SyncVals · Trajectory

open-drain-command-engine

codex gpt-5.5 ✓ 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

EvidenceAgent created a 16-state FSM implementation of the I2C command engine with two modules (TopModule wrapper + I2cCommandEngine). Test outcome: MISMATCH_COUNTS cmd_ack=0 busy=0 al=0 bit_out=0 scl=0 sda=0, mismatches=0 samples=2351, reward=1.0. The implementation correctly handles: (1) Synchronous reset clearing outputs, (2) Command gating (cmd=0 accepts while idle or busy, others only while busy), (3) Registered outputs and one-cycle cmd_ack pulses, (4) Multi-cycle setup phase transitions with SCL/SDA stretch awareness, (5) Arbitration loss detection on write-1 high and start wait phases, (6) Read sampling on SCL high edge. Agent self-tested with iverilog syntax check and a comprehensive testbench covering reset, illegal commands, start, write with arbitration, read sampling, and stop sequences.

Root causeAgent correctly interpreted a dense, complex protocol specification and implemented a fully functional FSM that passes exhaustive simulation testing with zero output mismatches across 2351 samples.

RecommendationN/A - task is fine

Trajectory

Tool-by-tool agent trajectory

14 tool calls · 2 tool types · 25 steps

<permissions instructions> Filesystem sandboxing defines which files can be read or written. `sandbox_mode` is `danger-full-access`: No filesystem sandboxing - all commands are permitted. Network access is enabled. Approval policy is currently never. Do not provide the `sandbox_permissions` for any reason, commands will be rejected. </permissions instructions><apps_instructions> ## Apps (Connectors) Apps (Connectors) can be explicitly triggered in user messages in the format `[$app-name](app://{connector_id})`. Apps can also be implicitly triggered as long as the context suggests usage of available apps. An app is equivalent to a set of MCP tools within the `codex_apps` MCP. An installed app's MCP tools are either provided to you already, or can be lazy-loaded through the `tool_search` tool. If `tool_search` is available, the apps that are searchable by `tools_search` will be listed by it. Do not additionally call list_mcp_resources or list_mcp_resource_templates for apps. </apps_instructions><skills_instructions> ## Skills A skill is a set of instructions provided through a `SKILL.md` source. Below is the list of skills that can be used. Each entry includes a name, description, and source locator. `file` locators are on the host filesystem, `environment resource` locators are owned by an execution environment, `orchestrator resource` locators are opaque non-filesystem resources, and `custom resource` locators use their provider's access mechanism. ### Available skills - imagegen: Generate or edit raster images when the task benefits from AI-created bitmap visuals such as photos, illustrations, textures, sprites, mockups, or transparent-background cutouts. Use when Codex should create a brand-new image, transform an existing image, or derive visual variants from references, and the output should be a bitmap asset rather than repo-native code or vector. Do not use when the task is better handled by editing existing SVG/vector/code-native assets, extending an established icon or logo system, or building the visual directly in HTML/CSS/canvas. (file: /tmp/codex-home/skills/.system/imagegen/SKILL.md) - openai-docs: Use when the user asks how to build with OpenAI products or APIs, asks about Codex itself or choosing Codex surfaces, needs up-to-date official documentation with citations, help choosing the latest model for a use case, or model upgrade and prompt-upgrade guidance; use OpenAI docs MCP tools for non-Codex docs questions, use the Codex manual helper first for broad Codex self-knowledge, and restrict fallback browsing to official OpenAI domains. (file: /tmp/codex-home/skills/.system/openai-docs/SKILL.md) - plugin-creator: Create and scaffold plugin directories for Codex with a required `.codex-plugin/plugin.json`, optional plugin folders/files, valid manifest defaults, and personal-marketplace entries by default. Use when Codex needs to create a new personal plugin, add optional plugin structure, generate or update marketplace entries for plugin ordering and availability metadata, or update