>::max()) {
+        return std::nullopt;
+    }
+    return static_cast<int>(*value);
+}
+
+template <class... Opts> bool all_present(const Opts &...opts) noexcept {
+    return (static_cast<bool>(opts) && ...);
+}
+
+int usage() {
+    std::cerr << "usage:\n  qsl-mdfeed subscribe <port> [count] [rcvbuf_bytes]\n  qsl-mdfeed "
+                 "publish <port> [seed] [orders]\n";
+    return 2;
+}
+
 int publish(std::uint16_t port, std::uint64_t seed, std::size_t orders) {
     qsl::engine::MatchingEngine engine;
     qsl::feed::MarketDataPublisher publisher;
@@ -74,22 +117,34 @@ int subscribe(std::uint16_t port, std::size_t count, int recv_buffer_bytes) {
 
 } // namespace
 
+int run_subscribe(int argc, char **argv) {
+    const auto port = parse_port(argv[2]);
+    const auto count = (argc >= 4) ? parse_u64(argv[3]) : std::optional<std::uint64_t>{20};
+    const auto rcvbuf = (argc >= 5) ? parse_rcvbuf(argv[4]) : std::optional<int>{0};
+    if (!all_present(port, count, rcvbuf)) {
+        return usage();
+    }
+    return subscribe(*port, static_cast<std::size_t>(*count), *rcvbuf);
+}
+
+int run_publish(int argc, char **argv) {
+    const auto port = parse_port(argv[2]);
+    const auto seed = (argc >= 4) ? parse_u64(argv[3]) : std::optional<std::uint64_t>{42};
+    const auto orders = (argc >= 5) ? parse_u64(argv[4]) : std::optional<std::uint64_t>{200};
+    if (!all_present(port, seed, orders)) {
+        return usage();
+    }
+    return publish(*port, *seed, static_cast<std::size_t>(*orders));
+}
+
 // qsl-mdfeed subscribe <port> [count] [rcvbuf_bytes]  -> receive datagrams and report gaps
 // qsl-mdfeed publish   <port> [seed] [orders]         -> run a synthetic flow, stream its data
 int main(int argc, char **argv) {
     if (argc >= 3 && std::string(argv[1]) == "subscribe") {
-        const auto port = static_cast<std::uint16_t>(std::stoul(argv[2]));
-        const std::size_t count = (argc >= 4) ? std::stoul(argv[3]) : 20;
-        const int recv_buffer_bytes = (argc >= 5) ? std::stoi(argv[4]) : 0;
-        return subscribe(port, count, recv_buffer_bytes);
+        return run_subscribe(argc, argv);
     }
     if (argc >= 3 && std::string(argv[1]) == "publish") {
-        const auto port = static_cast<std::uint16_t>(std::stoul(argv[2]));
-        const std::uint64_t seed = (argc >= 4) ? std::stoull(argv[3]) : 42;
-        const std::size_t orders = (argc >= 5) ? std::stoul(argv[4]) : 200;
-        return publish(port, seed, orders);
+        return run_publish(argc, argv);
     }
-    std::cerr << "usage:\n  qsl-mdfeed subscribe <port> [count] [rcvbuf_bytes]\n  qsl-mdfeed "
-                 "publish <port> [seed] [orders]\n";
-    return 2;
+    return usage();
 }
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 45669f1..9c862f5 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -82,6 +82,38 @@ add_test(
           -D "EXPECT_OUTPUT=orders must be an unsigned integer" -P
           "${CMAKE_CURRENT_LIST_DIR}/cmake/expect_command_failure.cmake")
 
+# CLI argument hardening: malformed numeric args must print usage and exit non-zero, not throw out
+# of main (std::terminate) or silently truncate an out-of-range port.
+add_test(
+  NAME qsl_client_invalid_port_fails
+  COMMAND ${CMAKE_COMMAND} -D "PROGRAM=$<TARGET_FILE:qsl-client>" -D "ARGS=99xyz" -D
+          "EXPECT_OUTPUT=usage:" -P
+          "${CMAKE_CURRENT_LIST_DIR}/cmake/expect_command_failure.cmake")
+
+add_test(
+  NAME qsl_client_out_of_range_port_fails
+  COMMAND ${CMAKE_COMMAND} -D "PROGRAM=$<TARGET_FILE:qsl-client>" -D "ARGS=70000" -D
+          "EXPECT_OUTPUT=usage:" -P
+          "${CMAKE_CURRENT_LIST_DIR}/cmake/expect_command_failure.cmake")
+
+add_test(
+  NAME qsl_mdfeed_invalid_count_fails
+  COMMAND
+    ${CMAKE_COMMAND} -D "PROGRAM=$<TARGET_FILE:qsl-mdfeed>" -D "ARGS=subscribe;9009;notanumber" -D
+    "EXPECT_OUTPUT=usage:" -P "${CMAKE_CURRENT_LIST_DIR}/cmake/expect_command_failure.cmake")
+
+add_test(
+  NAME qsl_mdfeed_out_of_range_port_fails
+  COMMAND ${CMAKE_COMMAND} -D "PROGRAM=$<TARGET_FILE:qsl-mdfeed>" -D "ARGS=publish;70000" -D
+          "EXPECT_OUTPUT=usage:" -P
+          "${CMAKE_CURRENT_LIST_DIR}/cmake/expect_command_failure.cmake")
+
+add_test(
+  NAME qsl_export_fixture_invalid_seed_fails
+  COMMAND ${CMAKE_COMMAND} -D "PROGRAM=$<TARGET_FILE:qsl-export-fixture>" -D "ARGS=abc" -D
+          "EXPECT_OUTPUT=usage:" -P
+          "${CMAKE_CURRENT_LIST_DIR}/cmake/expect_command_failure.cmake")
+
 # Shell unit tests for the shared artifact-publish helper (MAC sanitization +
 # trailing-whitespace/blank-line trimming). Portable (sed/awk/mktemp); the script
 # locates the repo via BASH_SOURCE, so it runs correctly by absolute path.