SyncValsverifier → artifact → classifier → verdict
SyncVals · Trajectory

round-robin-arbiter-grant

claude-code claude-opus-4-8 ✓ resolved GOOD_SUCCESS ↑ 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_SUCCESSLegitimate solve, implements the asked-for behavior; tests verify real functionality.
SubtypeLegitimate Solution
EvidenceVerified result passed: `TEST reference_equivalence: PASS (all 4583 samples matched reference)` and `score: 1.000000`. The agent wrote `/app/dut.sv` implementing circular arbitration with `start_ptr`, lock state, dead-air handling when `req[lock_idx]` drops, and synchronous reset; its own clean-sampled checks showed expected sequences such as all-four `0001,0010,0100,1000` and dead-air `gv=0 grant=0000`. Tests compare behavior cycle-by-cycle against `RefModule`, not superficial strings.
Root causeThe agent correctly implemented the specified four-requester round-robin arbiter and lock semantics using only the visible instruction. The hidden verifier exercised the behavioral contract over directed and randomized cycles and found zero mismatches.
RecommendationN/A - task is fine
Trajectory
Tool-by-tool agent trajectory
6 tool calls · 3 tool types · 10 steps
write a verilog module TopModule sharing one resource among four requesters. ports: clk, rst, req[3:0], lock, outputs grant[3:0], grant_valid. grant is one-hot; grant_valid marks cycles it names someone. the very first time all four ask at once, requester 0 wins. fairness is a circular queue: the one just served goes to the back. walk up from just past the last winner, wrapping 3 to 0; grant the first asker. all four asking forever gives 0,1,2,3,0,1,... a lone requester keeps winning; idle keeps your place. lock high on a granted cycle latches that winner: grant stays nailed to it, grant_valid high, while lock stays high; everyone else waits. lock with no grant does nothing. when lock drops the held winner still owns that cycle; sharing resumes next cycle just past it. a hold lives only while the held requester keeps asking. a held cycle with its req low, even the cycle lock drops, is dead air: grant 0, grant_valid 0, whoever else asks; the hold is over. sharing resumes next cycle just past the dead winner, it counts as served; a lock still high needs no re-arm, latching the next real winner by the start rule. rst is synchronous active-high: clears any hold, restarts the order. write plain synthesizable systemverilog to /app/dut.sv; no preprocessor directives, system tasks, or force/release; ascii only. graded by iverilog -g2012 against a hidden reference every cycle.
write · /app/dut.sv
module TopModule (
    input               clk,
    input               rst,
    input        [3:0]  req,
    input               lock,
    output logic [3:0]  grant,
    output logic        grant_valid
);

    // Sequential state
    logic [1:0] start_ptr;   // scan starts here (just past last winner)
    logic       locked;      // currently holding a locked winner
    logic [1:0] lock_idx;    // the held winner index

    // Combinational: scan req from start_ptr for first asker
    logic       win_found;
    logic [1:0] win_idx;
    logic [1:0] idx;
    integer     i;

    always_comb begin
        win_found = 1'b0;
        win_idx   = 2'b00;
        idx       = 2'b00;
        for (i = 0; i < 4; i = i + 1) begin
            idx = start_ptr + i[1:0];
            if (!win_found && req[idx]) begin
                win_found = 1'b1;
                win_idx   = idx;
            end
        end
    end

    // Combinational: this cycle's grant decision
    logic       cur_valid;
    logic [1:0] cur_idx;

    always_comb begin
        cur_valid = 1'b0;
        cur_idx   = 2'b00;
        if (locked) begin
            // Held winner owns the cycle only while it keeps asking.
            if (req[lock_idx]) begin
                cur_valid = 1'b1;
                cur_idx   = lock_idx;
            end
            // else: dead air -> stays 0
        end else begin
            if (win_found) begin
                cur_valid = 1'b1;
                cur_idx   = win_idx;
            end
        end
    end

    // Outputs
    always_comb begin
        grant_valid = cur_valid;
        grant       = 4'b0000;
        if (cur_valid)
            grant[cur_idx] = 1'b1;
    end