an existing local plugin during development with the CLI-driven cachebuster and reinstall flow. (file: /tmp/codex-home/skills/.system/plugin-creator/SKILL.md) - skill-creator: Guide for creating effective skills. This skill should be used when users want to create a new skill (or update an existing skill) that extends Codex's capabilities with specialized knowledge, workflows, or tool integrations. (file: /tmp/codex-home/skills/.system/skill-creator/SKILL.md) - skill-installer: Install Codex skills into $CODEX_HOME/skills from a curated list or a [code-host] repo path. Use when a user asks to list installable skills, install a curated skill, or install a skill from another repo (including private repos). (file: /tmp/codex-home/skills/.system/skill-installer/SKILL.md) ### How to use skills - Discovery: The list above is the skills available in this session (name + description + source locator). `file` entries live on the host filesystem, `environment resource` entries are owned by their execution environment, `orchestrator resource` entries must be accessed through `skills.list` and `skills.read`, and `custom resource` entries use their provider's access mechanism. - Trigger rules: If the user names a skill (with `$SkillName` or plain text) OR the task clearly matches a skill's description shown above, you must use that skill for that turn. Multiple mentions mean use them all. Do not carry skills across turns unless re-mentioned. - Missing/blocked: If a named skill isn't in the list or its source can't be read, say so briefly and continue with the best fallback. - How to use a skill (progressive disclosure): 1) After deciding to use a skill, the main agent must read its `SKILL.md` completely before taking task actions. For a `file` entry, open the listed path. For an `environment resource`, use the filesystem of the owning environment. For an `orchestrator resource`, call `skills.list` with `{"authority":{"kind":"orchestrator"}}`, select the matching package, and pass its `main_resource` to `skills.read`. If a read is truncated or paginated, continue until EOF. 2) When `SKILL.md` references another resource, use the same access mechanism. Resolve relative paths against a filesystem-backed skill directory. For orchestrator skills, pass the exact referenced resource identifier with the same authority and package to `skills.read`; do not treat `skill://` identifiers as filesystem paths. 3) If `SKILL.md` points to extra folders such as `references/`, use its routing instructions to identify the resources required for the task. The main agent must read each required instruction or reference file itself before acting on it. Do not delegate reading, summarizing, or interpreting skill instructions to a subagent. Subagents may still perform task work when the selected skill allows it. 4) For filesystem-backed skills, prefer running or patching provided scripts instead of retyping large code blocks. For orchestrator skills, use `skills.read` and the available tools; do not invent a local path. 5) Reuse provided assets or templates through the same source access mechanism instead of recreating them. - Coordination and sequencing: - If multiple skills apply, choose the minimal set that covers the request and state the order you'll use them. - Announce which skill(s) you're using and why (one short line). If you skip an obvious skill, say why. - Context hygiene: - Progressive disclosure applies to selecting relevant files, not partially reading a selected instruction file. Do not load unrelated references, scripts, or assets. - Avoid deep reference-chasing: prefer opening only files directly linked from `SKILL.md` unless you're blocked. - When variants exist (frameworks, providers, domains), pick only the relevant reference file(s) and note that choice. - Safety and fallback: If a skill can't be applied cleanly (missing files, unclear instructions), state the issue, pick the next-best approach, and continue. </skills_instructions>

<environment_context> <cwd>/app</cwd> <shell>bash</shell> <current_date>2026-06-20</current_date> <timezone>Etc/UTC</timezone> <filesystem><workspace_roots><root>/app</root></workspace_roots><permission_profile type="disabled"><file_system type="unrestricted" /></permission_profile></filesystem> </environment_context>

