# AGENTS.md

Guidance for LLM agents working in this repository. Human-facing documentation lives in
[README.md](README.md); the remaining work is tracked in [todo.md](todo.md).

## What this is

A ground-up TypeScript rewrite of `larvitsmpp` 0.4.0, published as `@larvit/smpp` 1.0.0. The branch
started from an orphan commit — no history from 0.4.0 is carried over. The 0.4.0 source is still
readable on the `master` branch of the same repository and is the reference for protocol behaviour,
not for structure or style.

The library's value is its very small API. Do not grow the public surface without being asked.

## Hard rules

These are not preferences. Breaking one is a defect.

1. **Nothing throws.** Every fallible function returns (or resolves to) a DTO carrying an optional
   `err`. No `throw`, no rejected promises, no exceptions as control flow. Node APIs that throw are
   wrapped at the boundary and converted into a result. Programmer errors (bad arguments) are
   results too.
2. **Log messages are static strings.** Every dynamic value goes into the log metadata. Never
   interpolate, never concatenate.
   - GOOD: `log.debug('sendSms() - splitting message', { parts: msgs.length, to });`
   - BANNED: `log.debug('sendSms() - splitting into ' + msgs.length + ' parts');`
3. **No `error` event.** Node makes an unhandled `error` event throw, which would break rule 1.
   Sessions emit `sessionError`, servers emit `serverError`.
4. **No casts, no non-null assertions.** `as`, `as unknown as` and `!` are all banned. Parse untyped
   input once through a type guard at the boundary; everything past it is typed. `noUncheckedIndexedAccess`
   is on, so every lookup into a record or buffer is `T | undefined` until you handle it — that is the
   point, not an obstacle to route around.

## Architecture

```
src/
	index.ts             Public surface. Named exports only, no default export.
	client.ts            client() -> { err, session }
	server.ts            server() -> { err, server }, server owns the listener + close()
	session.ts           Session: dispatch, events, and the collaborators below
	sms.ts               The live handle emitted as the 'sms' event (sendResp/sendDlr)
	dlr.ts               Delivery receipts: text and TLV parsing, receipt status codes
	dlr-merger.ts        DlrMerger: per-segment receipts counted into one MessageDlr
	expiring-groups.ts   ExpiringGroups: the capped, expiring store both of those share
	incoming-requests.ts Every request the peer sends: messages, receipts, links, unknown commands
	link-timers.ts       LinkTimers: the enquire_link heartbeat and the idle timeout
	log.ts               SmppLog, the logger contract, and silentLog — the default
	message.ts           Encoding detection, splitting, bit counting, SMPP date formatting
	pdu.ts               pduToObj / objToPdu / pduReturn — synchronous, result-returning
	pdu-framer.ts        PduFramer: a byte stream cut into complete PDUs
	pending-requests.ts  PendingRequests: sequence numbers, correlation, timeout, abort
	reassembly.ts        Reassembler: capped, expiring multipart groups
	reconnect-loop.ts    ReconnectLoop: backoff, retry timer, stopped-ness
	result.ts            Result<T> — the shape every fallible call returns
	send-sms.ts          submitSms composition and the submitSmParams builder
	send-window.ts       SendWindow: the maxOutstanding semaphore
	session-options.ts   SessionOptions, ReconnectOptions, bind direction and the session defaults
	udh.ts               User data header: the concatenation fields of a long SMS
	uuid.ts              uuidv7() — the ids the library generates for messages
	defs/
		commands.ts      The 33 commands, their ids and ordered parameter lists
		constants.ts     consts + constsById, and the SMPP version constants
		encodings.ts     GSM 03.38, LATIN1, UCS2, detection, data_coding resolution
		errors.ts        errors + errorsById (ESME_*)
		tlvs.ts          TLV definitions, tlvsById
		types.ts         Wire types: int8/int16/int32/string/cstring/buffer/arrays
```

Dependency direction is one way: `defs` knows nothing above it, `pdu` uses `defs`, `session` uses
`pdu`, and `client`/`server` use `session`. Nothing reaches back up.

