What happened

CLI-backend session resume (claude-cli) invalidates with reuse=invalidated:system-prompt whenever consecutive group messages toggle between "explicitly @-mentions the bot" and "doesn't mention the bot". Each invalidation abandons the resumed CLI session and spawns a fresh one, dropping continuity and re-paying warm-up.

This is the same disease as #99372 / #99595 / #99633 (per-turn facts leaking into the session-stable reuse fingerprint), on a new axis: per-message mention state. With the senderIsOwner axes fixed, this becomes the dominant remaining churn source — "hi @bot" followed by an un-mentioned follow-up is the most common two-message pattern in group chats.

Root cause

buildGroupChatContext bakes a per-message fact into the group context block:

• src/auto-reply/reply/groups.ts (buildGroupChatContext): when sessionCtx.ExplicitlyMentionedBot === true, it appends "The incoming message explicitly mentions your channel identity @...". ExplicitlyMentionedBot is a property of the current inbound message, not the session.
• src/auto-reply/reply/get-reply-run.ts (extraSystemPromptStatic): groupChatContext is one of the parts joined into extraSystemPromptStatic, which feeds cliSessionBindingFacts and is hashed into extraSystemPromptHash.
• src/agents/cli-session.ts (resolveCliSessionReuse): a changed extraSystemPromptHash returns invalidatedReason: "system-prompt", which deletes the stored binding and forces a fresh CLI session.

So the "static" prompt hash flips on every mention↔no-mention transition.

Related, same file: groupIntro is conditionally included in extraSystemPromptStatic (shouldInjectGroupIntro = isFirstTurnInSession || sessionEntry?.groupActivationNeedsSystemIntro, get-reply-run.ts). A fresh session's first turn hashes with the intro and the second turn without, guaranteeing one extra invalidation early in every new session. Same class: turn-varying input inside the session-stable hash.

Production evidence

Observed 2026-07-03 on a live Telegram group deployment running fix/cli-binding-tool-scope (abc6ee5) with temporary per-hash attribution logging in resolveCliSessionReuse:

promptToolNamesHash and messageToolPolicyHash stayed constant across all three turns, isolating the mention line as the changing content. The hash tracked mention state, not sender: the owner→non-owner flip at 23:10:05 did not flip the hash back.

Expected

The reuse fingerprint should only include session-stable facts. Per-message facts (mention state) belong in per-turn context — inboundMetaPrompt already exists for exactly this and is correctly excluded from extraSystemPromptStatic.

Fix shape

• Move the explicit-mention notice out of buildGroupChatContext into the per-turn inbound context (buildInboundMetaSystemPrompt path) so extraSystemPromptStatic stops depending on ExplicitlyMentionedBot.
• Decide groupIntro's contract: either include it unconditionally in the binding facts (stable content) or exclude it from the static hash; today's conditional inclusion churns turn 2 of every fresh session.
• Regression tests mirroring the production pattern: mention → no-mention → mention with a stable extraSystemPromptHash throughout.

Environment

• OpenClaw at abc6ee5633087a87207865aa2332c26c63f878c6 (main + fix: CLI agent session resume churns when owner and non-owner alternate in a group #99640)
• claude-cli backend (Claude Code 2.1.200), Telegram group, message_tool_only delivery