Mirrors the pattern already shipping in hindsight-integrations/openclaw:
probe `<api_url>/version` once per process, gate on Hindsight ≥ 0.5.0.
When supported, retains use a stable session-scoped `document_id`
(`session_id`) plus `update_mode='append'` so cross-process retains for
the same session merge into one document instead of producing
N-different-process-stamped duplicates. When unsupported (or probe
fails), fall back to the existing per-process unique
`f"{session_id}-{start_ts}"` document_id with no `update_mode` — the
resume-overwrite fix (#6654) keeps working unchanged on legacy servers.
Closes the dedup half of #20115. The proposed `document_id_strategy`
config knob isn't needed: auto-detection via the same /version probe
the OpenClaw plugin already uses gives the same outcome with no extra
config burden, and the choice is purely a function of what the server
can do.
Plumbing
--------
- Module-level helpers (`_meets_minimum_version`, `_fetch_hindsight_api_version`,
`_check_api_supports_update_mode_append`) cache the result per api_url
so every provider in the process gets one /version round-trip.
- One-time WARN logged when the API is older than 0.5.0, telling the
user to upgrade for cross-session deduplication.
- New instance helper `_resolve_retain_target(fallback_doc_id)` returns
`(document_id, update_mode)` based on cached capability. Wired into
`sync_turn` and the `on_session_switch` flush path.
- For local_embedded mode, the probe URL is taken from the running
client (`client.url`) so we hit the actual daemon port rather than
the configured default.
- `update_mode` is set on the per-item dict; `aretain_batch` already
threads `item['update_mode']` into the API call.
Tests
-----
- `TestUpdateModeAppendCapability` (5 cases): legacy fallback, modern
stable+append, per-url cache, one-time warn, flush-on-switch resolves
against the OLD session.
- Existing `_make_hindsight_provider` factory in the manager-side test
file extended to seed `_mode`/`_api_url`/`_api_key`/`_client` and stub
`_resolve_retain_target` so the bypass-init pattern keeps working.
E2E verified against installed `~/.hermes/hermes-agent`:
- Legacy probe (unreachable host) → `legacy-session-<ts>` doc_id,
no `update_mode`.
- Modern probe (live local_embedded 0.5.6 daemon) → stable
`modern-session` doc_id + `update_mode='append'`.
- `test_hermes_embedded_smoke.py` passes (90s).
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