diff --git a/.changeset/bump-js-yaml-5x.md b/.changeset/bump-js-yaml-5x.md
new file mode 100644
index 00000000..c035e50d
--- /dev/null
+++ b/.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"
+```
diff --git a/.github/workflows/integration.yml b/.github/workflows/integration.yml
index bc0188ec..1e5227db 100644
--- a/.github/workflows/integration.yml
+++ b/.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"
diff --git a/package-lock.json b/package-lock.json
index 66077374..8ade4f8f 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -15,7 +15,7 @@
         "@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"
       },
@@ -481,6 +481,29 @@
         "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",
@@ -1707,9 +1730,9 @@
       "license": "ISC"
     },
     "node_modules/js-yaml": {
-      "version": "4.2.0",
-      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-4.2.0.tgz",
-      "integrity": "sha512-ePWsvanv0DWuDRsW8dnt+R4jQ31SCRCQ7hhNcPXZPsoBZiemuZNYGf7adZdqX2D86j6rvKp3RpCxVTSb8WQlOw==",
+      "version": "5.2.1",
+      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-5.2.1.tgz",
+      "integrity": "sha512-zfLtNfQqxVqq3uaTqSkh4x4hZw3KHobGUA0fJUj4wawW8bsQLTVqpHdXSIzidh7o+4lEW36tANuAGdaFx6Zgnw==",
       "funding": [
         {
           "type": "github",
@@ -1725,7 +1748,7 @@
         "argparse": "^2.0.1"
       },
       "bin": {
-        "js-yaml": "bin/js-yaml.js"
+        "js-yaml": "bin/js-yaml.mjs"
       }
     },
     "node_modules/jsonfile": {
diff --git a/package.json b/package.json
index 8975a394..99a9b041 100644
--- a/package.json
+++ b/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"
   },
diff --git a/test/content.spec.js b/test/content.spec.js
index 7704afd0..072f6a52 100644
--- a/test/content.spec.js
+++ b/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",