Implement `/app/dut.sv`. Define `TopModule` with this port list: `input clk, rst, cmd_valid, bit_in, scl_i, sda_i; input [1:0] cmd; output cmd_ack, busy, al, bit_out, scl_drive_low, sda_drive_low;`. Define another module and instantiate it from `TopModule`. `rst` is synchronous active high and clears outputs to 0. All outputs are registered. `cmd_ack` is a one-cycle completion pulse. Accept a command only when none is active; `cmd=0` may be accepted while idle or busy, and other commands only while `busy` is high. Illegal commands leave outputs unchanged and do not acknowledge. The line controls are open-drain: 1 pulls low, 0 releases. The command-accept edge only loads the first registered phase; it must not also consume `scl_i` or sample `sda_i` for phase completion. Because every output is registered, when a later phase says "until `scl_i` is high", the clock edge that observes high only records that fact; the next visible line-drive/ack phase appears on the following clock. Command acceptance itself is a visible registered phase: for stop, write, and read, the line-drive values loaded on the accept edge must be held through one additional full clocked setup cycle before the SCL-release phase begins. `cmd=0` starts or restarts ownership: release both lines on accept, then in following clocked wait phase keep both released until `scl_i` is high, keep SCL released while pulling SDA low for one full cycle, then pull SCL low and pulse `cmd_ack`; `busy` is set from the accept edge. `cmd=1` stops: drive both low on accept, keep both low for the following setup cycle, release SCL until `scl_i` is high, then release SDA, clear `busy`, and pulse `cmd_ack` on the following clock. `cmd=2` writes `bit_in`: on accept drive SCL low and drive SDA low for 0 or released for 1, keep those drive values for the following setup cycle, release SCL until `scl_i` is high, then pull SCL low and acknowledge on the following clock. `cmd=3` reads: on accept drive SCL low and release SDA, keep those drive values for the following setup cycle, release SCL until `scl_i` is high, sample `sda_i` on that high-SCL edge into `bit_out`, then pull SCL low and acknowledge on the following clock. `al` is sticky until reset and asserts only during start or write-1 high-SCL phases when released SDA is observed low.

I’ll inspect the workspace and any existing `dut.sv` before editing, then I’ll implement the state machine and run whatever local checks are available.

shell

pwd && rg --files -g '!*'

output

Chunk ID: b37e48
Wall time: 0.0000 seconds
Process exited with code 1
Original token count: 2
Output:
/app

shell

sed -n '1,240p' /app/dut.sv

output

Chunk ID: e95161
Wall time: 0.0002 seconds
Process exited with code 2
Original token count: 14
Output:
sed: can't read /app/dut.sv: No such file or directory

shell

ls -la /app

output

Chunk ID: 9fd4f8
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 24
Output:
total 8
drwxr-xr-x 2 root root 4096 Jun 19 22:02 .
drwxr-xr-x 1 root root 4096 Jun 20 17:47 ..

There isn’t an existing source file in `/app`; I’m creating `dut.sv` from scratch. The key point I’m encoding is the extra registered setup cycle and the delayed transition after observing `scl_i` high, since those are where off-by-one clock bugs usually happen in this kind of bit controller.

I’ve settled on a small command engine under `TopModule`. `busy` is treated as bus ownership, separate from the internal “command in progress” state, so repeated start (`cmd=0`) can be accepted whenever the engine is idle even if `busy` is already set.

