fix: align truncation with effective history filtering

Author: roomote

src/core/condense/index.ts

@@ -544,19 +544,33 @@ export function getMessagesSinceLastSummary(messages: ApiMessage[]): ApiMessage[
  * @returns The filtered history that should be sent to the API
  */
 export function getEffectiveApiHistory(messages: ApiMessage[]): ApiMessage[] {
+	const effectiveHistoryWithIndices = getEffectiveApiHistoryWithIndices(messages)
+	return effectiveHistoryWithIndices.map(({ message }) => message)
+}
+
+export function getEffectiveApiHistoryIndices(messages: ApiMessage[]): number[] {
+	const effectiveHistoryWithIndices = getEffectiveApiHistoryWithIndices(messages)
+	return effectiveHistoryWithIndices.map(({ index }) => index)
+}
+
+function getEffectiveApiHistoryWithIndices(messages: ApiMessage[]): Array<{ message: ApiMessage; index: number }> {
 	// Find the most recent summary message
 	const lastSummary = findLast(messages, (msg) => msg.isSummary === true)
 
 	if (lastSummary) {
 		// Fresh start model: return only messages from the summary onwards
 		const summaryIndex = messages.indexOf(lastSummary)
-		let messagesFromSummary = messages.slice(summaryIndex)
+		let messagesFromSummary = messages.slice(summaryIndex).map((message, offset) => ({
+			message,
+			index: summaryIndex + offset,
+		}))
 
 		// Collect all tool_use IDs from assistant messages in the result
 		// This is needed to filter out orphan tool_result blocks that reference
 		// tool_use IDs from messages that were condensed away
 		const toolUseIds = new Set<string>()
-		for (const msg of messagesFromSummary) {
+		for (const { message } of messagesFromSummary) {
+			const msg = message
 			if (msg.role === "assistant" && Array.isArray(msg.content)) {
 				for (const block of msg.content) {
 					if (block.type === "tool_use" && (block as Anthropic.Messages.ToolUseBlockParam).id) {
@@ -568,7 +582,8 @@ export function getEffectiveApiHistory(messages: ApiMessage[]): ApiMessage[] {
 
 		// Filter out orphan tool_result blocks from user messages
 		messagesFromSummary = messagesFromSummary
-			.map((msg) => {
+			.map(({ message, index }) => {
+				const msg = message
 				if (msg.role === "user" && Array.isArray(msg.content)) {
 					const filteredContent = msg.content.filter((block) => {
 						if (block.type === "tool_result") {
@@ -582,22 +597,24 @@ export function getEffectiveApiHistory(messages: ApiMessage[]): ApiMessage[] {
 					}
 					// If some content was filtered, return updated message
 					if (filteredContent.length !== msg.content.length) {
-						return { ...msg, content: filteredContent }
+						return { message: { ...msg, content: filteredContent }, index }
 					}
 				}
-				return msg
+				return { message: msg, index }
 			})
-			.filter((msg): msg is ApiMessage => msg !== null)
+			.filter((entry): entry is { message: ApiMessage; index: number } => entry !== null)
 
 		// Still need to filter out any truncated messages within this range
 		const existingTruncationIds = new Set<string>()
-		for (const msg of messagesFromSummary) {
+		for (const { message } of messagesFromSummary) {
+			const msg = message
 			if (msg.isTruncationMarker && msg.truncationId) {
 				existingTruncationIds.add(msg.truncationId)
 			}
 		}
 
-		return messagesFromSummary.filter((msg) => {
+		return messagesFromSummary.filter(({ message }) => {
+			const msg = message
 			// Filter out truncated messages if their truncation marker exists
 			if (msg.truncationParent && existingTruncationIds.has(msg.truncationParent)) {
 				return false
@@ -626,17 +643,20 @@ export function getEffectiveApiHistory(messages: ApiMessage[]): ApiMessage[] {
 	// Filter out messages whose condenseParent points to an existing summary
 	// or whose truncationParent points to an existing truncation marker.
 	// Messages with orphaned parents (summary/marker was deleted) are included.
-	return messages.filter((msg) => {
-		// Filter out condensed messages if their summary exists
-		if (msg.condenseParent && existingSummaryIds.has(msg.condenseParent)) {
-			return false
-		}
-		// Filter out truncated messages if their truncation marker exists
-		if (msg.truncationParent && existingTruncationIds.has(msg.truncationParent)) {
-			return false
-		}
-		return true
-	})
+	return messages
+		.map((message, index) => ({ message, index }))
+		.filter(({ message }) => {
+			const msg = message
+			// Filter out condensed messages if their summary exists
+			if (msg.condenseParent && existingSummaryIds.has(msg.condenseParent)) {
+				return false
+			}
+			// Filter out truncated messages if their truncation marker exists
+			if (msg.truncationParent && existingTruncationIds.has(msg.truncationParent)) {
+				return false
+			}
+			return true
+		})
 }
 