**Parameter order is wire order.** The key order inside `cmds.*.params` is the order the fields are
written to and read from the buffer. Never sort those alphabetically — the alphabetical-ordering
convention applies everywhere else, but here it corrupts every PDU.

## Toolchain

Run everything through the container; never invoke node or npm on the host.

```bash
docker compose run --rm node npm install
docker compose run --rm node npm test
docker compose run --rm node npm run build
```

- Tests are `.ts` and run directly under Node's type stripping — no build step in the dev loop.
- Source imports use `.ts` extensions; `rewriteRelativeImportExtensions` emits `.js` into `dist`.
- `erasableSyntaxOnly` is on, so no enums, no namespaces, no parameter properties. Use `as const`
  objects plus union types.
- The published floor is Node 18, but the dev container runs Node 24 (type stripping needs it). CI
  compiles the tests and runs them on 18/20/22/24, so the floor is verified rather than asserted.
- `typescript` is pinned to the 6.x line because `typescript-eslint` peer-requires `<6.1.0`. Move to
  TypeScript 7 once that constraint lifts.

## Defects found in 0.4.0

Confirmed by reading the 0.4.0 source. The rewrite fixes all of them; each needs a regression test
naming the behaviour, and the wire-affecting ones are cross-checked against a reference
implementation (see todo.md).

| Defect | 0.4.0 behaviour |
| --- | --- |
| LATIN1 never decodes | `decodeMsg` loops `consts.ENCODING` without breaking, so `data_coding` 0x03 lands on the alias `ISO_8859_1`, which has no decoder, and silently falls back to ASCII |
| Short segments | `splitMsg` accumulates a full segment then pushes `msgPart.slice(0, -1)`, so every segment is one character short: 152 GSM characters instead of 153, 66 UCS2 instead of 67. Long messages are split into more segments than they need, and each extra segment is billed |
| DLR month off by one | `smppDate()` uses `getMonth()` (0-based) without `+1`, so January renders as `00` |
| Non-standard DLR status | Receipts emit `stat:UNDELIVERABLE`; the spec's field is 7 characters (`UNDELIV`) |
| Flash destroys UCS2 | `flash: true` overwrites `data_coding` with 0x10, discarding the UCS2 alphabet, which needs 0x18 |
| Shared concat reference | The concatenation reference counter is a module-level global shared by every session in the process |
| `send()` never times out | Each call adds a listener keyed on the sequence number; a peer that never answers leaks it and the promise never settles |
| `tls: true` is not TLS | Constructs a bare `new tls.Socket()` with no handshake instead of `tls.connect()` |
| Alphanumeric sender TON | `sendSms` hardcodes `source_addr_ton` to 1 (international) even for alphanumeric senders, which require TON 5 |
| Text-only DLRs refused | `deliver_sm` without both `message_state` and `receipted_message_id` TLVs is rejected with `ESME_RINVTLVSTREAM`, so Kannel-style receipts are unusable |
| Unbounded reassembly | Incomplete long-SMS groups are capped by nothing and swept only when other traffic arrives, after 24 hours |
| Dead DLR aggregation | `longSmsDlrs` is allocated to merge per-segment receipts and then never used |
| Trailing NULL truncation | `types.buffer.size()` subtracts one whenever the value's last octet is `0x00`, so the PDU is allocated one octet short while `sm_length` still reports the full length. Any UCS2 message ending in a character like U+4E00 or U+3000 goes out corrupt |
| Dormant filters | `defs.filters` is declared on commands and TLVs but never invoked anywhere. Dropped in the rewrite; SMPP time formatting is exported as `smppTime` instead |
| Unchecked reads | Wire reads index straight into the buffer, so a short or malformed PDU throws out of the codec. Reads are bounds-checked and return results now |
| Unrangechecked writes | Integer params are handed to `writeUInt8`/`writeUInt16BE` unvalidated, so an out-of-range value throws from inside Node |
| `submit_multi` missing `sm_length` | The field is commented out of the command table, so `short_message` never round-trips for that command |
| Per-parameter defaults never applied | `calcCmdLength` reads `paramType.default` (the wire type's) rather than the parameter's, so `interface_version: 0x50` on the bind commands did nothing and every bind declared version 0x00 |
| `source_telematics_id` width | Defined as a 2-octet integer; SMPP 3.4 5.3.2.8 makes it 1 octet, unlike `dest_telematics_id`, which really is 2 |
| Binary payloads decoded as text | `data_coding` 0x02, 0x04, 0x14 and 0xF4-0xF7 are 8-bit binary and land on the GSM 03.38 table, which rewrites every octet outside it. They resolve to LATIN1 now, so the payload survives as bytes |
| `ESME_RINVBCASTCHANIND` typo | Defined as `0x011`, three hex digits; the spec value is `0x0112` |

