Fast, hardware-free unit tests for pure and host-simulated logic, run with GoogleTest through PlatformIO. The native environment runs every suite except the KISS modem; native_kiss_modem builds that suite with its separate source filter. Hardware, real radio, AsyncTCP, Wi-Fi/MQTT, and SoftAP behavior still require integration or target testing; see "Local testing without hardware" in MQTT_IMPLEMENTATION.md.

Running

pio test -e native                      # all suites except KISS modem
pio test -e native_kiss_modem           # KISS modem suite
pio test -e native -f test_webconfig_keys   # a single suite

A green [PASSED] per suite means GoogleTest returned 0 (all assertions passed). PlatformIO's "0 test cases" line is just its Unity-style counter and does not reflect the GoogleTest count -- run the built binary directly (.pio/build/native/program) to see the per-assertion breakdown.

Suites

Suite	Source under test	Covers
`test_mqtt_presets`	`src/helpers/MQTTPresets.h`	preset lookup; table integrity (unique names, non-empty URLs, JWT-audience invariant, names fit the slot buffer); `mqttPresetNeedsSlotCredentials`; slot-count constants
`test_observer_validation`	`src/helpers/MQTTObserverValidation.h`	IATA (exactly 3 alphanumerics), owner key (64 hex), NTP hostname, and the buffer-fit check behind the #17 length validation -- including boundaries and nulls
`test_webconfig_keys`	`src/helpers/WebConfigKeys.h`	POST-key allowlist, secret detection, admin-password and browser-terminal command validation, slot-index bounds, and short-input guards
`test_webconfig_batch`	`src/helpers/WebConfigBatch.h`	config POST/replay/busy decisions; paced command drain; setup WiFi validation/IP handoff; reboot confirmation/fallback; stop/refcount gating; exact timing and rollover boundaries
`test_topic_template`	`src/helpers/MQTTTopicTemplate.h`	`{iata}/{device}/{token}/{type}` expansion, overflow/NUL-termination, and a buffer-size fuzz
`test_mqtt_topic_router`	`src/helpers/MQTTTopicRouter.h`	complete preset/custom topic-routing contract; MeshRank all types except raw; required identifiers; invalid inputs/slots; exact buffer boundaries
`test_mqtt_connection_policy`	`src/helpers/MQTTConnectionPolicy.h`	reconnect guard/backoff/stagger and breaker transitions; stable reset; JWT lifetime/renewal policy; exact timing boundaries and 32-bit `millis()` rollover
`test_wifi_reconnect_policy`	`src/helpers/WiFiReconnectPolicy.h`	five-minute forced reconnect cadence, connection resets, duplicate disconnect observations, and 32-bit `millis()` rollover
`test_radio_liveness`	`src/helpers/RadioLivenessTracker.h`, `src/helpers/radiolib/RxBoostedGainDefaults.h`	staged radio recovery, activity reset and rollover handling; target-specific RX boosted-gain defaults and SX126x precedence
`test_lr1110_rx_recovery`	`src/helpers/radiolib/LR1110RxRecovery.h`	LR1110 four-byte RX-buffer shift signature; captured and accumulated shifts; ordinary/scoped packet exclusions
`test_mqtt_packet_queue_policy`	`src/helpers/MQTTPacketQueuePolicy.h`	queue-full eviction; stale-disconnect flush; adaptive drain limits; bounded QoS0 retries; exact timing boundaries and 32-bit `millis()` rollover
`test_mqtt_packet_filter`	`src/helpers/MQTTPacketFilter.h`	per-slot 0-15 allowlist parsing/formatting, numeric and named spellings; exact bounds; membership; candidate/eligible split and retry-completion policy; pre-queue union gate; default-mask detection
`test_mqtt_runtime_buffer_lifecycle`	`src/helpers/MQTTRuntimeBufferLifecycle.h`	idempotent allocation/release; partial-allocation degradation; retry of only missing buffers
`test_mqtt_prefs_codec`	`src/helpers/MQTTPrefsStorage.h`, `src/helpers/MQTTPrefsCodec.h`	binary pre-slot/3-slot/6-slot migration fixtures; v1 header integrity; downgrade preservation; shortest-payload write policy (default filters stay downgrade-readable)
`test_mqtt_prefs_atomic_store`	`src/helpers/MQTTPrefsAtomicStore.h`	transactional MQTT writes and legacy `/node_prefs` handoff; exact short-write detection; begin/finish/rename failure cleanup; original-file preservation
`test_prefs_save_routing`	`src/helpers/PrefsSaveRouting.h`	runtime common/observer setters write only their owning preference image; mixed-owner setters and migrations can deliberately write both
