Use HF username for personal trace uploads

[lewtun]

This fixes personal trace uploads from the HF Space.

The Space was using the OAuth sub as Session.user_id, then formatting {hf_user}/ml-intern-sessions from that value. OAuth sub is not necessarily the user's HF namespace, so personal dataset creation/uploads could silently target an invalid repo path.

This keeps user_id for ownership and quotas, passes the authenticated HF username separately, and uses that username for personal trace dataset paths.

[github-actions]

Claude finished @lewtun's task in 1m 47s —— View job

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PR Review: Use HF username for personal trace uploads

• Gather context and read changed files
• Analyze code changes
• Post review findings

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No blocking issues — 1 P1
Verdict: ready to merge

What I checked

• _personal_trace_repo_id fallback logic (agent/core/session.py:469-481)
• _update_hf_identity refactor (backend/session_manager.py:332-346)
• User dict construction in backend/dependencies.py — confirmed user_id = OAuth sub, username = preferred_username
• All callers of create_session and ensure_session_loaded in backend/routes/agent.py
• hf_username propagation through AgentSession and Session
• Test coverage in tests/unit/test_personal_trace_repo.py

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P1 — Dead code in _update_hf_identity

backend/session_manager.py:339-340

if not hf_token:
    hf_token = None   # ← no-op: sets falsy to None, does nothing
if hf_token:
    ...

This is a leftover from the refactor of _update_hf_token (which had if not hf_token: return). The local rebind to None is a no-op and can be removed; the if hf_token: guard on line 341 is sufficient.

Fix this →

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Other observations

• Correct precedence logic (session.py:470): hf_user = self.hf_username or self.user_id correctly handles both OAuth (username ≠ sub) and CLI (username == user_id) paths. Tests confirm both cases.
• hf_username not persisted to MongoDB: this is intentional — it's re-derived from the request's OAuth token on each ensure_session_loaded call, which is correct.
• DEV_USER (dependencies.py:34): username: "dev" → trace repo resolves to "dev/ml-intern-sessions", same as before since user_id was already "dev". No regression.
• Test coverage: both the "HF username overrides OAuth subject" and "CLI fallback to user_id" cases are covered. The hf_username=None, user_id=None → None path is exercised by existing tests elsewhere.

[github-actions]

Claude finished @lewtun's task in 58s —— View job

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Re-review: Use HF username for personal trace uploads

• Check P1 fix from prior review
• Scan for new P0 issues

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No blocking issues
Verdict: ready to merge

P1 fix acknowledged

backend/session_manager.py:330–344 — dead if not hf_token: hf_token = None block removed. _update_hf_identity now cleanly guards both branches with if hf_token: / if hf_username:. ✓

What I checked on this commit

• _update_hf_identity implementation — no regressions, both fields updated atomically within their guards
• hf_username propagated through create_session / ensure_session_loaded / _create_session_sync — all call sites in backend/routes/agent.py pass user.get("username") ✓
• get_trajectory() (agent/core/session.py:368) — hf_username added to trace payload; this goes to the HF dataset file, not MongoDB, consistent with the intended design
• No new callers introduced that bypass the username propagation path