## Multipart sends and the send window

`sendSms` puts every segment of a message on the wire together instead of waiting for each response
in turn. This is not an optimisation: this library's own server holds segments until the whole
message is reassembled before it answers any of them, so sending them one-after-a-response
deadlocks. It follows that a message with more segments than `maxOutstanding` cannot be delivered to
a server that defers responses that way — real SMSCs answer each `submit_sm` immediately, so this
only bites when both ends are this library.

## GSM 7-bit is sent unpacked

Over SMPP the ESME puts one GSM character per octet in `short_message` and the SMSC packs it into
septets. The 140-octet limit applies to that packed result, not to what goes on the wire here, which
is why a concatenated segment is 153 characters plus a 6-octet UDH — 159 octets in `short_message`,
and entirely correct. Do not "fix" this to 134; that number is the packed payload size and would
truncate every long message by a fifth.

UCS2 is not packed, so there the two coincide: 67 characters = 134 octets, plus the 6-octet UDH is
exactly 140.

## Conventions

- Hard tabs. Alphabetical ordering for keys, imports and lists unless order is logic-significant.
  Two deliberate exceptions: command parameters are in wire order (above), and the `errors` and TLV
  tables are ordered by their numeric id so they can be diffed against the spec and gaps stay visible.
- Comments are the exception, not the default — see the root `CLAUDE.md` rules. Do not write file
  preambles or restate what the code says.
- Test data uses real randomised UUID v7 values, never `aaaa-0000` placeholders.
- `message_id` values the library generates are UUID v7.
- A test that needs a dummy peer must `resume()` its sockets. An unread socket never processes the
  peer's FIN, so `server.close()` hangs forever — that is a test bug, not a library one.
- `assert.equal` from `node:assert/strict` narrows its first argument, so a following `?.` on the
  same value is flagged as unnecessary. Assert once with `assert.ok(x)` and use plain access after.