    // Next-state
    always_ff @(posedge clk) begin
        if (rst) begin
            start_ptr <= 2'b00;
            locked    <= 1'b0;
            lock_idx  <= 2'b00;
        end else if (locked) begin
            if (req[lock_idx]) begin
                // held requester still asking; it owns this cycle
                start_ptr <= lock_idx + 2'd1;
                if (lock) begin
                    // keep holding
                    locked   <= 1'b1;
                    lock_idx <= lock_idx;
                end else begin
                    // lock dropped: owned this cycle, sharing resumes next
                    locked   <= 1'b0;
                end
            end else begin
                // dead air: hold is over, held slot counts as served
                start_ptr <= lock_idx + 2'd1;
                locked    <= 1'b0;
            end
        end else begin
            if (win_found) begin
                start_ptr <= win_idx + 2'd1;
                if (lock) begin
                    // latch this winner
                    locked   <= 1'b1;
                    lock_idx <= win_idx;
                end else begin
                    locked   <= 1'b0;
                end
            end else begin
                // idle: keep place
                start_ptr <= start_ptr;
                locked    <= 1'b0;
            end
        end
    end

endmodule

Compile dut.sv with iverilog

shell
iverilog -g2012 -o /tmp/dut.out dut.sv 2>&1 && echo "COMPILE OK"
output
dut.sv:21: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
COMPILE OK

[stdout]
dut.sv:21: sorry: constant selects in always_* processes are not currently supported (all bits will be included).
COMPILE OK

/app/dut.sv

− removed
            idx = start_ptr + i[1:0];
+ added
            idx = start_ptr + i;

Recompile dut.sv

shell
iverilog -g2012 -o /tmp/dut.out dut.sv 2>&1 && echo "COMPILE OK"
output
COMPILE OK

[stdout]
COMPILE OK
Let me write a quick self-check testbench to validate the key behaviors.

Run behavioral sanity testbench

shell
cat > /tmp/tb.sv << 'EOF'
module tb;
  logic clk=0, rst, lock;
  logic [3:0] req;
  logic [3:0] grant;
  logic grant_valid;
  TopModule d(.clk,.rst,.req,.lock,.grant,.grant_valid);
  always #5 clk=~clk;
  task step; @(posedge clk); #1; endtask
  initial begin
    rst=1; req=0; lock=0; step;
    rst=0;
    // Test 1: all four forever -> 0,1,2,3,0,1
    req=4'b1111; lock=0;
    for(int k=0;k<6;k++) begin
      $display("all4: gv=%b grant=%b (expect one-hot cycling 0..3)", grant_valid, grant); step;
    end
    // reset
    rst=1; step; rst=0;
    // Test 2: lone requester req2
    req=4'b0100;
    for(int k=0;k<3;k++) begin
      $display("lone2: gv=%b grant=%b (expect 0100)", grant_valid, grant); step;
    end
    // Test 3: idle keeps place -> was granting 2, ptr=3. idle a cycle, then all4 -> should grant 3
    req=0; $display("idle: gv=%b grant=%b (expect gv=0)", grant_valid, grant); step;
    req=4'b1111; $display("resume: gv=%b grant=%b (expect 1000 idx3)", grant_valid, grant); step;
    // Test 4: lock hold. reset, then req all, lock high on grant of 0 -> hold 0
    rst=1; step; rst=0;
    req=4'b1111; lock=1;
    $display("lockgrant: gv=%b grant=%b (expect 0001 idx0)", grant_valid, grant); step;
    $display("held1: gv=%b grant=%b (expect 0001 held)", grant_valid, grant); step;
    $display("held2: gv=%b grant=%b (expect 0001 held)", grant_valid, grant); step;
    // lock drops now, held still owns this cycle
    lock=0;
    $display("lockdrop: gv=%b grant=%b (expect 0001 owns)", grant_valid, grant); step;
    $display("afterdrop: gv=%b grant=%b (expect 0010 idx1 resume past 0)", grant_valid, grant); step;
    // Test 5: dead air. reset, hold req0 with lock, then drop req0 while locked
    rst=1; step; rst=0;
    req=4'b1111; lock=1;
    $display("dl_grant: gv=%b grant=%b (expect 0001)", grant_valid, grant); step;
    // now req0 goes low while locked -> dead air
    req=4'b1110;
    $display("deadair: gv=%b grant=%b (expect gv=0 grant=0)", grant_valid, grant); step;
    $display("resume2: gv=%b grant=%b (expect 0010 idx1)", grant_valid, grant); step;
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/tb.out /tmp/tb.sv /app/dut.sv && vvp /tmp/tb.out
output
all4: gv=0 grant=0000 (expect one-hot cycling 0..3)
all4: gv=1 grant=0010 (expect one-hot cycling 0..3)
all4: gv=1 grant=0100 (expect one-hot cycling 0..3)
all4: gv=1 grant=1000 (expect one-hot cycling 0..3)
all4: gv=1 grant=0001 (expect one-hot cycling 0..3)
all4: gv=1 grant=0010 (expect one-hot cycling 0..3)
lone2: gv=1 grant=0001 (expect 0100)
lone2: gv=1 grant=0100 (expect 0100)
lone2: gv=1 grant=0100 (expect 0100)
idle: gv=1 grant=0100 (expect gv=0)
resume: gv=0 grant=0000 (expect 1000 idx3)
lockgrant: gv=1 grant=0001 (expect 0001 idx0)
held1: gv=1 grant=0001 (expect 0001 held)
held2: gv=1 grant=0001 (expect 0001 held)
lockdrop: gv=1 grant=0001 (expect 0001 owns)
afterdrop: gv=1 grant=0010 (expect 0010 idx1 resume past 0)
dl_grant: gv=1 grant=0001 (expect 0001)
deadair: gv=1 grant=0001 (expect gv=0 grant=0)
resume2: gv=1 grant=0010 (expect 0010 idx1)
/tmp/tb.sv:45: $finish called at 226 (1s)

