diff --git a/.changeset/revert-js-yaml-4x.md b/.changeset/revert-js-yaml-4x.md
new file mode 100644
index 00000000..4d56b1c0
--- /dev/null
+++ b/.changeset/revert-js-yaml-4x.md
@@ -0,0 +1,5 @@
+---
+"@slack/slack-github-action": patch
+---
+
+fix: revert multiline yaml parsing indentation change
diff --git a/.github/workflows/integration.yml b/.github/workflows/integration.yml
index db008d92..bfb183c1 100644
--- a/.github/workflows/integration.yml
+++ b/.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: ./
diff --git a/package-lock.json b/package-lock.json
index 85170ee4..1b8d388a 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -16,13 +16,14 @@
         "@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",
@@ -462,29 +463,6 @@
         "js-yaml": "^4.1.1"
       }
     },
-    "node_modules/@changesets/parse/node_modules/js-yaml": {
-      "version": "4.3.0",
-      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-4.3.0.tgz",
-      "integrity": "sha512-1td788aAnnZ5qs7V2QIRl1owjtYpbKt749Y3xauqQgwIIGF/xXWz1wMTEBx5O3LK3lXLVuqXPdPxj2BoFHaW9Q==",
-      "dev": true,
-      "funding": [
-        {
-          "type": "github",
-          "url": "https://github.com/sponsors/puzrin"
-        },
-        {
-          "type": "github",
-          "url": "https://github.com/sponsors/nodeca"
-        }
-      ],
-      "license": "MIT",
-      "dependencies": {
-        "argparse": "^2.0.1"
-      },
-      "bin": {
-        "js-yaml": "bin/js-yaml.js"
-      }
-    },
     "node_modules/@changesets/pre": {
       "version": "2.0.2",
       "resolved": "https://registry.npmjs.org/@changesets/pre/-/pre-2.0.2.tgz",
@@ -943,6 +921,13 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@types/js-yaml": {
+      "version": "4.0.9",
+      "resolved": "https://registry.npmjs.org/@types/js-yaml/-/js-yaml-4.0.9.tgz",
+      "integrity": "sha512-k4MGaQl5TGo/iipqb2UDG2UwjXziSWkh0uysQelTlJpX1qGlpUZYm8PnO4DxG1qBomtJUdYJ6qR6xdIah10JLg==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/markup-js": {
       "version": "1.5.0",
       "resolved": "https://registry.npmjs.org/@types/markup-js/-/markup-js-1.5.0.tgz",
@@ -1701,9 +1686,9 @@
       "license": "ISC"
     },
     "node_modules/js-yaml": {
-      "version": "5.2.0",
-      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-5.2.0.tgz",
-      "integrity": "sha512-YeLUMlvR4Ou1B119LIaM0r65JvbOBooJDc9yEu0dClb/uSC5P4FrLU8OCCz/HXWvtPoIrR0dRzABTjo1sTN9Bw==",
+      "version": "4.3.0",
+      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-4.3.0.tgz",
+      "integrity": "sha512-1td788aAnnZ5qs7V2QIRl1owjtYpbKt749Y3xauqQgwIIGF/xXWz1wMTEBx5O3LK3lXLVuqXPdPxj2BoFHaW9Q==",
       "funding": [
         {
           "type": "github",
@@ -1719,7 +1704,7 @@
         "argparse": "^2.0.1"
       },
       "bin": {
-        "js-yaml": "bin/js-yaml.mjs"
+        "js-yaml": "bin/js-yaml.js"
       }
     },
     "node_modules/jsonfile": {
diff --git a/package.json b/package.json
index 0a3f6015..c1a3153c 100644
--- a/package.json
+++ b/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",
diff --git a/test/content.spec.js b/test/content.spec.js
index 5288d61b..7704afd0 100644
--- a/test/content.spec.js
+++ b/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",