## Decisions

- **A close arriving after our own `unbind` is a clean unbind, not an error.** Maintainer's call,
  2026-08-26: most SMSCs drop the socket instead of answering, so the documented shutdown would
  otherwise always report a failure. It does mask a socket that died mid-unbind for an unrelated
  reason, which is accepted — the peer sees the same TCP close either way.
- **The published surface is frozen at what `src/index.ts` exports today.** `Session` is exported and
  publicly constructible, which is why `SessionOptions` and `ReconnectOptions` are public too — that
  is correct, not a leak, and it has been raised twice. The collaborators `session.ts` delegates to
  (`Reassembler`, `PendingRequests`, `SendWindow`, `ReconnectLoop`, `LinkTimers`, `DlrMerger`,
  `submitSms`) stay unpublished so they can be reshaped.
- **The sub-3.4 optional-parameter rule is a predicate, not a chokepoint.** `acceptsOptionalParams()`
  is consulted by the library's own senders; `session.send({ tlvs })` is passed through as written,
  because silently stripping a caller's explicit TLVs off a deliberately public low-level surface
  would be worse than sending them. The guarantee is "what this library sends honours the rule",
  never "the session cannot send optional parameters to an old peer".
- **Both ends feed `peerInterfaceVersion`, and a peer that declared nothing is pre-3.4.**
  `acceptBind()` records what the ESME declared in its bind request; the client's `bind()` records
  the `sc_interface_version` the SMSC answered with. A peer that declared no version is recorded as
  `undeclaredInterfaceVersion` (0x00) and is sent no optional parameters — the spec reads an absent
  `sc_interface_version` as an SMSC that supports none. `undefined` is left to mean one thing only:
  no bind has been accepted on this session yet.
- **The library speaks SMPP 3.4 on the wire, and `defs/` keeps the 5.0 tables as a superset.**
  Maintainer's call, 2026-08-26: 3.4 is what SMSCs actually run, while the wider tables let the codec
  parse and build whatever a peer sends. The declared version is an option on both `client()` and
  `server()`, so an implementation that needs 5.0 throughout can have it. The threshold at or above
  which a peer may be sent optional parameters is fixed at 0x34 by the spec and is not the same
  constant as the declared version.
- **Bind direction is enforced on the library's own senders and on everything incoming, not on
  `send()`.** A receiver-bound ESME carries no `submit_sm` and a transmitter-bound one no
  `deliver_sm`; `sendSms()` and `sendDlr()` refuse locally, and an arriving PDU is answered
  `ESME_RINVBNDSTS`. `bindAllows()` is a predicate on the same footing as `acceptsOptionalParams()`,
  so the deliberately public low-level `send()` stays a passthrough. Only those two commands are
  policed, because they are the only ones the library sends and dispatches by direction.

- **The logger is a five-method contract this library declares, not a dependency.** `SmppLog` in
  `log.ts` is what the code actually calls (`debug`, `error`, `info`, `verbose`, `warn`), so an
  application can satisfy it with an object literal and `@larvit/smpp` ships with no runtime
  dependencies. `@larvit/log` implements it structurally and stays a devDependency, where
  `test/tls.test.ts` passing a real `Log` as the server's logger keeps that compatibility compiled.

- **`esm_class` decides what a `deliver_sm` is, and the body is only read when it names nothing.**
  Message type `MC_DELIVERY_RECEIPT` (0x04) makes it a receipt whatever the body parses to, so a
  receipt in a format `dlrFromPdu()` cannot read reaches `dlr` with `smsId` undefined instead of
  arriving as an inbound SMS. Any other named type — delivery or user acknowledgement, conversation
  abort, intermediate notification — is not a receipt and its body is not scraped. A message type of
  0, or one of the ten the spec reserves, keeps the scrape: SMSCs that send text-only receipts leave
  `esm_class` at 0, and reading that as the spec's "default message type" would lose every one of
  them. A non-empty `receipted_message_id` TLV marks a receipt on the same footing there, since
  nothing but a receipt carries one. What gets scraped is the decoded `short_message` with any UDH
  stripped; a receipt this library recognises never reaches the reassembler, so an SMSC that splits one across segments gets a
  `dlr` per segment rather than one merged report.
  The `message_state` TLV is authoritative only where it names a state in the table — SMPP reserves
  0x80-0xFF for MC-vendor-specific values, so an unnameable one keeps its raw `statusId` and leaves
  `statusMsg` to the body.

- **The TLS tests build their own self-signed certificate in DER** (`test/tls.test.ts`) instead of
  adding a devDependency or shelling out to openssl. Maintainer's call, 2026-08-26: the dev image
  `node:24.18.0-bookworm-slim` ships no openssl binary, so a shelled-out fixture would pass in CI
  and fail on every developer machine, and a committed key leaks in a public repository. Valid while
  the dev image has no openssl.