`test_mqtt_payload_builder`	`src/helpers/MQTTPayloadBuilder.cpp`	status/packet/raw JSON contracts; optional fields; escaping; RX metrics and path; score handling; exact buffer bounds; maximum representative payloads
`test_telemetry_history`	`src/helpers/TelemetryHistory.h`	30-minute rings; seven-day temperature/voltage and dynamically sized GPS retention; exact 1 C temperature/status encoding; separate Base64 series payloads; 14-bit GPS differentials; resize preservation, heap budgets, and 1-based paging bounds
`test_flood_filter_policy`	`src/helpers/FloodFilterPolicy.h`	unordered blacklist matching; ordered 1/2/3-byte pbyte rule prefixes; original incoming scope classes and canonical region-name identity; channel-authentication cache key comparison; priority ordering and terminal stop masks; bridge-bucket and regionless channel-target selector encoding; `require=region` and per-channel scope-gate truth tables; fast/slow timing; adding, replacing, and preserving packet scope
`test_logical_message_cache`	`src/helpers/LogicalMessageCache.h`	bounded logical-message mapping; stable retry timestamps; exact older retries after newer messages; stale and same-timestamp mismatch rejection
`test_cli_command_utils`	`src/helpers/CLICommandUtils.h`, `src/helpers/ContactListOrder.h`, `src/helpers/TerminalCommandTracker.h`, `src/helpers/TerminalDisplayFilter.h`	terminal verb/argument/path parsing; routed receive labels; quiet display defaults and independent emergency filtering; favorite-first contact ordering; single-command reply matching, round-trip timing, and rollover-safe expiration
`test_identity_generation`	`src/helpers/IdentityGeneration.h`	reserved-prefix rejection; bounded retries; final provisioned attempt; fail-closed exhaustion
`test_remote_cli_reply_cache`	`src/helpers/RemoteCliReplyCache.h`, `src/helpers/RemoteCliRequest.h`, `src/helpers/RemoteCliTimeout.h`	authenticated logical-request matching; bounded recent-reply history; backward-compatible retry identity; 300% response timeout; empty-response completion; on-air truncation and clearing
`test_companion_frame_queue`	`src/helpers/CompanionFrameQueue.h`	response/required/best-effort classification; reserved capacity; stable priority; safe eviction; message-waiting coalescing
`test_serial_mode_switch`	`src/helpers/ArduinoSerialInterface.cpp`, `src/helpers/MultiSerialInterface.h`	terminal/seeder control-sequence recognition and passthrough ownership; queued/atomic USB output under backpressure and short writes; partial-frame busy state; requester-affine replies, locked contact streams, and Bluetooth-only pairing routing
`test_ble_tx_stall_watchdog`	`src/helpers/BleTxStallWatchdog.h`	exact BLE fragment progress; blocked-reply timeout; rollover-safe elapsed time; disconnect recovery retry and completion
`test_atomic_file_writer`	`src/helpers/AtomicFileWriter.h`	verified temporary-file commit; short-write, readback, validation, and rename failures; preservation of the live file and stale-temp cleanup
`test_cad_timing`	`src/helpers/radiolib/CadTiming.h`, `LR2021SideDetectorConfig.h`, `RadioAirtime.h`	Cascade and slow-profile CAD deadlines; invalid airtime handling; bounded LR2021 side-detector parsing and LDRO recomputation
`test_companion_node_prefs`	`examples/companion_radio/NodePrefs.h`	independent device power saving, RXPS, Wi-Fi, and FEM preferences; one-time migration of the regressed power-saving default
`test_config_serializer`	`src/helpers/ConfigSerializer.cpp`, Companion `NodePrefs`	escaped config save/load, whitespace and malformed input, unknown fields, and FEM preference round trips
`test_deferred_cli_command`	`src/helpers/DeferredCliCommand.h`	copying authenticated command context, single-pending-command enforcement, clearing, and length rejection
`test_kiss_modem`	`examples/kiss_modem/KissModem.cpp`	KISS escaping/framing and packet metadata under partial writes, host TX backpressure, queue saturation, and radio completion; run with `native_kiss_modem`
`test_mesh_tables`	`src/helpers/SimpleMeshTables.h`	packet and ACK/multipart deduplication, scope-independent identity, route-prefix matching, and deterministic recent-repeater expiry/eviction