 /**

src/core/context-management/__tests__/truncation.spec.ts

@@ -94,6 +94,50 @@ describe("Non-Destructive Sliding Window Truncation", () => {
 			const result = truncateConversation(manyMessages, 0.5, "test-task-id")
 			expect(result.messagesRemoved).toBe(4)
 		})
+
+		it("should ignore orphan tool_result-only messages when truncating fresh-start history", () => {
+			const condenseId = "condense-1"
+			const freshStartMessages: ApiMessage[] = [
+				{ role: "user", content: "Original task", ts: 1000, condenseParent: condenseId },
+				{
+					role: "assistant",
+					content: [{ type: "tool_use", id: "tool-orphan", name: "read_file", input: { path: "a.ts" } }],
+					ts: 1100,
+					condenseParent: condenseId,
+				},
+				{
+					role: "user",
+					content: [{ type: "text", text: "Summary" }],
+					ts: 1200,
+					isSummary: true,
+					condenseId,
+				},
+				{
+					role: "user",
+					content: [{ type: "tool_result", tool_use_id: "tool-orphan", content: "orphan result" }],
+					ts: 1300,
+				},
+				{ role: "assistant", content: "Visible assistant 1", ts: 1400 },
+				{ role: "user", content: "Visible user 1", ts: 1500 },
+				{ role: "assistant", content: "Visible assistant 2", ts: 1600 },
+				{ role: "user", content: "Visible user 2", ts: 1700 },
+			]
+
+			const effectiveBefore = getEffectiveApiHistory(freshStartMessages)
+			expect(effectiveBefore).toHaveLength(5)
+
+			const result = truncateConversation(freshStartMessages, 0.5, "test-task-id")
+
+			expect(result.messagesRemoved).toBe(2)
+			expect(result.messages[3].truncationParent).toBeUndefined()
+			expect(result.messages[4].truncationParent).toBe(result.truncationId)
+			expect(result.messages[5].truncationParent).toBe(result.truncationId)
+
+			const effectiveAfter = getEffectiveApiHistory(result.messages)
+			expect(effectiveAfter).toHaveLength(4)
+			expect(effectiveAfter[0].isSummary).toBe(true)
+			expect(effectiveAfter[1].isTruncationMarker).toBe(true)
+		})
 	})
 
 	describe("getEffectiveApiHistory()", () => {

src/core/context-management/index.ts

@@ -8,6 +8,7 @@ import {
 	MAX_CONDENSE_THRESHOLD,
 	MIN_CONDENSE_THRESHOLD,
 	getEffectiveApiHistory,
+	getEffectiveApiHistoryIndices,
 	summarizeConversation,
 	SummarizeResponse,
 } from "../condense"
@@ -55,35 +56,6 @@ export type TruncationResult = {
 	messagesRemoved: number
 }
 
-function getEffectiveVisibleIndices(messages: ApiMessage[]): number[] {
-	const lastSummaryIndexReverse = [...messages].reverse().findIndex((message) => message.isSummary)
-	const firstEffectiveIndex = lastSummaryIndexReverse === -1 ? 0 : messages.length - lastSummaryIndexReverse - 1
-
-	const existingTruncationIds = new Set<string>()
-	for (const message of messages.slice(firstEffectiveIndex)) {
-		if (message.isTruncationMarker && message.truncationId) {
-			existingTruncationIds.add(message.truncationId)
-		}
-	}
-
-	const visibleIndices: number[] = []
-	for (let index = firstEffectiveIndex; index < messages.length; index++) {
-		const message = messages[index]
-
-		if (message.isTruncationMarker) {
-			continue
-		}
-
-		if (message.truncationParent && existingTruncationIds.has(message.truncationParent)) {
-			continue
-		}
-
-		visibleIndices.push(index)
-	}
-
-	return visibleIndices
-}
-
 /**
  * Truncates a conversation by tagging messages as hidden instead of removing them.
  *
@@ -108,7 +80,9 @@ export function truncateConversation(messages: ApiMessage[], fracToRemove: numbe
 	// Only truncate messages that are still part of the effective API history.
 	// Prior condensed history remains stored for rewind, but it should not consume
 	// the fallback truncation slice.
-	const visibleIndices = getEffectiveVisibleIndices(messages)
+	const visibleIndices = getEffectiveApiHistoryIndices(messages).filter(
+		(index) => !messages[index].isTruncationMarker,
+	)
 
 	// Calculate how many visible messages to truncate (excluding first visible message)
 	const visibleCount = visibleIndices.length