apply_patch

*** Begin Patch
*** Add File: /app/dut.sv
+module TopModule(
+    input clk,
+    input rst,
+    input cmd_valid,
+    input bit_in,
+    input scl_i,
+    input sda_i,
+    input [1:0] cmd,
+    output cmd_ack,
+    output busy,
+    output al,
+    output bit_out,
+    output scl_drive_low,
+    output sda_drive_low
+);
+
+    I2cCommandEngine engine (
+        .clk(clk),
+        .rst(rst),
+        .cmd_valid(cmd_valid),
+        .bit_in(bit_in),
+        .scl_i(scl_i),
+        .sda_i(sda_i),
+        .cmd(cmd),
+        .cmd_ack(cmd_ack),
+        .busy(busy),
+        .al(al),
+        .bit_out(bit_out),
+        .scl_drive_low(scl_drive_low),
+        .sda_drive_low(sda_drive_low)
+    );
+
+endmodule
+
+module I2cCommandEngine(
+    input clk,
+    input rst,
+    input cmd_valid,
+    input bit_in,
+    input scl_i,
+    input sda_i,
+    input [1:0] cmd,
+    output reg cmd_ack,
+    output reg busy,
+    output reg al,
+    output reg bit_out,
+    output reg scl_drive_low,
+    output reg sda_drive_low
+);
+
+    localparam [3:0]
+        ST_IDLE             = 4'd0,
+        ST_START_WAIT       = 4'd1,
+        ST_START_SDA_LOW    = 4'd2,
+        ST_START_DONE       = 4'd3,
+        ST_STOP_SETUP       = 4'd4,
+        ST_STOP_RELEASE_SCL = 4'd5,
+        ST_STOP_WAIT        = 4'd6,
+        ST_STOP_DONE        = 4'd7,
+        ST_WRITE_SETUP      = 4'd8,
+        ST_WRITE_RELEASE_SCL= 4'd9,
+        ST_WRITE_WAIT       = 4'd10,
+        ST_WRITE_DONE       = 4'd11,
+        ST_READ_SETUP       = 4'd12,
+        ST_READ_RELEASE_SCL = 4'd13,
+        ST_READ_WAIT        = 4'd14,
+        ST_READ_DONE        = 4'd15;
+
+    reg [3:0] state;
+    reg write_one;
+
+    always @(posedge clk) begin
+        if (rst) begin
+            state <= ST_IDLE;
+            write_one <= 1'b0;
+            cmd_ack <= 1'b0;
+            busy <= 1'b0;
+            al <= 1'b0;
+            bit_out <= 1'b0;
+            scl_drive_low <= 1'b0;
+            sda_drive_low <= 1'b0;
+        end else begin
+            cmd_ack <= 1'b0;
+
+            case (state)
+                ST_IDLE: begin
+                    if (cmd_valid) begin
+                        case (cmd)
+                            2'd0: begin
+                                busy <= 1'b1;
+                                scl_drive_low <= 1'b0;
+                                sda_drive_low <= 1'b0;
+                                state <= ST_START_WAIT;
+                            end
+
+                            2'd1: begin
+                                if (busy) begin
+                                    scl_drive_low <= 1'b1;
+                                    sda_drive_low <= 1'b1;
+                                    state <= ST_STOP_SETUP;
+                                end
+                            end
+
+                            2'd2: begin
+                                if (busy) begin
+                                    write_one <= bit_in;
+                                    scl_drive_low <= 1'b1;
+                                    sda_drive_low <= ~bit_in;
+                                    state <= ST_WRITE_SETUP;
+                                end
+                            end
+
+                            2'd3: begin
+                                if (busy) begin
+                                    scl_drive_low <= 1'b1;
+                                    sda_drive_low <= 1'b0;
+                                    state <= ST_READ_SETUP;
+                                end
+                            end
+                        endcase
+                    end
+                end
+
+                ST_START_WAIT: begin
+                    if (scl_i) begin
+                        if (!sda_i) begin
+                            al <= 1'b1;
+                        end
+                        state <= ST_START_SDA_LOW;
+                    end
+                end
+
+                ST_START_SDA_LOW: begin
+                    scl_drive_low <= 1'b0;
+                    sda_drive_low <= 1'b1;
+                    state <= ST_START_DONE;
+                end
+
+                ST_START_DONE: begin
+                    scl_drive_low <= 1'b1;
+                    sda_drive_low <= 1'b1;
+                    cmd_ack <= 1'b1;
+                    state <= ST_IDLE;
+                end
+
+                ST_STOP_SETUP: begin
+                    state <= ST_STOP_RELEASE_SCL;
+                end
+
+                ST_STOP_RELEASE_SCL: begin
+                    scl_drive_low <= 1'b0;
+                    sda_drive_low <= 1'b1;
+                    state <= ST_STOP_WAIT;
+                end
+
+                ST_STOP_WAIT: begin
+                    if (scl_i) begin
+                        state <= ST_STOP_DONE;
+                    end
+                end
+
+                ST_STOP_DONE: begin
+                    scl_drive_low <= 1'b0;
+                    sda_drive_low <= 1'b0;
+                    busy <= 1'b0;
+                    cmd_ack <= 1'b1;
+                    state <= ST_IDLE;
+                end
+
+                ST_WRITE_SETUP: begin
+                    state <= ST_WRITE_RELEASE_SCL;
+                end
+
+                ST_WRITE_RELEASE_SCL: begin
+                    scl_drive_low <= 1'b0;
+                    sda_drive_low <= ~write_one;
+                    state <= ST_WRITE_WAIT;
+                end
+
+                ST_WRITE_WAIT: begin
+                    if (scl_i) begin
+                        if (write_one && !sda_i) begin
+                            al <= 1'b1;
+                        end
+                        state <= ST_WRITE_DONE;
+                    end
+                end
+
+                ST_WRITE_DONE: begin
+                    scl_drive_low <= 1'b1;
+                    sda_drive_low <= ~write_one;
+                    cmd_ack <= 1'b1;
+                    state <= ST_IDLE;
+                end
+
+                ST_READ_SETUP: begin
+                    state <= ST_READ_RELEASE_SCL;
+                end
+
+                ST_READ_RELEASE_SCL: begin
+                    scl_drive_low <= 1'b0;
+                    sda_drive_low <= 1'b0;
+                    state <= ST_READ_WAIT;
+                end
+
+                ST_READ_WAIT: begin
+                    if (scl_i) begin
+                        bit_out <= sda_i;
+                        state <= ST_READ_DONE;
+                    end
+                end
+
+                ST_READ_DONE: begin
+                    scl_drive_low <= 1'b1;
+                    sda_drive_low <= 1'b0;
+                    cmd_ack <= 1'b1;
+                    state <= ST_IDLE;
+                end
+
+                default: begin
+                    state <= ST_IDLE;
+                end
+            endcase
+        end
+    end
+
+endmodule
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
A /app/dut.sv

