smpp-js/AGENTS.md

AGENTS.md

Guidance for LLM agents working in this repository. Human-facing documentation lives in README.md; the remaining work is tracked in 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 @larvit/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
	link-timers.ts       LinkTimers: the enquire_link heartbeat and the idle timeout
	log.ts               silentLog — the default when the application passes none
	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 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.

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
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".
• Only the server feeds peerInterfaceVersion. acceptBind() records what the peer declared; the client never reads sc_interface_version out of its bind response, so a client session is permissive. That is not a defect today — this library's ESME direction sends no TLVs at all — but anyone adding a client-side TLV owes the other half of the feed.
• 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.
• 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.