fix: revert multiline yaml parsing indentation change

.changeset/revert-js-yaml-4x.md

@@ -0,0 +1,5 @@
+---
+"@slack/slack-github-action": patch
+---
+
+fix: revert multiline yaml parsing indentation change

.github/workflows/integration.yml

@@ -103,6 +103,24 @@ jobs:
         env:
           MESSAGE_OUTPUT_TS: ${{ steps.message.outputs.ts }}
 
+      - name: "test(api): post a multiline message" # https://github.com/slackapi/slack-github-action/issues/637
+        id: multiline
+        uses: ./
+        with:
+          errors: true
+          method: chat.postMessage
+          token: ${{ secrets.SLACK_BOT_TOKEN }}
+          payload: |
+            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>"
+
+      - name: "test(api): confirm the multiline message was posted"
+        run: test -n "$MULTILINE_OUTPUT_TS"
+        env:
+          MULTILINE_OUTPUT_TS: ${{ steps.multiline.outputs.ts }}
+
       - name: "test(api): post a message with blocks"
         id: blocks
         uses: ./

package.json

@@ -49,13 +49,14 @@
     "@slack/webhook": "^7.2.0",
     "flat": "^6.0.1",
     "https-proxy-agent": "^9.0.0",
-    "js-yaml": "^5.2.0",
+    "js-yaml": "^4.2.0",
     "markup-js": "^1.5.21"
   },
   "devDependencies": {
     "@biomejs/biome": "^2.5.1",
     "@changesets/cli": "^2.31.0",
     "@types/flat": "^5.0.5",
+    "@types/js-yaml": "^4.0.9",
     "@types/markup-js": "^1.5.0",
     "@types/node": "^24.13.2",
     "@types/sinon": "^21.0.0",

test/content.spec.js

@@ -71,6 +71,21 @@ describe("content", () => {
       assert.deepEqual(config.content.values, expected);
     });
 
+    /**
+     * @see {@link https://github.com/slackapi/slack-github-action/issues/637}
+     */
+    it("parses multiline YAML from the input payload", async () => {
+      mocks.core.getInput
+        .withArgs("payload")
+        .returns('channel: C0123456789\ntext: "first line\n\nsecond line"');
+      const config = new Config(mocks.core);
+      const expected = {
+        channel: "C0123456789",
+        text: "first line\nsecond line",
+      };
+      assert.deepEqual(config.content.values, expected);
+    });
+
     it("parses complete JSON from the input payload", async () => {
       mocks.core.getInput.withArgs("payload").returns(`{
           "message": "this is wrapped",