The implementation is in place. I’m going to run syntax/lint checks now, and I’ll add a lightweight simulation if the installed tools make that quick enough.

shell

command -v iverilog

output

Chunk ID: 64bb9b
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 5
Output:
/usr/bin/iverilog

shell

command -v verilator

output

Chunk ID: c71b95
Wall time: 0.0000 seconds
Process exited with code 1
Original token count: 0
Output:

shell

sed -n '1,260p' /app/dut.sv

output

Chunk ID: bc5bad
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 1650
Output:
module TopModule(
    input clk,
    input rst,
    input cmd_valid,
    input bit_in,
    input scl_i,
    input sda_i,
    input [1:0] cmd,
    output cmd_ack,
    output busy,
    output al,
    output bit_out,
    output scl_drive_low,
    output sda_drive_low
);

    I2cCommandEngine engine (
        .clk(clk),
        .rst(rst),
        .cmd_valid(cmd_valid),
        .bit_in(bit_in),
        .scl_i(scl_i),
        .sda_i(sda_i),
        .cmd(cmd),
        .cmd_ack(cmd_ack),
        .busy(busy),
        .al(al),
        .bit_out(bit_out),
        .scl_drive_low(scl_drive_low),
        .sda_drive_low(sda_drive_low)
    );

endmodule

module I2cCommandEngine(
    input clk,
    input rst,
    input cmd_valid,
    input bit_in,
    input scl_i,
    input sda_i,
    input [1:0] cmd,
    output reg cmd_ack,
    output reg busy,
    output reg al,
    output reg bit_out,
    output reg scl_drive_low,
    output reg sda_drive_low
);

    localparam [3:0]
        ST_IDLE             = 4'd0,
        ST_START_WAIT       = 4'd1,
        ST_START_SDA_LOW    = 4'd2,
        ST_START_DONE       = 4'd3,
        ST_STOP_SETUP       = 4'd4,
        ST_STOP_RELEASE_SCL = 4'd5,
        ST_STOP_WAIT        = 4'd6,
        ST_STOP_DONE        = 4'd7,
        ST_WRITE_SETUP      = 4'd8,
        ST_WRITE_RELEASE_SCL= 4'd9,
        ST_WRITE_WAIT       = 4'd10,
        ST_WRITE_DONE       = 4'd11,
        ST_READ_SETUP       = 4'd12,
        ST_READ_RELEASE_SCL = 4'd13,
        ST_READ_WAIT        = 4'd14,
        ST_READ_DONE        = 4'd15;

    reg [3:0] state;
    reg write_one;

    always @(posedge clk) begin
        if (rst) begin
            state <= ST_IDLE;
            write_one <= 1'b0;
            cmd_ack <= 1'b0;
            busy <= 1'b0;
            al <= 1'b0;
            bit_out <= 1'b0;
            scl_drive_low <= 1'b0;
            sda_drive_low <= 1'b0;
        end else begin
            cmd_ack <= 1'b0;

            case (state)
                ST_IDLE: begin
                    if (cmd_valid) begin
                        case (cmd)
                            2'd0: begin
                                busy <= 1'b1;
                                scl_drive_low <= 1'b0;
                                sda_drive_low <= 1'b0;
                                state <= ST_START_WAIT;
                            end

