Check for compaction before prompt submission, not just after agent turn

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Check for compaction before submitting user prompt, not just after agent turn ends. This catches cases where user aborts mid-response and context is already near the limit.
+
 ### Changed
 
 - Improved system prompt documentation section with clearer pointers to specific doc files for custom models, themes, skills, hooks, custom tools, and RPC.

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

@@ -220,7 +220,7 @@ export class AgentSession {
 				this._resolveRetry();
 			}
 
-			await this._handleAgentEndCompaction(msg);
+			await this._checkCompaction(msg);
 		}
 	};
 
@@ -242,6 +242,18 @@ export class AgentSession {
 		return textBlocks.map((c) => (c as TextContent).text).join("");
 	}
 
+	/** Find the last assistant message in agent state (including aborted ones) */
+	private _findLastAssistantMessage(): AssistantMessage | null {
+		const messages = this.agent.state.messages;
+		for (let i = messages.length - 1; i >= 0; i--) {
+			const msg = messages[i];
+			if (msg.role === "assistant") {
+				return msg as AssistantMessage;
+			}
+		}
+		return null;
+	}
+
 	/** Emit hook events based on agent events */
 	private async _emitHookEvent(event: AgentEvent): Promise<void> {
 		if (!this._hookRunner) return;
@@ -416,6 +428,12 @@ export class AgentSession {
 			);
 		}
 
+		// Check if we need to compact before sending (catches aborted responses)
+		const lastAssistant = this._findLastAssistantMessage();
+		if (lastAssistant) {
+			await this._checkCompaction(lastAssistant, false);
+		}
+
 		// Expand slash commands if requested
 		const expandedText = expandCommands ? expandSlashCommand(text, [...this._fileCommands]) : text;
 
@@ -719,17 +737,22 @@ export class AgentSession {
 	}
 
 	/**
-	 * Handle compaction after agent_end.
+	 * Check if compaction is needed and run it.
+	 * Called after agent_end and before prompt submission.
+	 *
 	 * Two cases:
 	 * 1. Overflow: LLM returned context overflow error, remove error message from agent state, compact, auto-retry
-	 * 2. Threshold: Turn succeeded but context over threshold, compact, NO auto-retry (user continues manually)
+	 * 2. Threshold: Context over threshold, compact, NO auto-retry (user continues manually)
+	 *
+	 * @param assistantMessage The assistant message to check
+	 * @param skipAbortedCheck If false, include aborted messages (for pre-prompt check). Default: true
 	 */
-	private async _handleAgentEndCompaction(assistantMessage: AssistantMessage): Promise<void> {
+	private async _checkCompaction(assistantMessage: AssistantMessage, skipAbortedCheck = true): Promise<void> {
 		const settings = this.settingsManager.getCompactionSettings();
 		if (!settings.enabled) return;
 
-		// Skip if message was aborted (user cancelled)
-		if (assistantMessage.stopReason === "aborted") return;
+		// Skip if message was aborted (user cancelled) - unless skipAbortedCheck is false
+		if (skipAbortedCheck && assistantMessage.stopReason === "aborted") return;
 
 		const contextWindow = this.model?.contextWindow ?? 0;