[stdout]
all4: gv=0 grant=0000 (expect one-hot cycling 0..3)
all4: gv=1 grant=0010 (expect one-hot cycling 0..3)
all4: gv=1 grant=0100 (expect one-hot cycling 0..3)
all4: gv=1 grant=1000 (expect one-hot cycling 0..3)
all4: gv=1 grant=0001 (expect one-hot cycling 0..3)
all4: gv=1 grant=0010 (expect one-hot cycling 0..3)
lone2: gv=1 grant=0001 (expect 0100)
lone2: gv=1 grant=0100 (expect 0100)
lone2: gv=1 grant=0100 (expect 0100)
idle: gv=1 grant=0100 (expect gv=0)
resume: gv=0 grant=0000 (expect 1000 idx3)
lockgrant: gv=1 grant=0001 (expect 0001 idx0)
held1: gv=1 grant=0001 (expect 0001 held)
held2: gv=1 grant=0001 (expect 0001 held)
lockdrop: gv=1 grant=0001 (expect 0001 owns)
afterdrop: gv=1 grant=0010 (expect 0010 idx1 resume past 0)
dl_grant: gv=1 grant=0001 (expect 0001)
deadair: gv=1 grant=0001 (expect gv=0 grant=0)
resume2: gv=1 grant=0010 (expect 0010 idx1)
/tmp/tb.sv:45: $finish called at 226 (1s)
The displays are off-by-one due to a testbench sampling race (reading `grant` before the combinational block settles on the newly-assigned `req`). The actual per-cycle sequence is correct. Let me rewrite the TB to sample cleanly.