                            2'd1: begin
                                if (busy) begin
                                    scl_drive_low <= 1'b1;
                                    sda_drive_low <= 1'b1;
                                    state <= ST_STOP_SETUP;
                                end
                            end

                            2'd2: begin
                                if (busy) begin
                                    write_one <= bit_in;
                                    scl_drive_low <= 1'b1;
                                    sda_drive_low <= ~bit_in;
                                    state <= ST_WRITE_SETUP;
                                end
                            end

                            2'd3: begin
                                if (busy) begin
                                    scl_drive_low <= 1'b1;
                                    sda_drive_low <= 1'b0;
                                    state <= ST_READ_SETUP;
                                end
                            end
                        endcase
                    end
                end

                ST_START_WAIT: begin
                    if (scl_i) begin
                        if (!sda_i) begin
                            al <= 1'b1;
                        end
                        state <= ST_START_SDA_LOW;
                    end
                end

                ST_START_SDA_LOW: begin
                    scl_drive_low <= 1'b0;
                    sda_drive_low <= 1'b1;
                    state <= ST_START_DONE;
                end

                ST_START_DONE: begin
                    scl_drive_low <= 1'b1;
                    sda_drive_low <= 1'b1;
                    cmd_ack <= 1'b1;
                    state <= ST_IDLE;
                end

                ST_STOP_SETUP: begin
                    state <= ST_STOP_RELEASE_SCL;
                end

                ST_STOP_RELEASE_SCL: begin
                    scl_drive_low <= 1'b0;
                    sda_drive_low <= 1'b1;
                    state <= ST_STOP_WAIT;
                end

                ST_STOP_WAIT: begin
                    if (scl_i) begin
                        state <= ST_STOP_DONE;
                    end
                end

                ST_STOP_DONE: begin
                    scl_drive_low <= 1'b0;
                    sda_drive_low <= 1'b0;
                    busy <= 1'b0;
                    cmd_ack <= 1'b1;
                    state <= ST_IDLE;
                end

                ST_WRITE_SETUP: begin
                    state <= ST_WRITE_RELEASE_SCL;
                end

                ST_WRITE_RELEASE_SCL: begin
                    scl_drive_low <= 1'b0;
                    sda_drive_low <= ~write_one;
                    state <= ST_WRITE_WAIT;
                end

                ST_WRITE_WAIT: begin
                    if (scl_i) begin
                        if (write_one && !sda_i) begin
                            al <= 1'b1;
                        end
                        state <= ST_WRITE_DONE;
                    end
                end

                ST_WRITE_DONE: begin
                    scl_drive_low <= 1'b1;
                    sda_drive_low <= ~write_one;
                    cmd_ack <= 1'b1;
                    state <= ST_IDLE;
                end

                ST_READ_SETUP: begin
                    state <= ST_READ_RELEASE_SCL;
                end

                ST_READ_RELEASE_SCL: begin
                    scl_drive_low <= 1'b0;
                    sda_drive_low <= 1'b0;
                    state <= ST_READ_WAIT;
                end

                ST_READ_WAIT: begin
                    if (scl_i) begin
                        bit_out <= sda_i;
                        state <= ST_READ_DONE;
                    end
                end

                ST_READ_DONE: begin
                    scl_drive_low <= 1'b1;
                    sda_drive_low <= 1'b0;
                    cmd_ack <= 1'b1;
                    state <= ST_IDLE;
                end

                default: begin
                    state <= ST_IDLE;
                end
            endcase
        end
    end

endmodule

shell

iverilog -g2012 -Wall -o /tmp/dut_check.vvp /app/dut.sv

output

Chunk ID: 9333c5
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 0
Output:

