AIKIDO-2026-10391

TL;DR

Client codegen for event stream operations that include an initial-request message serializes that message to bytes and chains it to the stream before the MessageStreamAdapter. The adapter signs messages in its poll_next path, but the initial message is already bytes when chained, so it is never signed. Event stream operations that require SigV4 therefore send the initial-request message unsigned while subsequent messages are signed, allowing authentication failures or inconsistent validation by services that expect the initial message to be signed. The fix routes the initial message through the same marshalling and signing pipeline as regular events so it is signed by the adapter.