build!: parse yaml with more strict multiline indentation rules

.changeset/bump-js-yaml-5x.md

@@ -0,0 +1,17 @@
+---
+"@slack/slack-github-action": major
+---
+
+build: parse yaml with more strict multiline indentation rules
+
+Internal dependencies of [`js-yaml@v5`](https://github.com/nodeca/js-yaml/blob/master/CHANGELOG.md#500---2026-06-20) make YAML parsing more strict and compliant with the YAML specification. Indentation is now required for values that span multiple lines against the base value.
+
+See the YAML [line prefixes](https://yaml.org/spec/1.2.2/#63-line-prefixes) spec for the expected indentation rule:
+
+```diff
+  channel: "C0123"
+  text: "first line
+
+- second line"
++   second line"
+```

.github/workflows/integration.yml

@@ -114,7 +114,7 @@ jobs:
             channel: ${{ secrets.SLACK_CHANNEL_ID }}
             text: ":checkered_flag: A multiline message wraps across lines
 
-            with a blank line before <https://github.com/${{ github.repository }}|a link>"
+              with a blank line before <https://github.com/${{ github.repository }}|a link>"
 
       - name: "test(api): confirm the multiline message was posted"
         run: test -n "$MULTILINE_OUTPUT_TS"

package.json

@@ -48,7 +48,7 @@
     "@slack/web-api": "^8.0.0",
     "@slack/webhook": "^8.0.0",
     "flat": "^6.0.1",
-    "js-yaml": "^4.2.0",
+    "js-yaml": "^5.2.1",
     "markup-js": "^1.5.21",
     "undici": "^8.7.0"
   },

test/content.spec.js

@@ -77,7 +77,7 @@ describe("content", () => {
     it("parses multiline YAML from the input payload", async () => {
       mocks.core.getInput
         .withArgs("payload")
-        .returns('channel: C0123456789\ntext: "first line\n\nsecond line"');
+        .returns('channel: C0123456789\ntext: "first line\n\n  second line"');
       const config = new Config(mocks.core);
       const expected = {
         channel: "C0123456789",