Syntax passes under `iverilog -g2012 -Wall`. I’m adding a temporary self-checking testbench to exercise the main phase timing and arbitration latch behavior, then I’ll remove or leave it outside the workspace so `/app` only contains the requested design.

apply_patch

*** Begin Patch
*** Add File: /tmp/dut_tb.sv
+module tb;
+    reg clk = 1'b0;
+    reg rst = 1'b0;
+    reg cmd_valid = 1'b0;
+    reg bit_in = 1'b0;
+    reg scl_i = 1'b1;
+    reg sda_i = 1'b1;
+    reg [1:0] cmd = 2'b00;
+    wire cmd_ack;
+    wire busy;
+    wire al;
+    wire bit_out;
+    wire scl_drive_low;
+    wire sda_drive_low;
+
+    TopModule dut (
+        .clk(clk),
+        .rst(rst),
+        .cmd_valid(cmd_valid),
+        .bit_in(bit_in),
+        .scl_i(scl_i),
+        .sda_i(sda_i),
+        .cmd(cmd),
+        .cmd_ack(cmd_ack),
+        .busy(busy),
+        .al(al),
+        .bit_out(bit_out),
+        .scl_drive_low(scl_drive_low),
+        .sda_drive_low(sda_drive_low)
+    );
+
+    always #5 clk = ~clk;
+
+    task step;
+        begin
+            @(posedge clk);
+            #1;
+        end
+    endtask
+
+    task check;
+        input condition;
+        input [255:0] message;
+        begin
+            if (!condition) begin
+                $display("FAIL: %0s", message);
+                $finish;
+            end
+        end
+    endtask
+
+    task pulse_start;
+        begin
+            cmd = 2'd0;
+            cmd_valid = 1'b1;
+            scl_i = 1'b1;
+            sda_i = 1'b1;
+            step();
+            check(busy && !cmd_ack && !scl_drive_low && !sda_drive_low, "start accept");
+            cmd_valid = 1'b0;
+            step();
+            check(busy && !cmd_ack && !scl_drive_low && !sda_drive_low, "start wait observes high only");
+            step();
+            check(busy && !cmd_ack && !scl_drive_low && sda_drive_low, "start sda low setup");
+            step();
+            check(busy && cmd_ack && scl_drive_low && sda_drive_low, "start ack");
+            step();
+            check(busy && !cmd_ack && scl_drive_low && sda_drive_low, "start ack clears");
+        end
+    endtask
+
+    initial begin
+        rst = 1'b1;
+        step();
+        check(!cmd_ack && !busy && !al && !bit_out && !scl_drive_low && !sda_drive_low, "reset clears outputs");
+        rst = 1'b0;
+        step();
+
+        cmd = 2'd1;
+        cmd_valid = 1'b1;
+        step();
+        check(!cmd_ack && !busy && !scl_drive_low && !sda_drive_low, "illegal stop while idle unchanged");
+        cmd_valid = 1'b0;
+
+        pulse_start();
+
+        cmd = 2'd2;
+        bit_in = 1'b1;
+        cmd_valid = 1'b1;
+        sda_i = 1'b1;
+        step();
+        check(!cmd_ack && busy && scl_drive_low && !sda_drive_low, "write accept");
+        cmd_valid = 1'b0;
+        step();
+        check(!cmd_ack && busy && scl_drive_low && !sda_drive_low, "write setup");
+        step();
+        check(!cmd_ack && busy && !scl_drive_low && !sda_drive_low, "write release scl");
+        sda_i = 1'b0;
+        step();
+        check(!cmd_ack && busy && al && !scl_drive_low && !sda_drive_low, "write arbitration records on high");
+        step();
+        check(cmd_ack && busy && al && scl_drive_low && !sda_drive_low, "write ack after observed high");
+        step();
+
+        rst = 1'b1;
+        step();
+        rst = 1'b0;
+        step();
+        pulse_start();
+
+        cmd = 2'd3;
+        cmd_valid = 1'b1;
+        sda_i = 1'b0;
+        step();
+        check(!cmd_ack && busy && scl_drive_low && !sda_drive_low, "read accept");
+        cmd_valid = 1'b0;
+        step();
+        check(!cmd_ack && busy && scl_drive_low && !sda_drive_low, "read setup");
+        step();
+        check(!cmd_ack && busy && !scl_drive_low && !sda_drive_low, "read release scl");
+        sda_i = 1'b1;
+        step();
+        check(!cmd_ack && busy && bit_out && !scl_drive_low && !sda_drive_low, "read samples on high");
+        step();
+        check(cmd_ack && busy && bit_out && scl_drive_low && !sda_drive_low, "read ack after sample");
+        step();
+
+        cmd = 2'd1;
+        cmd_valid = 1'b1;
+        step();
+        check(!cmd_ack && busy && scl_drive_low && sda_drive_low, "stop accept");
+        cmd_valid = 1'b0;
+        step();
+        check(!cmd_ack && busy && scl_drive_low && sda_drive_low, "stop setup");
+        step();
+        check(!cmd_ack && busy && !scl_drive_low && sda_drive_low, "stop release scl");
+        step();
+        check(!cmd_ack && busy && !scl_drive_low && sda_drive_low, "stop observes high only");
+        step();
+        check(cmd_ack && !busy && !scl_drive_low && !sda_drive_low, "stop ack clears busy");
+
+        $display("PASS");
+        $finish;
+    end
+endmodule
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
A /tmp/dut_tb.sv

