SyncVals · Trajectory

debug-halt-step-fsm

claude-code claude-opus-4-8 ✗ failed BAD_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

BAD_FAILUREThe task is at fault, underspecified/contradictory instruction, brittle/flaky tests, or tests demanding undiscoverable behavior.

SubtypeAmbiguous Requirements

EvidenceInstruction says: "with dbus_ren_i high and address A at edge R, dbus_rdata_o presents location A at edge R+1". The agent implemented that as a registered request stage (`ren_d <= dbus_ren_i; addr_d <= dbus_addr_i; if (ren_d) dbus_rdata_o <= dbg_mem(addr_d);`). But the reference/test expects `if (dbus_ren_i) dbus_rdata_o <= rsel(dbus_addr_i, ...)` in the same clocked edge, and verifier failed with `reference_equivalence: FAIL (1196 mismatches in 2301 samples)`.

Root causeThe written dbus timing requirement reasonably implies a one-cycle read latency, but the oracle/reference model updates `dbus_rdata_o` from the current-cycle `dbus_ren_i` and address. The agent produced a plausible spec-faithful implementation that was rejected by tests coupled to the other interpretation.

RecommendationClarify the dbus timing in `instruction.md` to match the intended oracle, or change the oracle/tests to require true R+1 latency. In particular, state whether data may update on the same clock edge that samples `dbus_ren_i` or only on the following clock edge.

Trajectory

Tool-by-tool agent trajectory

3 tool calls · 2 tool types · 8 steps

A control block stops and resumes a core over a debug port. Ports: clk, rst, halt_req_i, resume_req_i, step_req_i, dbus_ren_i, dbus_addr_i (2 bits), dbus_rdata_o (8 bits), halted_o, fetch_o. rst is synchronous, active high, clearing all state and outputs to 0. All outputs are registered. While running, fetch_o is high every cycle and halted_o is low. When halt_req_i is sampled high at edge E while running, fetch_o stays high through edge E+1, is low from edge E+2, and halted_o is first high at edge E+2. In any state, with dbus_ren_i high and address A at edge R, dbus_rdata_o presents location A at edge R+1 and holds while dbus_ren_i is low. Address 0 reads 0x5A, address 1 reads 0xC3, address 2 reads 0x0F, address 3 reads 0x96. When step_req_i is sampled high at edge S while halted, fetch_o is high at edge S+1 only, and halted_o is low at edge S+1 and high again at edge S+2. When resume_req_i is sampled high at edge G while halted, halted_o is low and fetch_o high from edge G+1. If halt_req_i and step_req_i are both high at an edge while halted, the core stays halted and fetch_o stays low. Write module TopModule to /app/dut.sv.

I'll analyze the timing spec carefully, then write the module. Let me work out the state machine and verify the timing relationships before writing. My analysis of the timing (all outputs registered, so an input sampled at edge X first affects registered outputs one edge later): - **Halt** (sampled at E): fetch high at E, E+1; drops at E+2; halted first high at E+2 → needs 2 "still-fetching" transition states. - **Step** (sampled at S): stays halted at S; fetch high at S+1 only; halted again at S+2 → 1 request-latch state + 1 fetch state. - **Resume** (sampled at G): halted at G; running from G+1 → 1 transition state. - **dbus read** (sampled at R): data at R+1 → registered request + registered output (2 stages). Let me write it:

write · /app/dut.sv

module TopModule (
  input            clk,
  input            rst,
  input            halt_req_i,
  input            resume_req_i,
  input            step_req_i,
  input            dbus_ren_i,
  input      [1:0] dbus_addr_i,
  output reg [7:0] dbus_rdata_o,
  output reg       halted_o,
  output reg       fetch_o
);

  // FSM states
  localparam [2:0] RUN  = 3'd0, // running: fetch=1, halted=0
                   H1   = 3'd1, // halt in progress, still fetching
                   H2   = 3'd2, // halt in progress, still fetching
                   HALT = 3'd3, // halted: fetch=0, halted=1
                   SREQ = 3'd4, // step requested, still halted this cycle
                   SGO  = 3'd5, // step: single fetch cycle
                   RREQ = 3'd6;  // resume requested, still halted this cycle

  reg [2:0] state;

  // dbus read pipeline: request registered, then data presented next edge
  reg       ren_d;
  reg [1:0] addr_d;

  // debug memory contents
  function [7:0] dbg_mem(input [1:0] a);
    case (a)
      2'd0: dbg_mem = 8'h5A;
      2'd1: dbg_mem = 8'hC3;
      2'd2: dbg_mem = 8'h0F;
      2'd3: dbg_mem = 8'h96;
    endcase
  endfunction

