fix(slack-notify): fit messages into Slack's 4,000-character limit

steps/slack-notify/slackNotify.js

@@ -22,6 +22,32 @@
     );
 };
 
+// chat.update rejects `text` over 4,000 characters (`msg_too_long`), and a long
+// release changelog gets there. The body is cut at the last line break that fits,
+// so no mrkdwn link is split, and the cut is marked. A short closing paragraph
+// survives the cut: release flags such as the DB-migration warning live there.
+const MAX_TEXT_LENGTH = 4000;
+const MAX_KEPT_TAIL_LENGTH = 500;
+const TRUNCATION_MARK = '\n… (truncated)';
+
+const truncateText = (text, limit = MAX_TEXT_LENGTH) => {
+    if (text.length <= limit) {
+        return text;
+    }
+    const trimmed = text.trimEnd();
+    if (trimmed.length <= limit) {
+        return trimmed;
+    }
+    const tailStart = trimmed.lastIndexOf('\n\n');
+    const tail =
+        tailStart > 0 && trimmed.length - tailStart <= MAX_KEPT_TAIL_LENGTH
+            ? trimmed.slice(tailStart)
+            : '';
+    const room = limit - TRUNCATION_MARK.length - tail.length;
+    const lineEnd = trimmed.lastIndexOf('\n', room);
+    return `${trimmed.slice(0, lineEnd > 0 ? lineEnd : room)}${TRUNCATION_MARK}${tail}`;
+};
+
 // Posts a new message, or edits an existing one when `updateTs` is set
 // (chat.update) — used to reflect lifecycle status (started → cancelled →
 // completed) in the release's head message rather than only as thread replies.
@@ -79,7 +105,7 @@
         req.end();
     });
 
-module.exports = { isValidSlackTs, markdownToMrkdwn };
+module.exports = { isValidSlackTs, markdownToMrkdwn, truncateText };
 
 // Standalone runner
 if (require.main === module) {
@@ -103,7 +129,15 @@
 
     console.log('Sending Slack notification.');
 
-    sendMessage(token, channel, markdownToMrkdwn(message), threadTs, updateTs)
+    const text = markdownToMrkdwn(message);
+    const fittedText = truncateText(text);
+    if (fittedText.length < text.trimEnd().length) {
+        console.log(
+            `::warning::Slack message truncated from ${text.length} to ${fittedText.length} characters.`,
+        );
+    }
+
+    sendMessage(token, channel, fittedText, threadTs, updateTs)
         .then((ts) => {
             if (!isValidSlackTs(ts)) {
                 console.error('Slack returned an invalid ts.');
@@ -117,8 +151,8 @@
                 console.log(`::set-output name=ts::${ts}`);
             }
         })
-        .catch(() => {
-            console.error('Failed to send Slack message.');
+        .catch((error) => {
+            console.error(`Failed to send Slack message: ${error.message}`);
             process.exit(1);
         });
 }

steps/slack-notify/slackNotify.test.js

@@ -1,6 +1,6 @@
 const test = require('node:test');
 const assert = require('node:assert/strict');
-const { markdownToMrkdwn } = require('./slackNotify');
+const { markdownToMrkdwn, truncateText } = require('./slackNotify');
 
 test('converts markdown bold to mrkdwn and leaves mrkdwn bold alone', () => {
     assert.equal(
@@ -17,3 +17,33 @@ test('converts links, headers and list items', () => {
         '*Features*\n• <https://x.y/z|APP-1: title>',
     );
 });
+
+test('keeps text within the limit unchanged', () => {
+    assert.equal(truncateText('a\nb', 3), 'a\nb');
+});
+
+test('cuts over-limit text at the last line break that fits and marks it', () => {
+    const line = '• <https://x.y/z|APP-1: title>';
+    const text = Array.from({ length: 200 }, () => line).join('\n');
+    const truncated = truncateText(text);
+
+    assert.ok(truncated.length <= 4000);
+    assert.ok(truncated.endsWith('\n… (truncated)'));
+    const kept = truncated.slice(0, -'\n… (truncated)'.length).split('\n');
+    assert.ok(kept.every((keptLine) => keptLine === line));
+});
+
+test('keeps a short closing paragraph after the cut', () => {
+    const list = Array.from({ length: 200 }, () => '• item').join('\n');
+    const truncated = truncateText(`${list}\n\n⚠️ *Contains DB migrations*\n`, 100);
+
+    assert.ok(truncated.length <= 100);
+    assert.ok(truncated.endsWith('\n… (truncated)\n\n⚠️ *Contains DB migrations*'));
+});
+
+test('hard-cuts a single over-limit line', () => {
+    const truncated = truncateText('x'.repeat(5000));
+
+    assert.equal(truncated.length, 4000);
+    assert.ok(truncated.endsWith('\n… (truncated)'));
+});