feat(coding-agent): added snapcompact safety checks and UI integration

- Added validation to scan for non-ASCII characters before performing snap-compaction, falling back to LLM-based summarization if the unrenderable ratio is too high.
- Updated event handling and status reporting to explicitly support snapcompact actions, including specific error warnings and cancellation states in the UI.
- Updated session logic to default to snapcompact strategy when auto-compaction is enabled.

packages/coding-agent/src/modes/controllers/event-controller.ts

@@ -952,7 +952,9 @@ export class EventController {
 				? "Auto-handoff"
 				: event.action === "shake"
 					? "Auto-shake"
-					: "Auto context-full maintenance";
+					: event.action === "snapcompact"
+						? "Auto-snapcompact"
+						: "Auto context-full maintenance";
 		this.ctx.autoCompactionLoader = new Loader(
 			this.ctx.ui,
 			spinner => theme.fg("accent", spinner),
@@ -973,13 +975,16 @@ export class EventController {
 		}
 		const isHandoffAction = event.action === "handoff";
 		const isShakeAction = event.action === "shake";
+		const isSnapcompactAction = event.action === "snapcompact";
 		if (event.aborted) {
 			this.ctx.showStatus(
 				isHandoffAction
 					? "Auto-handoff cancelled"
 					: isShakeAction
 						? "Auto-shake cancelled"
-						: "Auto context-full maintenance cancelled",
+						: isSnapcompactAction
+							? "Auto-snapcompact cancelled"
+							: "Auto context-full maintenance cancelled",
 			);
 		} else if (isShakeAction) {
 			// Shake produces no CompactionResult; rebuild on success, suppress benign skips.
@@ -1018,6 +1023,8 @@ export class EventController {
 		} else if (event.skipped) {
 			// Benign skip: no model selected, no candidate models available, or nothing
 			// to compact yet. Not a failure — suppress the warning.
+		} else if (isSnapcompactAction) {
+			this.ctx.showWarning("Auto-snapcompact maintenance failed; continuing without maintenance");
 		} else {
 			this.ctx.showWarning("Auto context-full maintenance failed; continuing without maintenance");
 		}

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

@@ -159,7 +159,7 @@ import {
 import { MODEL_ROLE_IDS, MODEL_ROLES } from "../config/model-roles";
 import { expandPromptTemplate, type PromptTemplate } from "../config/prompt-templates";
 import type { Settings, SkillsSettings } from "../config/settings";
-import { onAppendOnlyModeChanged } from "../config/settings";
+import { getDefault, onAppendOnlyModeChanged } from "../config/settings";
 import { RawSseDebugBuffer } from "../debug/raw-sse-buffer";
 import { loadCapability } from "../discovery";
 import { expandApplyPatchToEntries, normalizeDiff, normalizeToLF, ParseError, previewPatch, stripBom } from "../edit";
@@ -9654,29 +9654,50 @@ export class AgentSession {
 			// summarizer path.
 			let snapcompactResult: snapcompact.CompactionResult | undefined;
 			if (action === "snapcompact" && compactionPrep.kind !== "fromHook") {
-				snapcompactResult = await snapcompact.compact(preparation, {
-					convertToLlm,
-					model: this.model,
-				});
-				const ctxWindow = this.model?.contextWindow ?? 0;
-				const budget =
-					ctxWindow > 0
-						? ctxWindow - effectiveReserveTokens(ctxWindow, compactionSettings)
-						: Number.POSITIVE_INFINITY;
-				const projected = this.#projectSnapcompactContextTokens(preparation, snapcompactResult);
-				if (projected > budget) {
-					logger.warn("Snapcompact still overflows the window; falling back to an LLM summary", {
+				const text = snapcompact.serializeConversation(
+					convertToLlm(preparation.messagesToSummarize.concat(preparation.turnPrefixMessages)),
+				);
+				const renderScan = snapcompact.scanRenderability(text);
+				if (renderScan.isSafe) {
+					snapcompactResult = await snapcompact.compact(preparation, {
+						convertToLlm,
+						model: this.model,
+						shape: snapcompact.resolveShape(this.model, this.settings.get("snapcompact.shape")),
+					});
+				} else {
+					logger.warn("Snapcompact disabled: high non-ASCII rate detected; falling back to an LLM summary", {
 						model: this.model?.id,
-						projected,
-						budget,
+						unrenderableRatio: renderScan.unrenderableRatio,
 					});
 					this.emitNotice(
 						"warning",
-						"snapcompact could not bring the context under the limit — using an LLM summary instead",
+						`snapcompact disabled: high non-ASCII rate detected (${(renderScan.unrenderableRatio * 100).toFixed(1)}%). Falling back to an LLM summary to prevent data loss.`,
 						"compaction",
 					);
 					action = "context-full";
-					snapcompactResult = undefined;
+				}
+
+				if (snapcompactResult) {
+					const ctxWindow = this.model?.contextWindow ?? 0;
+					const budget =
+						ctxWindow > 0
+							? ctxWindow - effectiveReserveTokens(ctxWindow, compactionSettings)
+							: Number.POSITIVE_INFINITY;
+					const projected = this.#projectSnapcompactContextTokens(preparation, snapcompactResult);
+					if (projected > budget) {
+						logger.warn("Snapcompact still overflows the window; falling back to an LLM summary", {
+							model: this.model?.id,
+							projected,
+							budget,
+						});
+						this.emitNotice(
+							"warning",
+							"snapcompact could not bring the context under the limit — using an LLM summary instead",
+							"compaction",
+						);
+						action = "context-full";
+						snapcompactResult = undefined;
+					}
 				}
 			}
 
@@ -10081,7 +10102,8 @@ export class AgentSession {
 				result: undefined,
 				aborted: false,
 				willRetry: false,
-				errorMessage: `Auto-shake failed: ${message}`,
+				errorMessage: message,
+				skipped: false,
 			});
 			// Overflow still needs recovery even if shake threw.
 			return reason === "overflow" ? "fallback" : COMPACTION_CHECK_NONE;

packages/coding-agent/test/acp-lazy-startup.test.ts

@@ -323,7 +323,7 @@ describe("ACP lazy startup", () => {
 				enabled: false,
 				subagents: false,
 				syncBacklog: "off",
-				immuneTurns: 1,
+				immuneTurns: 3,
 			});
 		}
 	});

packages/coding-agent/test/agent-session-handoff.test.ts

@@ -871,13 +871,13 @@ describe("AgentSession handoff", () => {
 		expect(events.filter(event => event.type === "auto_compaction_end")).toHaveLength(0);
 	});
 
-	it("restores context-full strategy when enabling auto-compaction from off strategy", () => {
+	it("restores default strategy when enabling auto-compaction from off strategy", () => {
 		session.settings.set("compaction.enabled", true);
 		session.settings.set("compaction.strategy", "off");
 
 		expect(session.autoCompactionEnabled).toBe(false);
 		session.setAutoCompactionEnabled(true);
-		expect(session.settings.get("compaction.strategy")).toBe("context-full");
+		expect(session.settings.get("compaction.strategy")).toBe("snapcompact");
 		expect(session.autoCompactionEnabled).toBe(true);
 	});