When SpotlightSearchTool is used with a custom LanguageModel backend (for example Apple’s ChatCompletionsLanguageModel from apple/foundation-models-utilities, pointed at an OpenAI-compatible server), the tool can never be successfully invoked. The model produces tool-call arguments that exactly match the format documented in the tool’s own description, but those arguments fail validation against the tool’s generated parameters JSON Schema, throwing LanguageModelSession.ToolCallError with underlying error “Failed to parse generated content.” The root cause is a mismatch between two things the framework sends to the model in the same tool definition: the human-readable description (“Call format”), which presents the top-level arguments as { root, modelComposition, … }, and the parameters JSON Schema (FullArguments), which requires { "query": { "type": "search", "value": { root, modelComposition, … } } }. A model that follows the description is guaranteed to fail the schema. Secondary observation (may be a separate issue or intended) CoreSpotlightSource.fetchAttributes appears to have no effect on which attributes are returned to the model on this agentic-search path. Even with fetchAttributes: [.title, .contentDescription] set on the source, results contain only default metadata (kMDItemTitle, kMDItemDisplayName, dates, identifiers) and omit kMDItemDescription. The description is returned only when the in-query SearchArguments.fetchAttributes explicitly lists it. The searchableIndexDelegate was never invoked in any configuration tried (including .dynamic). If the source-level fetchAttributes is meant to drive returned attributes, that also seems incorrect; otherwise, clarifying the docs would help. Therefore my question, is this just not supported or does the scheme need an update? Or is There a different way that should be done?

Using SpotlightSearchTool with a custom LanguageModel backend (Apple’s ChatCompletionsLanguageModel from foundation-models-utilities, pointed at an OpenAI-compatible server), every tool call fails with ToolCallError → "Failed to parse generated content." The model follows the tool’s documented "Call format" and emits { root, modelComposition, … }. But the generated parameters schema (FullArguments) requires { "query": { "type": "search", "value": { root, modelComposition, … } } }. Query is a QueryType union and a search must be wrapped in DiscriminatedSearch. Wrapping the args manually makes it parse and search correctly. So the description omits the query + type:"search" envelope the schema demands, which makes the tool uninvokable by any model that follows the documentation (it presumably works only with the on-device model trained on the real format). Is this a known issue / intended? Anyone gotten SpotlightSearchTool working with a non-Apple model? Secondary: CoreSpotlightSource.fetchAttributes seems to have no effect on returned attributes. kMDItemDescription only comes back when the in-query fetchAttributes requests it. Bug or expected?