diff --git a/eclair-core/src/main/scala/fr/acinq/eclair/wire/protocol/OfferTypes.scala b/eclair-core/src/main/scala/fr/acinq/eclair/wire/protocol/OfferTypes.scala
index eea4b8aa68..0428cadab2 100644
--- a/eclair-core/src/main/scala/fr/acinq/eclair/wire/protocol/OfferTypes.scala
+++ b/eclair-core/src/main/scala/fr/acinq/eclair/wire/protocol/OfferTypes.scala
@@ -314,8 +314,21 @@ object OfferTypes {
       Right(Offer(records))
     }
 
+    /**
+     * An offer string can be split with '+' to fit in places with a low character limit. This validates that the string adheres to the spec format to guard against copy-pasting errors.
+     * @return a lowercase string with '+' and whitespaces removed
+     */
+    private def validateFormat(s: String): String = {
+      val lowercase = s.toLowerCase
+      require(s == lowercase || s == s.toUpperCase)
+      require(lowercase.head == 'l')
+      require(Bech32.alphabet.contains(lowercase.last))
+      require(!lowercase.matches(".*\\+\\s*\\+.*"))
+      lowercase.replaceAll("\\+\\s*", "")
+    }
+
     def decode(s: String): Try[Offer] = Try {
-      val triple = Bech32.decodeBytes(s.toLowerCase, true)
+      val triple = Bech32.decodeBytes(validateFormat(s), true)
       val prefix = triple.getFirst
       val encoded = triple.getSecond
       val encoding = triple.getThird
diff --git a/eclair-core/src/test/resources/format-string-test.json b/eclair-core/src/test/resources/format-string-test.json
new file mode 100644
index 0000000000..869da95778
--- /dev/null
+++ b/eclair-core/src/test/resources/format-string-test.json
@@ -0,0 +1,62 @@
+[
+  {
+    "comment": "A complete string is valid",
+    "valid": true,
+    "string": "lno1pqps7sjqpgtyzm3qv4uxzmtsd3jjqer9wd3hy6tsw35k7msjzfpy7nz5yqcnygrfdej82um5wf5k2uckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg"
+  },
+  {
+    "comment": "Uppercase is valid",
+    "valid": true,
+    "string": "LNO1PQPS7SJQPGTYZM3QV4UXZMTSD3JJQER9WD3HY6TSW35K7MSJZFPY7NZ5YQCNYGRFDEJ82UM5WF5K2UCKYYPWA3EYT44H6TXTXQUQH7LZ5DJGE4AFGFJN7K4RGRKUAG0JSD5XVXG"
+  },
+  {
+    "comment": "+ can join anywhere",
+    "valid": true,
+    "string": "l+no1pqps7sjqpgtyzm3qv4uxzmtsd3jjqer9wd3hy6tsw35k7msjzfpy7nz5yqcnygrfdej82um5wf5k2uckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg"
+  },
+  {
+    "comment": "Multiple + can join",
+    "valid": true,
+    "string": "lno1pqps7sjqpgt+yzm3qv4uxzmtsd3jjqer9wd3hy6tsw3+5k7msjzfpy7nz5yqcn+ygrfdej82um5wf5k2uckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd+5xvxg"
+  },
+  {
+    "comment": "+ can be followed by whitespace",
+    "valid": true,
+    "string": "lno1pqps7sjqpgt+ yzm3qv4uxzmtsd3jjqer9wd3hy6tsw3+  5k7msjzfpy7nz5yqcn+\nygrfdej82um5wf5k2uckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd+\r\n 5xvxg"
+  },
+  {
+    "comment": "+ can be followed by whitespace, UPPERCASE",
+    "valid": true,
+    "string": "LNO1PQPS7SJQPGT+ YZM3QV4UXZMTSD3JJQER9WD3HY6TSW3+  5K7MSJZFPY7NZ5YQCN+\nYGRFDEJ82UM5WF5K2UCKYYPWA3EYT44H6TXTXQUQH7LZ5DJGE4AFGFJN7K4RGRKUAG0JSD+\r\n 5XVXG"
+  },
+  {
+    "comment": "Mixed case is invalid",
+    "valid": false,
+    "string": "LnO1PqPs7sJqPgTyZm3qV4UxZmTsD3JjQeR9Wd3hY6TsW35k7mSjZfPy7nZ5YqCnYgRfDeJ82uM5Wf5k2uCkYyPwA3EyT44h6tXtXqUqH7Lz5dJgE4AfGfJn7k4rGrKuAg0jSd5xVxG"
+  },
+  {
+    "comment": "+ must be surrounded by bech32 characters",
+    "valid": false,
+    "string": "lno1pqps7sjqpgtyzm3qv4uxzmtsd3jjqer9wd3hy6tsw35k7msjzfpy7nz5yqcnygrfdej82um5wf5k2uckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg+"
+  },
+  {
+    "comment": "+ must be surrounded by bech32 characters",
+    "valid": false,
+    "string": "lno1pqps7sjqpgtyzm3qv4uxzmtsd3jjqer9wd3hy6tsw35k7msjzfpy7nz5yqcnygrfdej82um5wf5k2uckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg+ "
+  },
+  {
+    "comment": "+ must be surrounded by bech32 characters",
+    "valid": false,
+    "string": "+lno1pqps7sjqpgtyzm3qv4uxzmtsd3jjqer9wd3hy6tsw35k7msjzfpy7nz5yqcnygrfdej82um5wf5k2uckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg"
+  },
+  {
+    "comment": "+ must be surrounded by bech32 characters",
+    "valid": false,
+    "string": "+ lno1pqps7sjqpgtyzm3qv4uxzmtsd3jjqer9wd3hy6tsw35k7msjzfpy7nz5yqcnygrfdej82um5wf5k2uckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg"
+  },
+  {
+    "comment": "+ must be surrounded by bech32 characters",
+    "valid": false,
+    "string": "ln++o1pqps7sjqpgtyzm3qv4uxzmtsd3jjqer9wd3hy6tsw35k7msjzfpy7nz5yqcnygrfdej82um5wf5k2uckyypwa3eyt44h6txtxquqh7lz5djge4afgfjn7k4rgrkuag0jsd5xvxg"
+  }
+]
diff --git a/eclair-core/src/test/scala/fr/acinq/eclair/wire/protocol/OfferTypesSpec.scala b/eclair-core/src/test/scala/fr/acinq/eclair/wire/protocol/OfferTypesSpec.scala
index 9228e1e917..988a22c391 100644
--- a/eclair-core/src/test/scala/fr/acinq/eclair/wire/protocol/OfferTypesSpec.scala
+++ b/eclair-core/src/test/scala/fr/acinq/eclair/wire/protocol/OfferTypesSpec.scala
@@ -319,4 +319,17 @@ class OfferTypesSpec extends AnyFunSuite {
       }
     }
   }
+
+  case class FormatTestVector(comment: String, valid: Boolean, string: String)
+
+  test("string format spec test vectors") {
+    implicit val formats: DefaultFormats.type = DefaultFormats
+
+    val src = Source.fromFile(new File(getClass.getResource(s"/format-string-test.json").getFile))
+    val testVectors = JsonMethods.parse(src.mkString).extract[Seq[FormatTestVector]]
+    src.close()
+    for (vector <- testVectors) {
+      assert(Offer.decode(vector.string).isSuccess == vector.valid, vector.comment)
+    }
+  }
 }