[borealisalastor-byte]

Cause

prepare_model_arg decides whether a value looks like JSON by inspecting the first character of origin_arg.strip():

if not origin_arg.strip()[0] in {'{', '['}:   # empty / whitespace-only -> IndexError

so an empty or whitespace-only value raised IndexError: string index out of range.

get_default_config builds the runtime additional_params from a dict comprehension whose result is wrapped in try/except Exception: pass, so one blank parameter made the whole mapping disappear:

config_raw = [{"key": "temperature", "val": 0}, {"key": "note", "val": ""}, {"key": "max_tokens", "val": 1024}]
runtime additional_params -> {}     # temperature and max_tokens dropped as well

check_llm skips blank values through its own truthiness filter, which is why the crash only shows on the runtime path; which values that filter should keep is tracked separately in #1446.

Fix

• blank strings are recognised explicitly and returned unchanged, before any index access;
• non-string inputs (0, False, None, numbers, dicts, lists) keep their current pass-through behaviour;
• the bare except: next to it becomes except Exception: so a JSON decode error stays contained.

Tests

backend/tests/test_prepare_model_arg.py pins the contract: blank values ("", " ", "\t", "\n") come back unchanged, JSON-looking strings are still parsed, malformed JSON and plain strings stay strings, non-string values keep their identity, and a config carrying one blank parameter keeps its siblings — the last two are exactly the reported failure mode.

Verification

• python -m pytest -q tests/test_prepare_model_arg.py -> 28 passed after the fix; the same file fails 11 cases against the unmodified source (IndexError in the blank-value cases), run in backend/ on a checkout of main at f10255b;
• ruff check and ruff format --check pass on the touched files;
• the full uv run pytest -q suite was not run in the sandbox where the change was prepared, which has no access to the private xpack index.

Fixes #1452