[bug] Go: serialization panics on nested map in LLM client options

[invakid404]

Product

BAML

Describe the bug

In the Go client, when using the BAML ClientRegistry to define an LLM client on the fly, BAML's internal serialization panics if the options parameter contains a nested map:

// ...
clientOptions := map[string]any{
	"api_key": apiKey,
	"model":   "claude-haiku-4-5-20251001",
	"metadata": map[string]any{
		"user_id": "some-metadata",
	},
}

clientRegistry := baml.NewClientRegistry()

clientRegistry.AddLlmClient("example", "anthropic", clientOptions)
clientRegistry.SetPrimaryClient("example")

result, err := b.ExtractResume(
	ctx,
	"...",
	b.WithClientRegistry(clientRegistry),
)
if err != nil {
	panic(err)
}
// ...

panic: encoding client registry: encoding client options: encoding client options field 'metadata': interface  does not implement InternalBamlSerializer

goroutine 1 [running]:
github.com/invakid404/baml-client-options-serialization-issue/baml_client.ExtractResume({0x10055fc60, 0x100828100}, {0x10040c625, 0x8e}, {0x14000039ed0, 0x1, 0x1400012e000?})
	/Users/inva/Coding/baml-client-options-serialization-issue/baml_client/functions.go:54 +0x550
main.runTest(0x1400012c000)
	/Users/inva/Coding/baml-client-options-serialization-issue/main.go:19 +0x15c
main.main()
	/Users/inva/Coding/baml-client-options-serialization-issue/main.go:51 +0x250
exit status 2

My brief investigation revealed that, for nested maps' values, the reflect package reports kind: reflect.Interface, which puts us in this branch. If the serialization logic had peeked at the .Elem() of the reflect value, the kind would've been correctly reported as reflect.String.

Reproduction Steps

Minimal reproducible example: https://github.com/invakid404/baml-client-options-serialization-issue

BAML Version

0.214.0

Language/Framework

Go

LLM Provider

None

LLM Model

No response

Operating System

macOS

Browser

None

Code Editor

None

[linear]

BAML-539