  // next-state and next-output (Moore decode of the state being entered)
  reg [2:0] next_state;
  reg       next_fetch;
  reg       next_halted;

  always @(*) begin
    next_state = state;
    case (state)
      RUN:  next_state = halt_req_i ? H1 : RUN;
      H1:   next_state = H2;
      H2:   next_state = HALT;
      HALT: begin
              if      (halt_req_i)   next_state = HALT;  // halt (and halt+step) keeps halted
              else if (step_req_i)   next_state = SREQ;
              else if (resume_req_i) next_state = RREQ;
              else                   next_state = HALT;
            end
      SREQ: next_state = SGO;
      SGO:  next_state = HALT;
      RREQ: next_state = RUN;
      default: next_state = RUN;
    endcase

    case (next_state)
      RUN, H1, H2, SGO: begin next_fetch = 1'b1; next_halted = 1'b0; end
      default:          begin next_fetch = 1'b0; next_halted = 1'b1; end
    endcase
  end

  always @(posedge clk) begin
    if (rst) begin
      state        <= RUN;
      fetch_o      <= 1'b0;
      halted_o     <= 1'b0;
      dbus_rdata_o <= 8'h00;
      ren_d        <= 1'b0;
      addr_d       <= 2'b00;
    end else begin
      state    <= next_state;
      fetch_o  <= next_fetch;
      halted_o <= next_halted;

      // debug bus read: sample request now, present data next edge, hold otherwise
      ren_d  <= dbus_ren_i;
      addr_d <= dbus_addr_i;
      if (ren_d)
        dbus_rdata_o <= dbg_mem(addr_d);
    end
  end

endmodule

Let me verify it compiles:

Compile the SystemVerilog module

shell

iverilog -g2012 -o /tmp/dut_test dut.sv 2>&1 && echo "COMPILE OK" || echo "(iverilog not available or error above)"

output

COMPILE OK

[stdout]
COMPILE OK

Compiles cleanly. Let me write a quick testbench to verify the exact timing behaviors:

Run timing testbench

shell

cat > /tmp/tb.sv << 'EOF'
module tb;
  reg clk=0, rst, halt_req_i=0, resume_req_i=0, step_req_i=0, dbus_ren_i=0;
  reg [1:0] dbus_addr_i=0;
  wire [7:0] dbus_rdata_o; wire halted_o, fetch_o;
  integer t=0;
  TopModule dut(.*);
  always #5 clk=~clk;
  // sample just before each posedge (at the "edge")
  task step; begin @(posedge clk); #1; t=t+1; end endtask
  initial begin
    rst=1; @(posedge clk); #1; t=0;
    $display("after reset t=%0d fetch=%b halted=%b", t, fetch_o, halted_o);
    rst=0;
    // run a few cycles
    step; $display("t=%0d fetch=%b halted=%b (running)", t, fetch_o, halted_o);
    step; $display("t=%0d fetch=%b halted=%b (running)", t, fetch_o, halted_o);
    // ---- HALT at edge E ----
    // assert halt_req to be sampled at next posedge = edge E
    halt_req_i=1; @(posedge clk); #1; halt_req_i=0; t=t+1; // this is edge E
    $display("HALT: edge E   t=%0d fetch=%b halted=%b (exp f1 h0)", t, fetch_o, halted_o);
    step; $display("      E+1   t=%0d fetch=%b halted=%b (exp f1 h0)", t, fetch_o, halted_o);
    step; $display("      E+2   t=%0d fetch=%b halted=%b (exp f0 h1)", t, fetch_o, halted_o);
    step; $display("      E+3   t=%0d fetch=%b halted=%b (exp f0 h1)", t, fetch_o, halted_o);
    // ---- STEP at edge S ----
    step_req_i=1; @(posedge clk); #1; step_req_i=0; t=t+1; // edge S
    $display("STEP: edge S   t=%0d fetch=%b halted=%b (exp f0 h1)", t, fetch_o, halted_o);
    step; $display("      S+1   t=%0d fetch=%b halted=%b (exp f1 h0)", t, fetch_o, halted_o);
    step; $display("      S+2   t=%0d fetch=%b halted=%b (exp f0 h1)", t, fetch_o, halted_o);
    // ---- HALT+STEP both high while halted ----
    halt_req_i=1; step_req_i=1; @(posedge clk); #1; halt_req_i=0; step_req_i=0; t=t+1;
    $display("H+S:  edge     t=%0d fetch=%b halted=%b (exp f0 h1 stay)", t, fetch_o, halted_o);
    step; $display("      +1    t=%0d fetch=%b halted=%b (exp f0 h1)", t, fetch_o, halted_o);
    // ---- DBUS read address 1 (=0xC3) at edge R ----
    dbus_ren_i=1; dbus_addr_i=1; @(posedge clk); #1; dbus_ren_i=0; t=t+1; // edge R
    $display("DBUS: edge R   t=%0d rdata=%02h (exp hold prev)", t, dbus_rdata_o);
    step; $display("      R+1   t=%0d rdata=%02h (exp C3)", t, dbus_rdata_o);
    step; $display("      R+2   t=%0d rdata=%02h (exp C3 hold)", t, dbus_rdata_o);
    // ---- read address 2 (=0x0F) ----
    dbus_ren_i=1; dbus_addr_i=2; @(posedge clk); #1; dbus_ren_i=0; t=t+1;
    step; $display("      addr2 R+1 rdata=%02h (exp 0F)", dbus_rdata_o);
    // ---- RESUME at edge G ----
    resume_req_i=1; @(posedge clk); #1; resume_req_i=0; t=t+1; // edge G
    $display("RESUME edge G  t=%0d fetch=%b halted=%b (exp f0 h1)", t, fetch_o, halted_o);
    step; $display("      G+1   t=%0d fetch=%b halted=%b (exp f1 h0)", t, fetch_o, halted_o);
    step; $display("      G+2   t=%0d fetch=%b halted=%b (exp f1 h0 running)", t, fetch_o, halted_o);
    $finish;
  end
endmodule
EOF
iverilog -g2012 -o /tmp/tbrun /tmp/tb.sv /app/dut.sv && vvp /tmp/tbrun

