[Python] UDS SecurityAccess/SessionControl timing out in Python (udsoncan/isotp) when ECU...

I am implementing a UDS flashing tool in Python using udsoncan and python-can-isotp.
The setup is:

• 
  Python client → sends UDS requests via Kvaser CAN interface.
  
  
• 
  CANoe CAPL script → acts as ECU, responds to diagnostic services.
  

The issue


Some services (e.g., ECUReset 0x11) work fine:

Python log:

Tx: <7E4> 0211010000000000
Rx: <7EC> 0251010000000000
Received positive response for ECUReset (0x11)


But other services like DiagnosticSessionControl (0x10) or SecurityAccess (0x27) timeout:

Python log:

Tx: <7E4> 0210020000000000
RAW CAN: ID=0x7EC, Data=065002003201f400
TimeoutException – Did not receive IsoTP frame in time


CANoe trace:

ID=7EC Data=06 50 02 00 32 01 F4 00


So Python clearly sees the CAN frame, but isotp/udsoncan discards it → timeout.

My CAPL code (simplified)


case 0x10: // DiagnosticSessionControl
{
byte resp[8];
resp[0] = 0x50;
resp[1] = data[1];
resp[2] = 0x00;
resp[3] = 0x32;
resp[4] = 0x01;
resp[5] = 0xF4;
CanTpSendData(gHandle, resp, 6);
write(">> Session Control 0x%02X", data[1]);
}
break;

case 0x27: // SecurityAccess
{
byte resp[8];
resp[0] = 0x67;
resp[1] = data[1];
resp[2] = 0x12;
resp[3] = 0x34;
resp[4] = 0x56;
resp[5] = 0x78;
CanTpSendData(gHandle, resp, 6); // send seed
write(">> Sent seed.");
}
break;

What I expect


For SessionControl (0x10) the response should be:

50 02 00 32 01 F4


(6 bytes total).

For SecurityAccess (0x27, sub 0x01) the response should be:

67 01 12 34 56 78


(6 bytes total).

What actually happens


CANoe sends 8-byte frames with trailing 00 padding, e.g.:

06 50 02 00 32 01 F4 00
06 67 01 12 34 56 78 00


Python isotp expects exactly 6 payload bytes, so it rejects the message → timeout.

My question

• 
  Why does CanTpSendData(gHandle, resp, 6) still result in an 8-byte CAN frame?
  
  
• 
  How can I make CANoe’s CAPL simulation send exactly the number of UDS bytes I specify (without padding), so udsoncan/isotp accepts it?
  
  
• 
  Is there a better practice for handling padding between Python isotp and CANoe CAPL?
  

Things I tried

• 
  CanTpSetPadding(gHandle, 0x00); (still sends 8-byte frames with extra zeros).
  
  
• 
  Initializing the array with zeros (byte resp[8] = {0} before filling.
  
  
• 
  Different values for len in CanTpSendData.
  

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