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I know how to check if a current contract's function is been called using CALL or DELEGATECALL (by comparing address(this) to the contract's address that must have been previously saved as an immutable variable).

However I was wondering if it would be possible to check if a DELEGATECALL has taken place at any point in the transaction, for instance:

user -> Contract A -> (delegatecall) -> Contract B -> (call) -> Contract C

As far as I know from Contract's C perspective, it is receiving a CALL from Contract A and I do not really see any way for Contract C to "detect" the DELEGATECALL that took place between Contract A and B....

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I think you are right, considering that you cannot control or write the logic in A and B, it seems impossible.

However, if we consider only your example of 3 contracts, there could be an expensive way to detect if A is a proxy or A has DELEGATECALL in it's code from smart contract C. When A (the caller) has DELEGATECALL or has bytecode similar to some of the popular proxy patterns, A may have used DELEGATECALL to call C. It can be done very expensive by copy A's bytecode into memory while executing contract C and analyze it. I think you may find this EXTCODECOPY opcode interesting: link . Assumming you can write your logic to analyze all the bytecode getting from EXTCODECOPY (msg.sender) in pure solidity to check if its bytecode is similar to a proxy, you may have a chance to say "it's probably delegate call".

But coming back to the question "any point" means it may be a long call chain and the DELEGATECALL could happen 5-10 calls in the chain before the target contract C. I think it's impossible on smart contract. The only way is to use evm tracing tx to check the execution trace like everyone is using.

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