Say what is true for a TLV too, and cover the to half of the check

README.md

@@ -288,11 +288,11 @@ await session.sendSms({
 ```
 
 **Addresses.** `sourceAddrTon` and `destinationAddrTon` default to 5 for an alphanumeric address
-and 1 for a numeric one; the NPI fields default to 0. An address is latin1: `Kaffeé` goes out as the
-six octets that spell it, `4B 61 66 66 65 E9`, and an address you received always sends back. One
-outside `/^[\u0001-ÿ]*$/` is refused, naming the character and its index — strip or
-transliterate it first. An SMSC may still refuse a non-ASCII sender of its own accord, which reaches
-you as `ESME_RINVSRCADR`.
+and 1 for a numeric one; the NPI fields default to 0. An address is latin1: `Kaffeé` goes out as
+the six octets that spell it, `4B 61 66 66 65 E9`, and an address you received always sends back.
+One outside `/^[\u0001-\u00FF]*$/` is refused, naming the character and its index — strip or
+transliterate it first. An SMSC may still refuse a non-ASCII sender of its own accord, which
+reaches you as a refusal such as `ESME_RINVSRCADR`.
 
 **Encoding.**
 
@@ -346,9 +346,10 @@ you formatted. Refused before anything goes out: an invalid `Date`, `NaN`, `Infi
 count, and a count past 99 days 23:59:59, since a count in seconds is spelled in days and below.
 Name a later instant as a `Date`, which goes out absolute.
 
-**What gets checked.** The library checks what it composes: an address you gave as `from` or `to`, an
-alphabet or a time you named, a string body under a `data_coding` you named. What you formed yourself, a `Buffer` body or a stamp you
-formatted, passes through as written. The same rule holds for `session.send()`.
+**What gets checked.** The library checks what it composes: an address you gave as `from` or `to`,
+an alphabet or a time you named, a string body under a `data_coding` you named. What you formed
+yourself, a `Buffer` body or a stamp you formatted, passes through as written. The same rule holds
+for `session.send()`.
 
 ## Session
 

docs/decisions.md

@@ -483,24 +483,21 @@ rule and an index of the titles below.
   [#16](https://gitea.larvit.se/larvit/smpp-js/pulls/16). 3.4 calls these fields ASCII, so goal 3
   settles the read alone — its generous clause is scoped to reading, and its sender clause is strict.
   Goal 1 settles the write, being 3.4 as SMSCs actually run it: an operator routing an alphanumeric
-  sender through the upper half is traffic to keep, and Node's `ascii` write already emitted the low
-  octet, so `Kaffeé` went out as `4B 61 66 66 65 E9` before this change and goes out as the same
-  octets after it. Naming the write latin1 is what makes the round trip idempotent, and no peer sees
-  a change. Goal 2 settles the refusals, both of them a `size()` that would have agreed with a
-  `write()` that put something else on the wire: a character past `U+00FF` written as its low octet,
-  and a caller's own `U+0000`, which the peer reads as the end of the field, shifting every mandatory
-  field behind it under a `command_length` that counted the whole string. Goal 4 settles them twice
-  over, since for `一`, `　` and most emoji that low octet is `0x00` and the field went out malformed
-  on the operator's parser. `wantText()` and
-  `wantCstringText()` are the only two places that decide it, which is why the `dest_address` and
+  sender through the upper half is traffic to keep, and Node's `ascii` write already put those octets
+  on the wire, so naming the write latin1 makes the round trip idempotent and no peer sees a change.
+  Goal 2 settles the refusals, both of them a `size()` that would have agreed with a `write()` that
+  put something else on the wire: a character past `U+00FF` written as its low octet, and a caller's
+  own `U+0000`, which a mandatory field's reader takes as the end of the field. Goal 4 settles them
+  twice over: for a great many characters that low octet is `0x00`, and the PDU went out malformed
+  on the operator's parser. `wantText()` and `wantCstringText()` are the only two places that decide
+  it, which is why the `dest_address` and
   `unsuccess_sme` structures write their embedded addresses through `cstring.write()` rather than
   reaching past it into `writeCstring()`. Rejected: reading latin1 and leaving the write spelled
   ASCII, which leaves two halves agreeing only by accident. Rejected: refusing the upper half on send
   to stay strict to 3.4's ASCII, which would be a new restriction taking away traffic this library
-  already sends and operators already accept, on no defect; whether an SMSC wants a non-ASCII sender
-  stays its own call, answered as `ESME_RINVSRCADR`. Rejected:
-  refusing `U+0000` in every text field, which would buy one spelling by taking a legitimate octet
-  away from the length-prefixed Octet String, whose length octet is what ends it.
