Why does fri_strep_list must start with 0x0?

[tdelabro]

The verification of my proof fails with error:

Caused by:
    0: Fri Error
    1: invalid first fri step

Looking at the code it appears this error is only raised in one place in the code, in crates/fri/src/config.rs:

impl Config {
    pub fn validate(
        &self,
        log_n_cosets: Felt,
        n_verifier_friendly_commitment_layers: Felt,
    ) -> Result<Felt, Error> {
        ...
        if *self.fri_step_sizes.first().ok_or(Error::FirstFriStepInvalid)? != Felt::ZERO {
            return Err(Error::FirstFriStepInvalid);
        }
        ...
    }

I debugged this and found the following value: fri_step_sizes: [0x4, 0x4, 0x4, 0x4], which match the content of my proof.json:

	"proof_parameters" : 
	{
		"field" : "PrimeField0",
		"stark" : 
		{
			"fri" : 
			{
				"fri_step_list" : 
				[
					4,
					4,
					4,
					4
				],
				"last_layer_degree_bound" : 32,
				"n_queries" : 18,
				"proof_of_work_bits" : 30
			},
			"log_n_cosets" : 4
		},
		"use_extension_field" : false
	},

This value is inherited from my cpu_air_params.json file

{
    "field": "PrimeField0",
    "stark": {
        "fri": {
            "fri_step_list": [
                0,
                4,
                4,
                4,
                4
            ],
            "last_layer_degree_bound": 32,
            "n_queries": 18,
            "proof_of_work_bits": 30
        },
        "log_n_cosets": 4
    },
    "use_extension_field": false
}

which is fed to the prover:

cpu_air_prover \
  --out_file=$PROOF_OUTPUT_DIR/proof.json \
  --private_input_file=$TRACE_OUTPUT_DIR/private-input.json \
  --public_input_file=$TRACE_OUTPUT_DIR/public-input.json \
  --prover_config_file=$CFG_AND_PARAMS_DIR/cpu_air_prover_config.json \
  --parameter_file=$CFG_AND_PARAMS_DIR/cpu_air_params.json \
  --generate_annotations

Your code seems to expect the first value to be 0. Why is this a requirement?
If it has to be a requirement can you add it to the list of requirements you have for the prover config?

[tdelabro]

I changed my cpu_air_params.json to "fri_step_list": [0, 4, 4, 4, 4]" and it now crashes later in the verification:
At crates/stark/src/oods.rs:31:18 which is:

pub fn verify_oods<Layout: LayoutTrait>(
    oods: &[Felt],
    interaction_elements: &Layout::InteractionElements,
    public_input: &PublicInput,
    constraint_coefficients: &[Felt],
    oods_point: &Felt,
    trace_domain_size: &Felt,
    trace_generator: &Felt,
) -> Result<(), OodsVerifyError> {
    let composition_from_trace = Layout::eval_composition_polynomial(
        interaction_elements,
        public_input,
        &oods[0..oods.len() - 2], // < here
        constraint_coefficients,
        oods_point,
        trace_domain_size,
        trace_generator,
    )?;
    ...
}

[Okm165]

First FRI layer is for gather_first_layer_queries we denote it as a layer but with no folding action happening that is why 0x0.
It is analogous to the cairo0 STARK verifier

[Okm165]

I changed my cpu_air_params.json to "fri_step_list": [0, 4, 4, 4, 4]" and it now crashes later in the verification: At crates/stark/src/oods.rs:31:18 which is:

pub fn verify_oods<Layout: LayoutTrait>(
    oods: &[Felt],
    interaction_elements: &Layout::InteractionElements,
    public_input: &PublicInput,
    constraint_coefficients: &[Felt],
    oods_point: &Felt,
    trace_domain_size: &Felt,
    trace_generator: &Felt,
) -> Result<(), OodsVerifyError> {
    let composition_from_trace = Layout::eval_composition_polynomial(
        interaction_elements,
        public_input,
        &oods[0..oods.len() - 2], // < here
        constraint_coefficients,
        oods_point,
        trace_domain_size,
        trace_generator,
    )?;
    ...
}

I need more information to help, what is the layout u use?
How can i reproduce it? Do u have repo with it?
Issue continues here #55