fix(coding-agent): compact oversized trailing tool results

Fall back to the latest safe assistant boundary when trailing tool results exceed the retained-token budget, allowing older history to compact before the next provider request.

Closes #9740

packages/coding-agent/CHANGELOG.md

@@ -21,6 +21,7 @@
 
 ### Fixed
 
+- Fixed mid-run threshold compaction silently skipping oversized trailing tool results ([#9740](https://github.com/earendil-works/pi/issues/9740)).
 - Fixed signal-terminated local shell commands being reported as successful with partial output ([#9577](https://github.com/earendil-works/pi/issues/9577) by [@BrendanJMurphy](https://github.com/BrendanJMurphy)).
 - Fixed local clipboard failures reporting success when the terminal ignored the fallback OSC 52 write, and added platform-specific setup guidance when no clipboard backend works ([#9618](https://github.com/earendil-works/pi/issues/9618)).
 - Capped agent-level retry backoff at `retry.maxAgentDelayMs` (60s by default) so long retry runs stay responsive during prolonged transient outages ([#8826](https://github.com/earendil-works/pi/issues/8826)).

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

@@ -442,13 +442,10 @@ export function findCutPoint(
 
 		// Check if we've exceeded the budget
 		if (accumulatedTokens >= keepRecentTokens) {
-			// Find the closest valid cut point at or after this entry
-			for (let c = 0; c < cutPoints.length; c++) {
-				if (cutPoints[c] >= i) {
-					cutIndex = cutPoints[c];
-					break;
-				}
-			}
+			// Prefer the closest valid cut point at or after this entry. If trailing
+			// tool results exceed the budget by themselves, keep their preceding
+			// assistant tool call instead of falling back to the first message.
+			cutIndex = cutPoints.find((candidate) => candidate >= i) ?? cutPoints[cutPoints.length - 1];
 			break;
 		}
 	}

packages/coding-agent/test/compaction.test.ts

@@ -373,6 +373,42 @@ describe("findCutPoint", () => {
 		expect(customFitsBudget.isSplitTurn).toBe(false);
 		expect(customFitsBudget.turnStartIndex).toBe(-1);
 	});
+
+	// Regression test for #9740.
+	it("should fall back to the latest valid cut point before oversized trailing tool results", () => {
+		const oldUser = createMessageEntry(createUserMessage("old history"));
+		const oldAssistant = createMessageEntry(createAssistantMessage("old answer"));
+		const currentUser = createMessageEntry(createUserMessage("read the large file"));
+		const toolCall = createMessageEntry({
+			...createAssistantMessage(""),
+			content: [{ type: "toolCall", id: "call-1", name: "read", arguments: { path: "big.txt" } }],
+			stopReason: "toolUse",
+		});
+		const toolResult = createMessageEntry({
+			role: "toolResult",
+			toolCallId: "call-1",
+			toolName: "read",
+			content: [{ type: "text", text: "x".repeat(8000) }],
+			isError: false,
+			timestamp: Date.now(),
+		});
+		const entries = [oldUser, oldAssistant, currentUser, toolCall, toolResult];
+
+		const result = findCutPoint(entries, 0, entries.length, 1000);
+		expect(result).toEqual({
+			firstKeptEntryIndex: 3,
+			turnStartIndex: 2,
+			isSplitTurn: true,
+		});
+
+		const preparation = prepareCompaction(entries, {
+			...DEFAULT_COMPACTION_SETTINGS,
+			keepRecentTokens: 1000,
+		});
+		expect(preparation?.firstKeptEntryId).toBe(toolCall.id);
+		expect(preparation?.messagesToSummarize).toEqual([oldUser.message, oldAssistant.message]);
+		expect(preparation?.turnPrefixMessages).toEqual([currentUser.message]);
+	});
 });
 
 describe("buildSessionContext", () => {

packages/coding-agent/test/suite/agent-session-compaction.test.ts

@@ -471,10 +471,11 @@ describe("AgentSession compaction characterization", () => {
 	});
 
 	// Regression coverage for #8133: model overrides must also apply between assistant turns.
+	// Regression coverage for #9740: an oversized trailing tool result must still produce a cut point.
 	it.each([false, true])(
-		"compacts after a tool result in the same run (model override: %s)",
+		"compacts after an oversized tool result in the same run (model override: %s)",
 		async (modelOverride) => {
-			const toolResult = `large-tool-result:${"x".repeat(6800)}`;
+			const toolResult = `large-tool-result:${"x".repeat(8000)}`;
 			const largeTool: AgentTool = {
 				name: "large_result",
 				label: "Large result",
@@ -532,8 +533,8 @@ describe("AgentSession compaction characterization", () => {
 			const agentStartsBefore = harness.eventsOfType("agent_start").length;
 			await harness.session.prompt("run the large tool");
 
-			expect(order).toEqual(["compaction", "provider"]);
-			expect(observedSettings).toEqual([{ enabled: true, reserveTokens: 400, keepRecentTokens: 1750 }]);
+			expect(order.slice(0, 2)).toEqual(["compaction", "provider"]);
+			expect(observedSettings[0]).toEqual({ enabled: true, reserveTokens: 400, keepRecentTokens: 1750 });
 			expect(harness.eventsOfType("agent_start")).toHaveLength(agentStartsBefore + 1);
 			expect(harness.eventsOfType("compaction_start").at(-1)).toEqual({
 				type: "compaction_start",