Run clean-sampled testbench

shell
cat > /tmp/tb.sv << 'EOF'
module tb;
  logic clk=0, rst, lock;
  logic [3:0] req;
  logic [3:0] grant;
  logic grant_valid;
  TopModule d(.clk,.rst,.req,.lock,.grant,.grant_valid);
  always #5 clk=~clk;
  // drive at negedge, sample just before posedge
  task drive(input logic r, input [3:0] q, input logic l);
    @(negedge clk); rst=r; req=q; lock=l;
    #1; $display("t=%0t rst=%b req=%b lock=%b -> gv=%b grant=%b", $time, r,q,l, grant_valid, grant);
  endtask
  initial begin
    // reset
    drive(1,4'b0000,0);
    $display("-- all four forever (expect 0,1,2,3,0,1) --");
    for(int k=0;k<6;k++) drive(0,4'b1111,0);
    $display("-- lone req2 (expect 0100 x3) --");
    drive(1,0,0);
    for(int k=0;k<3;k++) drive(0,4'b0100,0);
    $display("-- idle keeps place: grant2 -> ptr=3; idle; all4 -> expect idx3 --");
    drive(0,4'b0100,0); // grant2, ptr->3
    drive(0,4'b0000,0); // idle -> gv0
    drive(0,4'b1111,0); // expect 1000 (idx3)
    drive(0,4'b1111,0); // expect 0001 (idx0)
    $display("-- lock hold idx0, then drop --");
    drive(1,0,0);
    drive(0,4'b1111,1); // grant0, latch
    drive(0,4'b1111,1); // held0
    drive(0,4'b1111,1); // held0
    drive(0,4'b1111,0); // lock drop: still owns idx0
    drive(0,4'b1111,0); // resume idx1
    $display("-- dead air: hold idx0, drop its req while locked --");
    drive(1,0,0);
    drive(0,4'b1111,1); // grant0 latch
    drive(0,4'b1110,1); // req0 low while locked -> dead air gv0
    drive(0,4'b1111,1); // resume idx1, and lock still high re-latches
    drive(0,4'b1111,1); // held1
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/tb.out /tmp/tb.sv /app/dut.sv && vvp /tmp/tb.out
output
t=11 rst=1 req=0000 lock=0 -> gv=0 grant=0000
-- all four forever (expect 0,1,2,3,0,1) --
t=21 rst=0 req=1111 lock=0 -> gv=1 grant=0001
t=31 rst=0 req=1111 lock=0 -> gv=1 grant=0010
t=41 rst=0 req=1111 lock=0 -> gv=1 grant=0100
t=51 rst=0 req=1111 lock=0 -> gv=1 grant=1000
t=61 rst=0 req=1111 lock=0 -> gv=1 grant=0001
t=71 rst=0 req=1111 lock=0 -> gv=1 grant=0010
-- lone req2 (expect 0100 x3) --
t=81 rst=1 req=0000 lock=0 -> gv=0 grant=0000
t=91 rst=0 req=0100 lock=0 -> gv=1 grant=0100
t=101 rst=0 req=0100 lock=0 -> gv=1 grant=0100
t=111 rst=0 req=0100 lock=0 -> gv=1 grant=0100
-- idle keeps place: grant2 -> ptr=3; idle; all4 -> expect idx3 --
t=121 rst=0 req=0100 lock=0 -> gv=1 grant=0100
t=131 rst=0 req=0000 lock=0 -> gv=0 grant=0000
t=141 rst=0 req=1111 lock=0 -> gv=1 grant=1000
t=151 rst=0 req=1111 lock=0 -> gv=1 grant=0001
-- lock hold idx0, then drop --
t=161 rst=1 req=0000 lock=0 -> gv=0 grant=0000
t=171 rst=0 req=1111 lock=1 -> gv=1 grant=0001
t=181 rst=0 req=1111 lock=1 -> gv=1 grant=0001
t=191 rst=0 req=1111 lock=1 -> gv=1 grant=0001
t=201 rst=0 req=1111 lock=0 -> gv=1 grant=0001
t=211 rst=0 req=1111 lock=0 -> gv=1 grant=0010
-- dead air: hold idx0, drop its req while locked --
t=221 rst=1 req=0000 lock=0 -> gv=0 grant=0000
t=231 rst=0 req=1111 lock=1 -> gv=1 grant=0001
t=241 rst=0 req=1110 lock=1 -> gv=0 grant=0000
t=251 rst=0 req=1111 lock=1 -> gv=1 grant=0010
t=261 rst=0 req=1111 lock=1 -> gv=1 grant=0010
/tmp/tb.sv:39: $finish called at 261 (1s)

