t8y2/dbx · error · RuntimeError

command returned no JSON ({' '.join(command)}): {completed.s

Error message

command returned no JSON ({' '.join(command)}):
{completed.stdout}

What it means

run_json() expects the benchmark command to emit at least one JSON object on stdout, scanning output lines from the end. If no line parses as a JSON dict it raises RuntimeError including the command and full stdout, indicating the process succeeded (exit 0) but produced unparseable or non-JSON output.

Source

Thrown at agents/drivers/iotdb/bench/run.py:127

        env=environment,
        text=True,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE,
        check=False,
        timeout=env_float("BENCH_COMMAND_TIMEOUT", 180.0),
    )
    if completed.returncode != 0:
        raise RuntimeError(
            f"command failed ({' '.join(command)}):\nstdout:\n{completed.stdout}\nstderr:\n{completed.stderr}"
        )
    for line in reversed(completed.stdout.splitlines()):
        try:
            value = json.loads(line)
        except json.JSONDecodeError:
            continue
        if isinstance(value, dict):
            return value
    raise RuntimeError(f"command returned no JSON ({' '.join(command)}):\n{completed.stdout}")


def measure_rss(command: list[str], environment: dict[str, str]) -> float:
    process = subprocess.Popen(
        command,
        env=environment | {"IOTDB_BENCH_MODE": "hold"},
        text=True,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE,
    )
    stderr_lines: list[str] = []

    def drain_stderr() -> None:
        assert process.stderr is not None
        stderr_lines.extend(line.rstrip() for line in process.stderr)

    threading.Thread(target=drain_stderr, daemon=True).start()
    try:

View on GitHub (pinned to c0390bff16)

Solutions

1. Inspect the stdout embedded in the error to see what the command actually printed
2. Confirm the benchmark tool is the expected version that prints a JSON object to stdout
3. Capture the output to a file and locate the JSON line; check for CRLF or non-JSON banners corrupting lines
4. Run the command manually and verify output; add --json/--output-format flag if the tool supports one

Example fix

// before
print("bench done")  # tool prints only text -> RuntimeError: command returned no JSON
// after
print(json.dumps({"ready": True, "metrics": metrics}))  # ensure dict JSON on stdout

def preflight_json_output(cmd: list[str], env: dict) -> None:
    probe = subprocess.run(cmd + ["--dry-run"] if supports_dry_run else cmd, env=env,
                          capture_output=True, text=True)
    if not any(_is_json_dict(line) for line in probe.stdout.splitlines()):
        raise SystemExit("tool does not emit a JSON object on stdout; check version/flags")

def _is_json_dict(line: str) -> bool:
    try:
        return isinstance(json.loads(line), dict)
    except json.JSONDecodeError:
        return False

try:
    result = run_json(command, environment)
except RuntimeError as e:
    print(f"no JSON produced; raw output:\n{e}")
    raise SystemExit(1)  # inspect stdout in the message for format changes

When it happens

Trigger: The subprocess printed only human-readable logs/progress text, printed JSON to stderr instead of stdout, or emitted a JSON array/scalar (not a dict); every reversed line fails json.loads or isinstance(value, dict).

Common situations: Benchmark tool version changed its output format; a wrapper script added banner text only; JSON mixed with log lines but the dict line is last-line-first parsing missed a trailing carriage return; tool writes results to a file instead of stdout.

Related errors

• agent exited before ready
• timed out waiting for agent readiness
• agent exited during {method}
• timed out waiting for agent readiness
• agent exited before {method}

AI-assisted analysis of t8y2/dbx@c0390bff16 (2026-09-05). Data as JSON: /api/errors/ada24c3d86a40f7a. Report an issue: GitHub.