SyncValsverifier → artifact → classifier → verdict
SyncVals · Trajectory

byte-serial-round-scheduler

claude-code claude-opus-4-8 ✗ failed GOOD_FAILURE ↑ View task
Solved from the instruction alone, tests/ and solution/ were withheld from the agent's workspace and restored only for grading.
Reward = tests/test.sh exit code (0 → resolved); the classification below is post-hoc and cannot change it.
Classification , post-hoc; cannot change the reward
GOOD_FAILUREHonest miss, the agent ran correctly but couldn't solve it. Expected for a hard task; the task is sound.
SubtypeWrong Approach
EvidenceInstruction specifies sequential slots: "A round is sixteen byte slots then one key-mix slot" and "otherwise the next round starts next edge." Reference/testbench compares ready/result_valid/block_out every clock against a slot-by-slot RefModule. Agent instead wrote a combinational full-round implementation: `function [127:0] byte_slots` applies all 16 byte slots in one function and `wire [127:0] round_out = byte_slots(state, sf) ^ key_reg;`, then advances one whole round per clock. Verifier output: `reference_equivalence: FAIL (15924 mismatches in 16537 samples)`.
Root causeThe agent collapsed the required byte-slot/key-mix schedule into one round per clock, causing protocol timing and output mismatches. The task instructions and tests are aligned around the slot-by-slot registered schedule, so this is an implementation error rather than a task defect.
RecommendationN/A - task is fine
Trajectory
Tool-by-tool agent trajectory
4 tool calls · 2 tool types · 8 steps
A block engine transforms a 128-bit block under a start/ready protocol. Ports (1 bit unless width shown): clk rst_n start block_in 128 round_key0 128 num_rounds 4 reseed reseed_round 4 ready result_valid block_out 128 rst_n is active-low synchronous; while low at a rising edge outputs clear (ready high, result_valid low). start, while ready is high, loads block_in, round_key0 and num_rounds (1-15), else ignored; ready stays low until the finishing key-mix slot. Engine state is 128 bits, seeded by block_in. A round is sixteen byte slots then one key-mix slot. Byte slots visit columns 0 to 3; column c is state bits [32*c+31:32*c], its [31:24] byte first down to [7:0] last. A slot replaces only its byte with rol1(byte xor feedback) xor 8'h63; slot 0's feedback is the round's start feedback, else the previous slot's byte. round_index counts completed rounds. Round 0's start feedback is {4'b0, round_index}; a later round's is fixed at its preceding key-mix slot by reseed there (not reseed during the round): high gives {4'b0, reseed_round}, low {4'b0, round_index}. The key-mix slot xors state with round_key0. On the slot completing num_rounds, block_out becomes that post-xor state, result_valid and ready asserting that cycle; otherwise the next round starts next edge. block_out holds between operations, changing only on a finishing slot. All outputs are registered. Submit plain ASCII Verilog; $ system tasks and define/include directives are rejected. Write TopModule to /app/dut.sv.
write · /app/dut.sv
module TopModule(
    input             clk,
    input             rst_n,
    input             start,
    input      [127:0] block_in,
    input      [127:0] round_key0,
    input      [3:0]   num_rounds,
    input             reseed,
    input      [3:0]   reseed_round,
    output reg        ready,
    output reg        result_valid,
    output reg [127:0] block_out
);

    // Engine registers
    reg [127:0] state;
    reg [127:0] key_reg;
    reg [3:0]   nr_reg;
    reg [3:0]   ridx;      // index of the round currently being computed
    reg [7:0]   sf;        // start feedback for the round currently being computed
    reg         running;

    // Combinational: apply the sixteen byte slots of one round.
    function [127:0] byte_slots;
        input [127:0] s;
        input [7:0]   startfb;
        reg [127:0] cur;
        reg [7:0]   fb;
        reg [7:0]   orig, t, nb;
        integer     slot, col, bidx, lowbit;
        begin
            cur = s;
            fb  = startfb;
            for (slot = 0; slot < 16; slot = slot + 1) begin
                col    = slot / 4;
                bidx   = slot % 4;
                lowbit = 32*col + (3 - bidx)*8;
                orig   = cur[lowbit +: 8];
                t      = orig ^ fb;                 // byte xor feedback
                nb     = {t[6:0], t[7]} ^ 8'h63;    // rol1 then xor 0x63
                cur[lowbit +: 8] = nb;
                fb     = nb;                        // next slot feeds on this slot's byte
            end
            byte_slots = cur;
        end
    endfunction

    // Full round = byte slots, then key-mix slot (xor round_key0).
    wire [127:0] round_out = byte_slots(state, sf) ^ key_reg;

    always @(posedge clk) begin
        if (!rst_n) begin
            ready        <= 1'b1;
            result_valid <= 1'b0;
            running      <= 1'b0;
            block_out    <= 128'b0;
        end else begin
            result_valid <= 1'b0;
            if (ready && start) begin
                // Load a new operation; state seeded by block_in.
                state   <= block_in;
                key_reg <= round_key0;
                nr_reg  <= num_rounds;
                ridx    <= 4'd0;
                sf      <= 8'd0;          // round 0 start feedback = {4'b0, round_index=0}
                ready   <= 1'b0;
                running <= 1'b1;
            end else if (running) begin
                if ((ridx + 4'd1) == nr_reg) begin
                    // Finishing key-mix slot of the last round.
                    block_out    <= round_out;
                    result_valid <= 1'b1;
                    ready        <= 1'b1;
                    running      <= 1'b0;
                end else begin
                    // Advance to next round; fix its start feedback via reseed here.
                    state <= round_out;
                    ridx  <= ridx + 4'd1;
                    sf    <= reseed ? {4'd0, reseed_round}
                                    : {4'd0, (ridx + 4'd1)};
                end
            end
        end
    end

endmodule
Let me verify it compiles.

