Tested on these test suites: https://github.com/Thiht/vscode-venom/tree/master/sample

With the following command: /tmp/venom-v1.1.0-beta.5 run --format=json --output-dir=/tmp/out '**/*.venom.yml'

Venom creates 1 report per test suite:

$ ls -1
test_results.error.venom.yml.json
test_results.failure.venom.yml.json
test_results.success.venom.yml.json

That's a bit annoying to parse, is it possible to get back to a unique file?

Also there's a bug, because I have 2 test suites named success.venom.yml (sample/success.venom.yml and sample/nested/success.venom.yml) but it generates only one report because of the naming scheme test_results.<filename>.json (can be fixed if you go back to a unique file :p )

Last bug I see is that in the line numbering is broken in the error value message.
Before:

"failures": [
  {
    "value": "\u001b[33mTestcase \"Sample test case #2\", step #0: Assertion \"result.systemout ShouldEqual \\\"Good Bye\\\"\" failed. expected: Good Bye  got: Hello (failure.venom.yml:13)\u001b[0m",
    "type": "",
    "message": ""
  }
],

(note the failure.venom.yml:13 at the end of the message)

After:

"errors": [
  {
    "value": "Testcase \"Sample test case #2\", step #0-0: Assertion \"result.systemout ShouldEqual \\\"Good Bye\\\"\" failed. expected: Good Bye  got: Hello (:0)"
  }
],

If not in the error message, can you include the line number in the assertion itself? If you use gopkg.in/yaml.v3 it's possible to get the line number at parse time with a custom unmarshaller:

func (a *Assertion) UnmarshalYAML(value *yaml.Node) error {
	if err := value.Decode(&a.Raw); err != nil {
		return err
	}
	a.LineNumber = value.Line

        // ...

see: https://github.com/go-yaml/yaml/blob/v3.0.1/yaml.go#L372