output

after reset t=0 fetch=0 halted=0
t=1 fetch=1 halted=0 (running)
t=2 fetch=1 halted=0 (running)
HALT: edge E   t=3 fetch=1 halted=0 (exp f1 h0)
      E+1   t=4 fetch=1 halted=0 (exp f1 h0)
      E+2   t=5 fetch=0 halted=1 (exp f0 h1)
      E+3   t=6 fetch=0 halted=1 (exp f0 h1)
STEP: edge S   t=7 fetch=0 halted=1 (exp f0 h1)
      S+1   t=8 fetch=1 halted=0 (exp f1 h0)
      S+2   t=9 fetch=0 halted=1 (exp f0 h1)
H+S:  edge     t=10 fetch=0 halted=1 (exp f0 h1 stay)
      +1    t=11 fetch=0 halted=1 (exp f0 h1)
DBUS: edge R   t=12 rdata=00 (exp hold prev)
      R+1   t=13 rdata=c3 (exp C3)
      R+2   t=14 rdata=c3 (exp C3 hold)
      addr2 R+1 rdata=0f (exp 0F)
RESUME edge G  t=17 fetch=0 halted=1 (exp f0 h1)
      G+1   t=18 fetch=1 halted=0 (exp f1 h0)
      G+2   t=19 fetch=1 halted=0 (exp f1 h0 running)
/tmp/tb.sv:46: $finish called at 196 (1s)

[stdout]
after reset t=0 fetch=0 halted=0
t=1 fetch=1 halted=0 (running)
t=2 fetch=1 halted=0 (running)
HALT: edge E   t=3 fetch=1 halted=0 (exp f1 h0)
      E+1   t=4 fetch=1 halted=0 (exp f1 h0)
      E+2   t=5 fetch=0 halted=1 (exp f0 h1)
      E+3   t=6 fetch=0 halted=1 (exp f0 h1)
STEP: edge S   t=7 fetch=0 halted=1 (exp f0 h1)
      S+1   t=8 fetch=1 halted=0 (exp f1 h0)
      S+2   t=9 fetch=0 halted=1 (exp f0 h1)
H+S:  edge     t=10 fetch=0 halted=1 (exp f0 h1 stay)
      +1    t=11 fetch=0 halted=1 (exp f0 h1)
DBUS: edge R   t=12 rdata=00 (exp hold prev)
      R+1   t=13 rdata=c3 (exp C3)
      R+2   t=14 rdata=c3 (exp C3 hold)
      addr2 R+1 rdata=0f (exp 0F)
RESUME edge G  t=17 fetch=0 halted=1 (exp f0 h1)
      G+1   t=18 fetch=1 halted=0 (exp f1 h0)
      G+2   t=19 fetch=1 halted=0 (exp f1 h0 running)
/tmp/tb.sv:46: $finish called at 196 (1s)

All timing requirements verified against the spec. Every expectation matches: - **Reset**: outputs cleared to 0. - **Running**: `fetch_o=1`, `halted_o=0` every cycle. - **Halt** (edge E): fetch stays high through E+1, low from E+2; `halted_o` first high at E+2. ✓ - **Step** (edge S): still halted at S, `fetch_o` high at S+1 *only*, `halted_o` low at S+1 and high again at S+2. ✓ - **Halt+Step both high while halted**: stays halted, fetch stays low. ✓ - **dbus read** (edge R): data presented at R+1 (`0xC3` for addr 1, `0x0F` for addr 2) and held while `dbus_ren_i` low. ✓ - **Resume** (edge G): halted at G, running from G+1. ✓ The module is written to `/app/dut.sv`. It uses a Moore FSM (`RUN → H1 → H2 → HALT` for halting, `SREQ → SGO` for stepping, `RREQ` for resume) with all outputs registered, plus a two-stage registered read path for the debug bus to get the R+1 latency. After reset the core auto-runs (there's no separate start command, and reset clears outputs to 0 for that cycle).

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_compiles: PASS (compiled with held-back reference/testbench)
TEST simulation_completed: PASS (Mismatches: 1196 in 2301 samples)
TEST reference_equivalence: FAIL (1196 mismatches in 2301 samples)
{"score": 0.000000, "results_file": "/tmp/h2t-grader-home.kZMP0W/runtime/cb_verifier_311-16780-14414-9041_311/results.jsonl"}

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_eadcf1b44802453c. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_eadcf1b44802453c · verifier authoritative; classifier explanatory.