Perp order fails on mainnet with InstructionFallbackNotFound (0x65)

[alejandroprismajacuzzi]

Hi team, I’ve isolated a reproducible issue when placing perp orders on mainnet using the Drift SDK.

Environment:

• Network: mainnet-beta
• Node.js: v18
• SDK tested: 2.163.0-beta.0 and 2.156.0
• RPC: Triton (authenticated HTTP + WS)
• Wallet: correctly loaded
• User account initialized and funded

What works:

• driftClient.subscribe() → OK
• user account detected → OK
• free collateral available → OK
• programId matches expected → OK

What fails:

• placePerpOrder(...)
• placeOrders([...])

Both fail with the same error:

Simulation failed:
InstructionFallbackNotFound (Anchor error 101)
custom program error: 0x65

Logs show the instruction reaches the Drift program but is not recognized.

Observations:

• Happens across multiple SDK versions
• Same result regardless of order parameters
• Likely related to instruction encoding / SDK mismatch

Happy to provide a minimal reproducible script if needed.

Thanks!