Compile the Verilog module

shell
iverilog -g2012 -o /tmp/dut.out dut.sv 2>&1 && echo "COMPILE OK" || echo "iverilog not available or error"
output
COMPILE OK

[stdout]
COMPILE OK
Let me cross-check behavior against an independent Python reference model.

Run simulation testbench

shell
cat > /tmp/tb.sv <<'EOF'
module tb;
  reg clk=0, rst_n, start, reseed;
  reg [127:0] block_in, round_key0;
  reg [3:0] num_rounds, reseed_round;
  wire ready, result_valid;
  wire [127:0] block_out;
  TopModule dut(.clk(clk),.rst_n(rst_n),.start(start),.block_in(block_in),
    .round_key0(round_key0),.num_rounds(num_rounds),.reseed(reseed),
    .reseed_round(reseed_round),.ready(ready),.result_valid(result_valid),.block_out(block_out));
  always #5 clk=~clk;
  integer i;
  initial begin
    rst_n=0; start=0; reseed=0; reseed_round=0; block_in=0; round_key0=0; num_rounds=1;
    @(posedge clk); #1 rst_n=1;
    // Test A
    block_in=128'h0123456789abcdef_fedcba9876543210;
    round_key0=128'hdeadbeef_cafebabe_00112233_44556677;
    num_rounds=4; reseed=1; reseed_round=4'd7;
    @(negedge clk); start=1; @(negedge clk); start=0;
    wait(result_valid); #1;
    $display("A block_out=%h", block_out);
    @(posedge clk);
    // Test B single round no reseed, zeros
    @(negedge clk);
    block_in=0; round_key0=0; num_rounds=1; reseed=0;
    start=1; @(negedge clk); start=0;
    wait(result_valid); #1;
    $display("B block_out=%h", block_out);
    #20 $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/sim /app/dut.sv /tmp/tb.sv && vvp /tmp/sim
output
A block_out=8bbbe308a3b0f2d47a872434819275a9
B block_out=0569b10063a528330569b10063a52833
/tmp/tb.sv:29: $finish called at 106 (1s)

[stdout]
A block_out=8bbbe308a3b0f2d47a872434819275a9
B block_out=0569b10063a528330569b10063a52833
/tmp/tb.sv:29: $finish called at 106 (1s)

Python reference model

shell
cat > /tmp/model.py <<'EOF'
def rol1(x): return ((x<<1)|(x>>7))&0xff
def byte_slots(state, sf):
    cur=[(state>>(8*i))&0xff for i in range(16)]  # cur[i] = byte at bits[8i+7:8i]
    fb=sf
    for slot in range(16):
        col=slot//4; bidx=slot%4
        lowbit=32*col+(3-bidx)*8
        idx=lowbit//8
        orig=cur[idx]
        t=orig^fb
        nb=rol1(t)^0x63
        cur[idx]=nb
        fb=nb
    s=0
    for i in range(16): s|=cur[i]<<(8*i)
    return s
def run(block_in,key,num_rounds,reseed,reseed_round):
    state=block_in
    for r in range(num_rounds):
        if r==0: sf=0
        else:
            sf = reseed_round if reseed else r
        state=byte_slots(state,sf)^key
    return state
A=run(0x0123456789abcdeffedcba9876543210,0xdeadbeefcafebabe0011223344556677,4,1,7)
B=run(0,0,1,0,0)
print("A %032x"%A)
print("B %032x"%B)
EOF
python3 /tmp/model.py
output
A 8bbbe308a3b0f2d47a872434819275a9
B 0569b10063a528330569b10063a52833

[stdout]
A 8bbbe308a3b0f2d47a872434819275a9
B 0569b10063a528330569b10063a52833
Compiles cleanly and matches an independent reference model on both a 4-round reseed case and a single-round case. The implementation in `/app/dut.sv`: - **Protocol**: registered `ready`/`result_valid`/`block_out`, synchronous active-low reset clears `ready=1`, `result_valid=0`. `start` is honored only while `ready` is high, loading `block_in`→state, `round_key0`, `num_rounds`. - **Round engine**: one full round per cycle , sixteen byte slots (column 0→3, top byte `[31:24]` down to `[7:0]`), each replacing its byte with `rol1(byte ^ feedback) ^ 8'h63`, chaining feedback from the previous slot's resulting byte and seeding slot 0 from the round's start feedback. Then the key-mix slot xors `round_key0`. - **Start feedback**: round 0 uses `{4'b0, 0}`; later rounds are fixed at the preceding key-mix slot by sampling `reseed` there , high → `{4'b0, reseed_round}`, low → `{4'b0, round_index}`. - **Finish**: on the key-mix slot completing `num_rounds`, `block_out` takes the post-xor state and `result_valid`/`ready` assert that (registered) cycle; `block_out` holds otherwise.
Final diff , what the agent changed

No file changes, the agent left the workspace untouched.

Verifier output , the proof behind the reward
tests/test.sh
TEST dut_present: PASS (/app/dut.sv exists)
TEST dut_forbidden: PASS (no forbidden constructs)
TEST dut_elaborates: PASS (DUT elaborates standalone)
TEST dut_compiles: PASS (compiled with held-back reference/testbench)
TEST simulation_completed: PASS (Mismatches: 15924 in 16537 samples)
TEST reference_equivalence: FAIL (15924 mismatches in 16537 samples)
{"score": 0.000000, "results_file": "/tmp/h2t-grader-home.Xxw7bk/runtime/cb_verifier_312-7256-13782-5412_312/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_10944e8c3b2b4b47. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_10944e8c3b2b4b47 · verifier authoritative; classifier explanatory.