fix: prevent stream_async() from yielding non-serializable invocation_state fields#2066
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ModelStreamEvent.prepare() merged the full invocation_state dict into the event dict before it was yielded to callers. Because invocation_state can contain non-JSON-serializable objects (Agent instance, OTel spans), those objects appeared in every streamed event, causing repr() serialization on the wire and hitting payload limits in runtimes like AgentCore. The fix snapshots the event data before prepare() is called and yields that snapshot instead of the merged dict. The callback_handler still receives the fully-merged dict for backward compatibility. Fixes strands-agents#1928
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Description
stream_async()yields each event dict directly after callingevent.prepare(invocation_state=...). Theprepare()method merges the fullinvocation_statemapping into the event dict, which includes non-serializable objects such as theAgentinstance, OTel span objects, message history, and the model object. Callers that attempt to serialize these events (for example, AgentCore's SSE stream) raiseTypeError: Object of type Agent is not JSON serializable.The fix snapshots
event.as_dict()beforeprepare()runs, so callers ofstream_async()only ever receive the serializable event fields. Thecallback_handlerstill receives the full merged dict (preserving existing behavior).Related Issues
Resolves #1928
Documentation PR
N/A
Type of Change
Bug fix
Testing
hatch run prepareChecklist
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