Add model.reasoning_echo (default false) and per-fallback-entry
reasoning_echo to preserve assistant reasoning_content when
replaying history to custom providers and OpenAI-compatible gateways
that proxy thinking-mode models (Kimi K3, GLM-5.2, DeepSeek, etc.)
but are not matched by the built-in host-based _REASONING_ECHO_RULES.
The flag is per-active-provider, not a global toggle:
- Primary: read from model.reasoning_echo at init and switch_model
- Fallback: set by try_activate_fallback from the fallback entry
- Restore: restore_primary_runtime copies the switch_model snapshot
Unlike PR #76019 global agent.reasoning_echo toggle, the
per-provider flag travels with the active provider — falling back to
a strict provider (Mistral, Groq, Cerebras) correctly strips
reasoning_content even when the primary had the flag enabled,
because the flag is False for the strict fallback.
Complements PR #27361 (dynamic detection) which fires after the first
API response; this PR covers turn-1 and history-replay-on-fresh-session
where dynamic detection has not fired yet.
Closes#76018
Refs: #27297, #27361, #76019
Signed-off-by: Yingliang Zhang <zhangyingliang@outlook.com>
Yingliang Zhang