`test_mqtt_lifecycle`	`src/helpers/MQTTLifecycle.h`	idempotent start/stop, initialization rollback, cooperative stop acknowledgment and timeout, callback ownership, restart, and the OTA flash barrier
`test_mqtt_reply_format`	`src/helpers/MQTTReplyFormat.h`	bounded formatted appends, exact-fit and one-byte buffers, truncation, NUL termination, and invalid starting positions
`test_packet_manager`	`src/Packet.cpp`, `src/Dispatcher.cpp`, `src/helpers/StaticPoolPacketManager.cpp`	truncated-packet rejection, unavailable-radio behavior, scoped RX-delay replacement, queue/CAD scheduling, and staged radio/TX recovery
`test_persistent_store_format`	`src/helpers/PersistentStoreFormat.h`	contact-page headers and CRCs, dirty-page state, stable slot allocation, and bounded resumable legacy migration across power loss
`test_power_management`	`src/helpers/PowerManagementUtils.h`	median filtering of a brownout outlier and valid-reading requirements for the boot lock
`test_region_names`	`src/helpers/RegionNameUtils.h`	canonical public-region markers while preserving distinct private and differently named regions
`test_routing_policy`	`src/helpers/RoutingPolicy.h`	scoped/unscoped flood hop limits and selection of direct, path-return, mirrored-scope, default-scope, or unscoped replies
`test_rs232_uart`	`src/helpers/bridges/RS232UartUtils.h`	stopping the active UART peripheral before reassigning its pins
`test_security_session_timer`	`src/helpers/nrf52/SecuritySessionTimer.h`	two-minute security-session expiry, cancellation, restart, and `millis()` rollover
`test_trace_path_helpers`	`src/helpers/TracePathHelpers.h`	round-trip route construction, hash-width conversion, raw path parsing/limits, and terminal trace timeout bounds
`test_user_gpio`	`src/helpers/UserGpio.cpp`, `UserGpioReplyTracker.h`	board-approved pins, get/set/reset, timed nonblocking transitions, duplicate suppression, rollover, and completion-reply routing
`test_utf8_helpers`	`src/helpers/UTF8Helpers.h`	byte-limit truncation at complete code-point boundaries and rejection of malformed or truncated UTF-8
`test_wifi_ota_seeder_policy`	`src/helpers/WiFiOtaSeederPolicy.h`, `WiFiOtaSeederStatus.h`	listener state versus network availability, serial/TCP folder ownership, detach detection, and bounded status formatting
`test_ota`	`src/helpers/ota/`	v2 application/v3 bootloader parser separation; legacy XIAO, generic internal, and exact MeshTower V2 SD embedded identity, vector, capability, explicit-confirmation, codec-isolation, scratch-headroom/shared-slot no-EndF gates, and no-autofetch gates; container and EndF integrity; protocol codecs; transfer, resume, and layered apply safety; adaptive 2-to-4 block-request window growth and stall contraction; active-transfer priority classification
`test_trace_retry`	`src/Mesh.cpp` retry and relay policy	app-v2 and boot-v3 traffic sharing `PAYLOAD_TYPE_OTA=0x0C` and the TempRadio suspend policy; opaque OTA relay behavior; background discovery priority; immediate primary transfer relay, receive-delay bypass, fast CAD retry, and no generic flood retry; trace and non-OTA flood retry timing
`test_utils`	`src/Utils.cpp`	`Utils::toHex` (upstream)

Conventions (and how to add a suite)

Each test/test_<name>/ directory builds into its own GoogleTest program and must define its own main() (::testing::InitGoogleTest + RUN_ALL_TESTS).
Tests are host-only: include only pure headers. Arduino/crypto stubs live in test/mocks/ (on the include path via -I test/mocks).
Firmware headers are included from src (via -I src, e.g. #include "helpers/MQTTPresets.h"). Some are guarded or ESP-flavored, so a suite may need shims before the include -- e.g. test_mqtt_presets does #define WITH_MQTT_BRIDGE 1 (the preset table is behind that flag) and #define PROGMEM (the embedded CA-cert strings are PROGMEM-qualified).
To add a suite: create test/test_<name>/test_<name>.cpp with a main(), and add any host-only source it links to the native env's build_src_filter in platformio.ini (header-only code needs no source entry). No other wiring.
Keep logic testable by extracting pure functions into headers (as MQTTObserverValidation.h / WebConfigKeys.h / MQTTTopicTemplate.h do) and having the firmware call the same functions.