BoundaryML/baml · error

error decoding type, checked type not found:

Error message

error decoding type, checked type not found: 

What it means

When decoding a type descriptor received over the Go/C++ FFI boundary, baml's Go serde encounters a CheckedType (a BAML `check` expression result type). It serializes the inner type to a name via typeToString and looks it up in the caller-provided TypeMap under the "CHECKED_TYPES." prefix. This panic means the generated TypeMap shipped with the generated Go code has no entry for that checked type, so decoding cannot proceed and it fails fast with a panic.

Source

Thrown at engine/language_client_go/baml_go/serde/decode.go:426

	}

	if optional, ok := type_.(*cffi.CFFIFieldTypeHolder_OptionalType); ok {
		optionalType := optional.OptionalType
		goType := convertFieldTypeToGoType(optionalType.Value, typeMap)
		if goType == typeMap.typeMap["INTERNAL.nil"] {
			// Pointer to nil / any is same as nil / any
			return goType
		}
		return reflect.PointerTo(goType)
	}

	if checked, ok := type_.(*cffi.CFFIFieldTypeHolder_CheckedType); ok {
		checkedType := checked.CheckedType
		serializeType := typeToString(checkedType.Value)
		goType, ok := typeMap.typeMap["CHECKED_TYPES."+serializeType]
		if !ok {
			panic("error decoding type, checked type not found: " + serializeType + ", typeMap=" + fmt.Sprintf("%+v", typeMap))
		}
		return goType
	}

	if streamState, ok := type_.(*cffi.CFFIFieldTypeHolder_StreamStateType); ok {
		streamStateType := streamState.StreamStateType
		return convertFieldTypeToGoType(streamStateType.Value, typeMap)
	}

	if list, ok := type_.(*cffi.CFFIFieldTypeHolder_ListType); ok {
		listType := list.ListType
		goElementType := convertFieldTypeToGoType(listType.ItemType, typeMap)
		if goElementType == typeMap.typeMap["INTERNAL.nil"] {
			return reflect.TypeOf([]any{})
		}
		return reflect.SliceOf(goElementType)
	}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Regenerate the Go client with `baml-cli generate` (or `baml generate`) so the TypeMap includes the new checked type
2. Update the baml-cli and baml_go runtime packages to matching versions (go get -u github.com/boundaryml/baml/...) so serialization names align
3. If the check is new in the .baml file, confirm the generated baml_go client directory is actually the one being imported (stale module cache / replace directive)
4. As a last resort, remove or rename the check expression in the .baml file to a supported output shape and regenerate

Example fix

// before: stale generated code missing the check entry, decode panics
client, _ := baml.NewGoClient(ctx, baml.LoadCollector()) // old generated TypeMap
baml.ExtractResume(ctx, client, text) // panic: checked type not found

// after: regenerate then run
cd b/baml_src && baml-cli generate  # refresh generated TypeMap with CHECKED_TYPES entries
baml.ExtractResume(ctx, client, text)

// Regeneration check: ensure generated TypeMap contains CHECKED_TYPES entries
func validateCheckedTypes(tm baml_go.TypeMap) error {
	for name := range requiredCheckedTypesFromBamlSrc() {
		if _, ok := tm.TypeMap["CHECKED_TYPES."+name]; !ok {
			return fmt.Errorf("missing generated checked type %q — run baml-cli generate", name)
		}
	}
	return nil
}

When it happens

Trigger: Calling a BAML function whose output (or a nested field) contains a check (e.g. `check ... expr`), while the Go TypeMap built by baml-cli generate lacks a "CHECKED_TYPES.<serialized-type>" key — typically because the generated client code is stale relative to the baml file or the TypeMap was constructed without checked-type entries.

Common situations: Editing a .baml file to add or change a check() expression and re-running the app without regenerating the Go client; version mismatch between the baml CLI that produced cffi payloads and the generated Go code; hand-built TypeMaps used in tests that omit CHECKED_TYPES entries.

Related errors

• error decoding value, type alias not found:
• error decoding value, unknown field type:
• error decoding value, unknown literal type:
• ObjectValue is not yet supported:
• error decoding value:

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/d5891aa7342f0c99. Report an issue: GitHub.