The temporary testbench checks reset, illegal command rejection, start, write-1 arbitration, read sampling, and stop timing. I’m running it now against the new design.

shell

iverilog -g2012 -Wall -o /tmp/dut_tb.vvp /app/dut.sv /tmp/dut_tb.sv && vvp /tmp/dut_tb.vvp

output

Chunk ID: 103996
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 13
Output:
PASS
/tmp/dut_tb.sv:143: $finish called at 316 (1s)

The self-checking simulation passed. I’m cleaning up the temporary testbench artifact outside `/app`; the delivered workspace change remains just `/app/dut.sv`.

apply_patch

*** Begin Patch
*** Delete File: /tmp/dut_tb.sv
*** End Patch


Exit code: 0
Wall time: 0 seconds
Output:
Success. Updated the following files:
D /tmp/dut_tb.sv

shell

iverilog -g2012 -Wall -o /tmp/dut_check.vvp /app/dut.sv

output

Chunk ID: ed7e56
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 0
Output:

shell

git -C /app diff -- /app/dut.sv

output

Chunk ID: 65e084
Wall time: 0.0005 seconds
Process exited with code 127
Original token count: 11
Output:
/bin/bash: line 1: git: command not found

shell

ls -la /app

output

Chunk ID: fb62e4
Wall time: 0.0000 seconds
Process exited with code 0
Original token count: 36
Output:
total 16
drwxr-xr-x 1 root root 4096 Jun 20 17:49 .
drwxr-xr-x 1 root root 4096 Jun 20 17:47 ..
-rw-r--r-- 1 root root 6597 Jun 20 17:49 dut.sv

Implemented [dut.sv](/app/dut.sv) with `TopModule` instantiating `I2cCommandEngine`. It uses registered outputs, synchronous active-high reset, one-cycle `cmd_ack`, delayed SCL-high observation phases, the extra setup cycle for stop/write/read, repeated start handling, legal command gating, and sticky `al`. Verification: - `iverilog -g2012 -Wall -o /tmp/dut_check.vvp /app/dut.sv` passed. - A temporary self-checking simulation for reset/start/write/read/stop timing passed. - `git` is not installed in the container, so I could not show a diff.

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

MISMATCH_COUNTS cmd_ack=0 busy=0 al=0 bit_out=0 scl=0 sda=0
__CHIPBENCH_MISMATCH_SUMMARY__: mismatches=0 samples=2351

Reproduce this trial: git checkout 2f94510 && PYTHONPATH=src python3 scripts/build_site.py , then open trial/trial_c43fd5aeca484c89. Re-running the agent live requires EVAL_PLATFORM_ENABLE_OAUTH_SMOKE=1 and is non-deterministic.

Trial trial_c43fd5aeca484c89 · verifier authoritative; classifier explanatory.