fix(session): dropped recovered overflow error turns

Removed recoverable context-overflow and incomplete-response assistant errors from both active context and persisted session history before compaction/promotion schedules the retry.

Fixes #3747

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Fixed recoverable context-overflow compaction keeping the failed assistant error turn in visible session history after scheduling the retry. ([#3747](https://github.com/can1357/oh-my-pi/issues/3747))
+
 ## [16.2.4] - 2026-06-28
 
 ### Fixed

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

@@ -9482,12 +9482,7 @@ export class AgentSession {
 		const errorIsFromBeforeCompaction =
 			compactionEntry !== null && assistantMessage.timestamp < new Date(compactionEntry.timestamp).getTime();
 		if (sameModel && !errorIsFromBeforeCompaction && AIError.isContextOverflow(assistantMessage, contextWindow)) {
-			// Remove the error message from agent state (it IS saved to session for history,
-			// but we don't want it in context for the retry)
-			const messages = this.agent.state.messages;
-			if (messages.length > 0 && messages[messages.length - 1].role === "assistant") {
-				this.agent.replaceMessages(messages.slice(0, -1));
-			}
+			await this.#discardRecoverableAssistantTurn(assistantMessage);
 
 			// Try context promotion first - switch to a larger model and retry without compacting
 			const promoted = await this.#tryContextPromotion(assistantMessage);
@@ -9512,10 +9507,7 @@ export class AgentSession {
 		// otherwise compaction/handoff. Unlike overflow, the *input* is fine, so we
 		// allow the handoff strategy to actually run.
 		if (sameModel && !errorIsFromBeforeCompaction && assistantMessage.stopReason === "length") {
-			const messages = this.agent.state.messages;
-			if (messages.length > 0 && messages[messages.length - 1].role === "assistant") {
-				this.agent.replaceMessages(messages.slice(0, -1));
-			}
+			await this.#discardRecoverableAssistantTurn(assistantMessage);
 
 			const promoted = await this.#tryContextPromotion(assistantMessage);
 			if (promoted) {
@@ -9797,6 +9789,11 @@ export class AgentSession {
 		}
 	}
 
+	async #discardRecoverableAssistantTurn(assistantMessage: AssistantMessage): Promise<void> {
+		await this.#waitForSessionMessagePersistence(assistantMessage);
+		this.#discardAssistantTurn(assistantMessage);
+	}
+
 	/**
 	 * Drop an assistant turn from BOTH the live agent context and the persisted
 	 * session branch (reparenting the leaf to the turn's parent), so a discarded

packages/coding-agent/test/agent-session-auto-compaction-progress-guard.test.ts

@@ -382,6 +382,37 @@ describe("AgentSession auto-compaction progress guard", () => {
 		expect(noProgress.length).toBe(0);
 	});
 
+	it("removes the visible overflow error before retrying after compaction", async () => {
+		seedPriorTurns();
+		const continueSpy = vi.spyOn(session.agent, "continue").mockResolvedValue();
+		vi.spyOn(session.agent, "prompt").mockResolvedValue(undefined as never);
+		vi.spyOn(session, "getContextUsage").mockReturnValue({ tokens: 150000, contextWindow: 200000, percent: 75 });
+
+		const { promise: compactionDone, resolve: onCompactionDone } = Promise.withResolvers<void>();
+		session.subscribe(event => {
+			if (event.type === "auto_compaction_end") onCompactionDone();
+		});
+
+		const assistantMsg = overflowAssistant();
+		session.agent.emitExternalEvent({ type: "message_end", message: assistantMsg });
+		session.agent.emitExternalEvent({ type: "agent_end", messages: [assistantMsg] });
+
+		await compactionDone;
+		await session.waitForIdle();
+
+		expect(continueSpy).toHaveBeenCalledTimes(1);
+		expect(sessionManager.getBranch()).not.toContainEqual(
+			expect.objectContaining({
+				type: "message",
+				message: expect.objectContaining({
+					role: "assistant",
+					stopReason: "error",
+					errorMessage: assistantMsg.errorMessage,
+				}),
+			}),
+		);
+	});
+
 	it("retries a small-window overflow when the default reserve exceeds the model window", async () => {
 		// Bundled 4k/8k models can be smaller than the default absolute reserve
 		// (16,384). Retry fit must clamp that reserve; otherwise the budget goes