Anchor Program Integration

Use this skill when building raw Solana instructions that interact with Anchor programs, especially when bypassing Anchor's method builders.

Key Concept: What Anchor Method Builders Do

When you use Anchor's method builder pattern:

await program.methods['myInstruction'](params)
  .accounts({ ... })
  .remainingAccounts([...])
  .instruction();

Anchor automatically:

1. •Serializes instruction data with the correct discriminator
2. •Orders accounts according to the IDL
3. •Appends macro-injected accounts (like #[event_cpi])

#[event_cpi] Macro

The #[event_cpi] macro is used for emitting events via CPI (Cross-Program Invocation).

What It Does

When an instruction has #[event_cpi], Anchor:

1. •Expects two additional accounts at the END of the instruction accounts (but BEFORE remaining_accounts when using method builder)
2. •These accounts are: event_authority PDA and program ID

Account Positions

With Anchor method builder: Accounts are auto-appended Without method builder (raw instruction): You must add them manually at positions [N] and [N+1] where N is the number of main instruction accounts

// For an instruction with 7 main accounts:
// [0-6] = main accounts
// [7]   = event_authority (PDA with seed "__event_authority")
// [8]   = program ID
// [9+]  = remaining_accounts

Deriving event_authority

const [eventAuthorityPda] = PublicKey.findProgramAddressSync(
  [Buffer.from("__event_authority")],
  PROGRAM_ID  // The program that has #[event_cpi]
);

Instruction Data Serialization

Anchor instructions start with an 8-byte discriminator:

// Discriminator = SHA256("global:instruction_name").slice(0, 8)
const discriminator = crypto
  .createHash("sha256")
  .update("global:receive_message")
  .digest()
  .slice(0, 8);

For Borsh serialization of Vec<u8> parameters:

// Format: [4-byte length (u32 LE)][data bytes]
buffer.writeUInt32LE(data.length, offset);
data.copy(buffer, offset + 4);

Common Pitfalls

1. •Wrong event_authority position: Must be immediately after main accounts, not at end of all accounts
2. •Wrong program for PDA derivation: Each program has its OWN event_authority PDA
3. •Missing discriminator: Raw instructions need the 8-byte discriminator prefix
4. •Wrong Borsh encoding: Vec<u8> needs length prefix, not raw bytes