fix(agent): budget branch tool results after truncation

packages/agent/src/compaction/branch-summarization.ts

@@ -29,6 +29,7 @@ import {
 	SUMMARIZATION_SYSTEM_PROMPT,
 	serializeConversation,
 	stripReadSelector,
+	truncateToolResultForSummary,
 	upsertFileOperations,
 } from "./utils";
 
@@ -206,6 +207,19 @@ function getMessageFromEntry(entry: SessionEntry): AgentMessage | undefined {
 	}
 }
 
+function estimateBranchSummaryTokens(message: AgentMessage): number {
+	if (message.role !== "toolResult") return estimateTokens(message);
+	const text = message.content
+		.filter((c): c is { type: "text"; text: string } => c.type === "text")
+		.map(c => c.text)
+		.join("");
+	if (!text) return 0;
+	return estimateTokens({
+		...message,
+		content: [{ type: "text", text: truncateToolResultForSummary(text) }],
+	});
+}
+
 /**
  * Prepare entries for summarization with token budget.
  *
@@ -251,7 +265,7 @@ export function prepareBranchEntries(entries: SessionEntry[], tokenBudget: numbe
 		// Extract file ops from assistant messages (tool calls)
 		extractFileOpsFromMessage(message, fileOps);
 
-		const tokens = estimateTokens(message);
+		const tokens = estimateBranchSummaryTokens(message);
 
 		// Check budget before adding
 		if (tokenBudget > 0 && totalTokens + tokens > tokenBudget) {

packages/agent/src/compaction/utils.ts

@@ -199,13 +199,12 @@ export function upsertFileOperations(
 const TOOL_RESULT_MAX_CHARS = 2000;
 
 /**
- * Truncate text to a maximum character length for summarization.
- * Keeps the beginning and appends a truncation marker.
+ * Truncate tool results to the same representation used in summarization prompts.
  */
-function truncateForSummary(text: string, maxChars: number): string {
-	if (text.length <= maxChars) return text;
-	const truncatedChars = text.length - maxChars;
-	return `${text.slice(0, maxChars)}\n\n[... ${truncatedChars} more characters truncated]`;
+export function truncateToolResultForSummary(text: string): string {
+	if (text.length <= TOOL_RESULT_MAX_CHARS) return text;
+	const truncatedChars = text.length - TOOL_RESULT_MAX_CHARS;
+	return `${text.slice(0, TOOL_RESULT_MAX_CHARS)}\n\n[... ${truncatedChars} more characters truncated]`;
 }
 
 /**
@@ -241,7 +240,7 @@ export function serializeConversation(messages: Message[], dialect?: Dialect): s
 				if (!text) continue;
 				processed.push({
 					...msg,
-					content: [{ type: "text", text: truncateForSummary(text, TOOL_RESULT_MAX_CHARS) }],
+					content: [{ type: "text", text: truncateToolResultForSummary(text) }],
 				});
 				continue;
 			}
@@ -293,7 +292,7 @@ export function serializeConversation(messages: Message[], dialect?: Dialect): s
 				.map(c => c.text)
 				.join("");
 			if (content) {
-				const text = truncateForSummary(content, TOOL_RESULT_MAX_CHARS);
+				const text = truncateToolResultForSummary(content);
 				parts.push(`[Tool Result]: ${text}`);
 			}
 		}

packages/agent/test/branch-summarization.test.ts

@@ -190,4 +190,44 @@ describe("branch summarization", () => {
 		expect(userMessages[0].content).toBe("OLDER_USEFUL_FACT_4076");
 		expect(messages.some(m => m.role === "toolResult")).toBe(false);
 	});
+
+	test("large informative tool results are budgeted after summary truncation", () => {
+		const informativeBlob = `IMPORTANT_LARGE_TOOL_FACT_4112\n${"x".repeat(20_000)}`;
+		const entries: SessionEntry[] = [
+			{
+				type: "message",
+				id: "assistant-1",
+				parentId: null,
+				timestamp: new Date(0).toISOString(),
+				message: {
+					role: "assistant",
+					content: [{ type: "toolCall", id: "call-read", name: "read", arguments: { path: "big.txt" } }],
+					api: "mock",
+					provider: "mock",
+					model: "mock-model",
+					usage: ZERO_USAGE,
+					stopReason: "toolUse",
+					timestamp: 0,
+				},
+			},
+			{
+				type: "message",
+				id: "tool-1",
+				parentId: "assistant-1",
+				timestamp: new Date(1).toISOString(),
+				message: {
+					role: "toolResult",
+					toolCallId: "call-read",
+					toolName: "read",
+					content: [{ type: "text", text: informativeBlob }],
+					isError: false,
+					timestamp: 1,
+				},
+			},
+		];
+
+		const { messages } = prepareBranchEntries(entries, 700);
+
+		expect(messages.some(m => m.role === "toolResult")).toBe(true);
+	});
 });