fix: detect content rewrites in syncMessages, reset append-only cache on model switch

packages/coding-agent/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Fixed
 
 - Fixed clipboard image paste (Ctrl+V) silently failing on WSL2 by routing image reads through a `powershell.exe` bridge when WSL interop is detected, since `arboard` returns `ContentNotAvailable` under WSLg ([#1280](https://github.com/can1357/oh-my-pi/issues/1280))
+- Fixed append-only context mode not being recomputed after model switches — the mode was frozen at session construction time using the initial model's provider, so `provider.appendOnlyContext=auto` left append-only enabled after switching away from DeepSeek (or disabled after switching to DeepSeek) for the rest of the session
 
 ## [15.2.4] - 2026-05-22
 ### Breaking Changes

packages/coding-agent/src/session/agent-session.ts

@@ -5954,6 +5954,10 @@ export class AgentSession {
 		const enable = appendOnlySetting === "on" || (appendOnlySetting === "auto" && model.provider === "deepseek");
 		if (enable && !this.agent.appendOnlyContext) {
 			this.agent.setAppendOnlyContext(new AppendOnlyContextManager());
+		} else if (enable && this.agent.appendOnlyContext) {
+			// Model changed but mode stays enabled — reset cached prefix + log
+			// so the next turn rebuilds for the new model's normalization.
+			this.agent.appendOnlyContext.invalidateForModelChange();
 		} else if (!enable && this.agent.appendOnlyContext) {
 			this.agent.setAppendOnlyContext(undefined);
 		}