UTF-8 characters garbled when using Flow External Services POST

Question

Overview

Working on a simple integration using "external services" to POST to a custom endpoint. Discovered that all UTF-8 characters, including many common currency symbols, are coming through as question marks or other nonsense characters.

The MIME type for the external service is set to application/json.

After discovering this issue, I did a bunch of testing against postman-echo.com/post and hookbin.com.

I do have 'enhanced' external services enabled (not the legacy). I have tested using application/json;charset=UTF-8 and it has no impact.

Two examples:

Example 1:

acute (é), grave (è), circumflex (â, î or ô), tilde (ñ)

comes through as

acute (�), grave (�), circumflex (�, � or �), tilde (�)

Example 2:

¥ · £ · € · $ · ¢ · ₡ · ₢ · ₣ · ₤ · ₥ · ₦ · ₧ · ₨ · ₩ · ₪ · ₫ · ₭ · ₮ · ₯ · ₹

comes through as

� � � � ? � $ � � � ? � ? � ? � ? � ? � ? � ? � ? � ? � ? � ? � ? � ? � ? � ?",

---

It seems like a bug but regular support is unlikely to will not take this up, so posting here to see if anyone has successfully used external services to POST strings with UTF-8 characters? Pasting a truncated external service definition below that I've been using for testing.

{
   "swagger":"2.0",
   "info":{
      "description":"testing string encoding external services",
      "version":"0.1",
      "title":"testingmadness"
   },
   "paths":{
      "/":{
         "post":{
            "consumes":[
               "application/json"
            ],
            "produces":[
               "text/html"
            ],
            "parameters":[
               {
                  "in":"body",
                  "name":"body",
                  "required":false,
                  "schema":{
                     "$ref":"#/definitions/Model0"
                  }
               }
            ],
            "responses":{
               "200":{
                  "description":"Definition generated from Swagger Inspector",
                  "schema":{
                     "type":"string"
                  }
               }
            }
         }
      }
   },
   "definitions":{
      "Model0":{
         "properties":{
            "ttl":{
               "type":"string"
            }
         }
      }
   }
}

UPDATE - Package to Test

I've added the flow + ext service + named credential to a package if anyone with premiere support is willing to push on this, as its a rather bodacious bug.

https://test.salesforce.com/packaging/installPackage.apexp?p0=04tc0000000GktN

To configure / test

1. install package in sandbox
2. create a 'bin' on hookbin.com update
3. update named credential to point at the bin
4. run the flow ('UTF8 Encoding Test 3')

Answer 1

PM for External Services indicated that this is a known issue which will be addressed by SUmmer 21

The team is working on addressing a number of issues here where we are currently ignoring the "consumes" directive, including the charset. By default Latin-1 is used and thus any other Unicode characters are coming across as Latin-1 encoded which is not UTF-8 which then show as garbled characters on the server or in flow in case the "produces" directive has UTF-8 as the charset.

You can expect better support here in the Summer '21 release