Chat model streaming event protocol.
Defines a content-block-centric event model for streaming chat model responses.
Events carry LangChain ContentBlock types on lifecycle boundaries
and explicit delta variants for incremental updates during streaming.
Design Principles
-
Content-block deltas have explicit merge semantics. Text,
reasoning, and data deltas append to named fields. Generic block deltas
shallow-merge fields onto the active content block.
-
Lifecycle completeness. Every streamable entity has explicit start and finish
events. Consumers never need to infer boundaries from absence of events.
-
Interleaving allowed. Content blocks may interleave (e.g., parallel tool calls).
The only invariant: a block's start precedes its deltas, and its deltas precede
its finish. No ordering constraint between different blocks.
-
Provider passthrough. Native provider events that don't map to standard types
are forwarded as ProviderEvent rather than silently dropped.
Lifecycle Contract
MessageStart
-> ContentBlockStart(index=0, content=...)
-> ContentBlockDelta(index=0, delta={ type: "text-delta", text: "Hello" })
-> ContentBlockDelta(index=0, delta={ type: "text-delta", text: " world" })
-> ContentBlockFinish(index=0, content=...)
-> UsageUpdate(...)
-> MessageFinish(reason, usage?)