[stdout]
t=11 rst=1 req=0000 lock=0 -> gv=0 grant=0000
-- all four forever (expect 0,1,2,3,0,1) --
t=21 rst=0 req=1111 lock=0 -> gv=1 grant=0001
t=31 rst=0 req=1111 lock=0 -> gv=1 grant=0010
t=41 rst=0 req=1111 lock=0 -> gv=1 grant=0100
t=51 rst=0 req=1111 lock=0 -> gv=1 grant=1000
t=61 rst=0 req=1111 lock=0 -> gv=1 grant=0001
t=71 rst=0 req=1111 lock=0 -> gv=1 grant=0010
-- lone req2 (expect 0100 x3) --
t=81 rst=1 req=0000 lock=0 -> gv=0 grant=0000
t=91 rst=0 req=0100 lock=0 -> gv=1 grant=0100
t=101 rst=0 req=0100 lock=0 -> gv=1 grant=0100
t=111 rst=0 req=0100 lock=0 -> gv=1 grant=0100
-- idle keeps place: grant2 -> ptr=3; idle; all4 -> expect idx3 --
t=121 rst=0 req=0100 lock=0 -> gv=1 grant=0100
t=131 rst=0 req=0000 lock=0 -> gv=0 grant=0000
t=141 rst=0 req=1111 lock=0 -> gv=1 grant=1000
t=151 rst=0 req=1111 lock=0 -> gv=1 grant=0001
-- lock hold idx0, then drop --
t=161 rst=1 req=0000 lock=0 -> gv=0 grant=0000
t=171 rst=0 req=1111 lock=1 -> gv=1 grant=0001
t=181 rst=0 req=1111 lock=1 -> gv=1 grant=0001
t=191 rst=0 req=1111 lock=1 -> gv=1 grant=0001
t=201 rst=0 req=1111 lock=0 -> gv=1 grant=0001
t=211 rst=0 req=1111 lock=0 -> gv=1 grant=0010
-- dead air: hold idx0, drop its req while locked --
t=221 rst=1 req=0000 lock=0 -> gv=0 grant=0000
t=231 rst=0 req=1111 lock=1 -> gv=1 grant=0001
t=241 rst=0 req=1110 lock=1 -> gv=0 grant=0000
t=251 rst=0 req=1111 lock=1 -> gv=1 grant=0010
t=261 rst=0 req=1111 lock=1 -> gv=1 grant=0010
/tmp/tb.sv:39: $finish called at 261 (1s)
All behaviors match the spec exactly: - **All four forever** → `0001,0010,0100,1000,0001,0010` = 0,1,2,3,0,1 ✓ - **Lone requester** (req2) → keeps winning `0100` ✓ - **Idle keeps place** → after serving 2 (ptr→3), an idle cycle gives `gv=0`, then resumes at idx3 (`1000`) then idx0 ✓ - **Lock hold** → latches idx0, `gv` high while lock high; when lock drops the held winner still owns that cycle, sharing resumes at idx1 next cycle ✓ - **Dead air** → held req0 goes low while locked → `gv=0, grant=0000`; slot counts as served, sharing resumes at idx1, and a still-high lock re-latches the next real winner with no re-arm ✓ The module is written to `/app/dut.sv` and compiles cleanly under `iverilog -g2012`. Design summary: - **State**: `start_ptr` (scan origin = just past last winner, resets to 0 so the first all-ask grants 0), `locked`/`lock_idx` (the latched hold). - **Arbitration** is combinational: scan `req` circularly from `start_ptr`, first asker wins; when held, the output is nailed to `lock_idx` as long as it keeps asking. - **Outputs** are combinational (`grant` one-hot, `grant_valid`), so they track the current cycle's `req`. `lock` only influences next-state latching, never gates the current grant , which is why the held winner still owns the lock-drop cycle. - **Reset** is synchronous active-high: clears the hold and sets `start_ptr` to 0.
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: 0 in 4583 samples)
TEST reference_equivalence: PASS (all 4583 samples matched reference)
{"score": 1.000000, "results_file": "/tmp/h2t-grader-home.9z7F1C/runtime/cb_verifier_325-24459-31879-20187_325/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_208bea750e8b4442. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_208bea750e8b4442 · verifier authoritative; classifier explanatory.