+  already sends and operators already accept, on no defect. Rejected: refusing `U+0000` in every
+  text field, which would buy one spelling by taking a legitimate octet away from the
+  length-prefixed Octet String, whose length octet is what ends it.
 
 ## The session's life
 

src/defs/types.ts

@@ -96,7 +96,7 @@ function pastLatin1(text: string): { err: Error } | undefined {
 
 function wantText(value: ParamValue): Result<{ text: string }> {
 	if (typeof value !== 'number' && typeof value !== 'string') {
-		return { err: new Error(`Expected a string, got ${typeof value}`) };
+		return { err: new Error(`Expected a string or a number, got ${typeof value}`) };
 	}
 
 	const text = String(value);
@@ -114,9 +114,7 @@ function wantCstringText(value: ParamValue): Result<{ text: string }> {
 	if (index === -1) return { text };
 
 	return {
-		err: new Error(
-			`U+0000 at index ${String(index)} would end the C-Octet String there, so the peer would read every field behind it shifted`,
-		),
+		err: new Error(`U+0000 at index ${String(index)} would end the C-Octet String there`),
 	};
 }
 

test/unsendable.test.ts

@@ -350,19 +350,20 @@ describe('an address the field cannot carry', () => {
 		const smsc = await dummySmsc(t);
 		const session = await bindToSmsc(t, smsc.port, { reconnect: false });
 
-		const refusals: [string, RegExp][] = [
-			['46701113311\u0000EVIL', /U\+0000 at index 11/],
-			['Kaffe一', /"一" \(U\+4E00\) at index 5/],
-			['😀', /"😀" \(U\+1F600\) at index 0/],
+		const refusals: ['from' | 'to', string, RegExp][] = [
+			['from', '46701113311\u0000EVIL', /U\+0000 at index 11/],
+			['from', 'Kaffe一', /"一" \(U\+4E00\) at index 5/],
+			['from', '😀', /"😀" \(U\+1F600\) at index 0/],
+			['to', 'Kaffe一', /"一" \(U\+4E00\) at index 5/],
 		];
 
-		for (const [sender, names] of refusals) {
-			const sent = await session.sendSms({ from: sender, message: 'Hello world', to });
+		for (const [option, address, names] of refusals) {
+			const sent = await session.sendSms({ from, message: 'Hello world', to, [option]: address });
 
-			assert.ok(sent.err instanceof Error, sender);
-			assert.match(sent.err.message, /^from: /, 'names the option the caller wrote, not the wire field');
+			assert.ok(sent.err instanceof Error, address);
+			assert.match(sent.err.message, new RegExp(`^${option}: `), 'names the option the caller wrote');
 			assert.match(sent.err.message, names);
-			assert.deepEqual(sent.smsIds, [], sender);
+			assert.deepEqual(sent.smsIds, [], address);
 		}
 
 		assert.deepEqual(smsc.octets, [], 'an address the field cannot carry never reaches the socket');

todo.md

@@ -189,6 +189,12 @@ and is also what the panel ranked hardest — two methods, one answer.
 
 ### Correctness, ahead of everything below
 
+- [ ] **Refuse a non-finite number where a text field coerces one.** `wantText()` in `defs/types.ts`
+      stringifies a number so `message_id: 123` writes `"123"`, which is deliberate and tested. It
+      takes `NaN` and `Infinity` on the same path, so `sendSms({ from: NaN })` puts the literal sender
+      `NaN` on the wire and reports success — goal 2. Gate the numeric branch on `Number.isFinite`.
+      Predates the latin1 guard; found by the stability review of #16.
+
 - [ ] **Answer `alert_notification` and `outbind` by not answering them.** Both are response-less in
       SMPP 3.4, both fall through `route()`'s default into `unhandled()`, which calls
       `sendReturn(pduObj, 'ESME_RINVCMDID')`; `pduReturn()` then finds no response command, and the