[Cookie0103]

HDF5 trace loading still interprets every {/[-prefixed string as JSON. For example, an EvolutionTrace.llm_response of {"code": "result = 42"} is returned as a dictionary after an export/load round trip; valid Python expression code such as [1, 2] also changes type. This corrupts the original code/response payload used for trace analysis.

Extend the encoding-key marker already used for file-level metadata to each trace group. Only attributes explicitly encoded as JSON are decoded, while unmarked legacy trace groups retain their current loading behavior. This follows the metadata fix in #580/#574 and addresses the remaining per-trace path.

Tests cover an actual EvolutionTrace with JSON-looking code, response and change-description strings, numeric datasets, JSON-encoded lists/nested artifacts, and an old file without the marker.

Validation (Python 3.12, optional h5py installed):

• Before the fix: the new EvolutionTrace round-trip test failed (1 failure out of 8 HDF5 tests).
• Related trace suites: 39 tests passed.
• OPENAI_API_KEY=test-key-for-unit-tests python -m unittest discover tests: 1072 tests ran, OK (2 skipped).
• Repository pre-commit hooks (isort/Black) and git diff --check passed.

No live model request was used. AI assistance: Codex investigated, implemented and validated this contribution on behalf of the account owner, who has not independently reviewed or tested it.