pipecrab-lm: the provider-neutral structured-generation interface.

[LanguageModel] is the swappable LM capability the conversation loop drives: a [Conversation] in, a [ModelStream] of [ModelDelta]s out incrementally — text to append, or a complete [ToolCall]. Every item is a preemption point, so a barge-in Interrupt stops the reply within a single delta. Concrete engines stay behind the trait, so the pipeline never names one.

ModelDelta vs ModelFrame

A [ModelDelta] is the internal protocol between a [LanguageModel] and [LmStage], not a pipeline frame. The stage translates deltas into native frames: visible text becomes agent Transcripts (cumulative partials, then a final) because a transcript is prose; a tool call becomes ModelFrame::ToolCall because it is structured protocol; the lifecycle becomes GenerationStarted / GenerationFinished. Tool-call syntax, JSON, and provider metadata never enter a transcript.

Tools

[ToolDefinition::parameters] is a [serde_json::Value] so a framework's schema reaches a hosted adapter without a string round trip. Core stays JSON-free: a [ToolCall] carries validated arguments as JSON text, and [ModelDelta::tool_call] normalizes a JSON object into it. Provider-specific streaming, tool-call fragments, and constrained output are parsed inside the implementation; the stage sees only complete calls. [LmStage::with_tools] / add_tools configure tools on the stage — validated (duplicate names rejected) and passed to every generation. An adapter that wraps a higher-level agent (e.g. Rig) keeps its own registered tools internal, so the stage neither reads nor copies them.

The chat-context types ([Message], [Conversation]) preserve structured assistant turns, tool calls, tool results, and external events so a hosted adapter can reconstruct valid provider history.

Platform-neutral and wasm32-checkable: the concrete engines live elsewhere (a native llama.cpp context, a hosted Rig agent, a browser Worker), each behind this trait, so the interface itself carries no backend dependency and compiles for both the host and wasm32-unknown-unknown.