fix(agent): respect ephemeral_system_prompt over stored DB prompt (#58774)

[isheng-eqi]

Problem

/personality saves to config but does not affect the current CLI session because _restore_or_build_system_prompt() unconditionally restores the session-DB stored prompt when it matches the current runtime identity — even when the caller explicitly set ephemeral_system_prompt.

Root Cause

In agent/conversation_loop.py, _restore_or_build_system_prompt at the stored-prompt fast path (L326-330):

if stored_prompt and _stored_prompt_matches_runtime(agent, stored_prompt):
    agent._cached_system_prompt = stored_prompt
    return

No check for ephemeral_system_prompt — stored prompt always wins.

Fix

Check ephemeral_system_prompt before the stored-prompt fast path. When set (truthy), it takes precedence. The prefix cache will miss for that turn, but that is the expected trade-off for a deliberate personality switch.

Test plan

• 3 new tests in tests/agent/test_system_prompt_restore.py:
  • ephemeral overrides stored prompt
  • ephemeral=None falls through to stored (default behavior preserved)
  • ephemeral="" falls through to stored (empty string is falsy, not an override)
• All 11 existing tests continue to pass (14/14 total)

Related

Closes #58774

[tonydwb]

Code Review Summary

Verdict: Approved

Changes

Fixes #58774: when an agent has an explicit ephemeral_system_prompt set (e.g., via /personality), the stored prompt matching logic now short-circuits and uses the ephemeral override instead of reusing the cached prefix. This ensures personality switches take effect immediately rather than waiting for the cache to expire.

Quality

• Clean fix: the getattr(agent, "ephemeral_system_prompt", None) guard is placed correctly after the _stored_prompt_matches_runtime check, so existing session continuity is preserved when no explicit personality override exists
• The comment explains the cache-miss trade-off explicitly
• Test guard with explicit None on MagicMock is correct

Suggestions

• None — this is a well-scoped, well-documented fix

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Reviewed by Hermes Agent

[tonydwb]

Code Review

COMMENT: fix(agent): respect ephemeral_system_prompt over stored DB prompt (#58774)

Clean fix (56 additions). Prior COMMENT review present. No security concerns, well-scoped.

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Reviewed by Hermes Agent

[teknium1]

Thanks for investigating the session-prompt path. This is an automated hermes-sweeper review; current main already provides the requested behavior through the cache-safe ephemeral-overlay design.

• agent/system_prompt.py:456-457 intentionally keeps ephemeral_system_prompt out of the cached and stored prompt.
• agent/conversation_loop.py:851-870 adds the ephemeral prompt only while constructing the API request, after restoring the byte-stable base prompt.
• The /personality CLI path persists the selected overlay and recreates the agent (hermes_cli/cli_commands_mixin.py:1023-1026), while agent setup passes it as ephemeral_system_prompt (hermes_cli/cli_agent_setup_mixin.py:359).
• Promoting the overlay into _cached_system_prompt would conflict with the existing API assembly, which appends that same overlay, resulting in duplicate personality text.

The cache-safe implementation was introduced in d682f320b35a13084371a541a835e1d988c982b8 and is present in v2026.7.1.