fix: filter orphan tool_result blocks after sliding window truncation (no summary)

When sliding window truncation removes assistant messages containing tool_use
blocks, user messages with tool_result blocks referencing those removed IDs
were being sent to the API, causing Anthropic to reject the request with
'unexpected tool_use_id in tool_result blocks'.

The existing orphan filtering logic only ran in the summary (condensation)
code path. This adds the same filtering to the no-summary code path in
getEffectiveApiHistory(), ensuring tool_use/tool_result pairs remain atomic
regardless of how truncation occurs.

Fixes #11029

src/core/condense/__tests__/index.spec.ts

@@ -583,6 +583,134 @@ describe("getEffectiveApiHistory", () => {
 	})
 })
 
+describe("getEffectiveApiHistory - orphan tool_result filtering after truncation (no summary)", () => {
+	it("should filter orphan tool_result blocks when truncation removes assistant tool_use messages", () => {
+		const truncationId = "trunc-1"
+		const messages: ApiMessage[] = [
+			// Truncation marker
+			{
+				role: "user",
+				content: [{ type: "text", text: "[Previous context truncated]" }],
+				isTruncationMarker: true,
+				truncationId,
+			},
+			// Assistant message with tool_use, hidden by truncation
+			{
+				role: "assistant",
+				content: [{ type: "tool_use", id: "tool-1", name: "read_file", input: { path: "test.ts" } }],
+				truncationParent: truncationId,
+			},
+			// User message with tool_result referencing truncated tool_use
+			{
+				role: "user",
+				content: [{ type: "tool_result", tool_use_id: "tool-1", content: "file contents" }],
+				truncationParent: truncationId,
+			},
+			// Visible assistant message with tool_use
+			{
+				role: "assistant",
+				content: [
+					{ type: "tool_use", id: "tool-2", name: "write_file", input: { path: "out.ts", content: "code" } },
+				],
+			},
+			// Visible user message with tool_result for tool-2
+			{
+				role: "user",
+				content: [{ type: "tool_result", tool_use_id: "tool-2", content: "file written" }],
+			},
+		]
+
+		const result = getEffectiveApiHistory(messages)
+
+		// Should have: truncation marker, visible assistant, visible user (3 messages)
+		// Truncated assistant and user are filtered by truncationParent
+		expect(result).toHaveLength(3)
+		expect(result[0].isTruncationMarker).toBe(true)
+		expect(result[1].role).toBe("assistant")
+		expect(result[2].role).toBe("user")
+		const userContent = result[2].content as any[]
+		expect(userContent[0].tool_use_id).toBe("tool-2")
+	})
+
+	it("should filter orphan tool_result when user message survives truncation but referenced assistant is truncated", () => {
+		const truncationId = "trunc-1"
+		const messages: ApiMessage[] = [
+			// Truncation marker
+			{
+				role: "user",
+				content: [{ type: "text", text: "[Previous context truncated]" }],
+				isTruncationMarker: true,
+				truncationId,
+			},
+			// Assistant message with tool_use, hidden by truncation
+			{
+				role: "assistant",
+				content: [{ type: "tool_use", id: "tool-orphan", name: "read_file", input: { path: "test.ts" } }],
+				truncationParent: truncationId,
+			},
+			// User message with orphan tool_result - NOT tagged with truncationParent
+			// This is the bug scenario: truncation removed the assistant but not the user message
+			{
+				role: "user",
+				content: [{ type: "tool_result", tool_use_id: "tool-orphan", content: "file contents" }],
+			},
+			// Visible conversation continues
+			{
+				role: "assistant",
+				content: "Here is the result.",
+			},
+		]
+
+		const result = getEffectiveApiHistory(messages)
+
+		// The orphan tool_result user message should be removed entirely
+		// Result: truncation marker, assistant text (2 messages)
+		expect(result).toHaveLength(2)
+		expect(result[0].isTruncationMarker).toBe(true)
+		expect(result[1].role).toBe("assistant")
+		expect(result[1].content).toBe("Here is the result.")
+	})
+
+	it("should keep non-orphan content in mixed user message after truncation", () => {
+		const truncationId = "trunc-1"
+		const messages: ApiMessage[] = [
+			{
+				role: "user",
+				content: [{ type: "text", text: "[Previous context truncated]" }],
+				isTruncationMarker: true,
+				truncationId,
+			},
+			{
+				role: "assistant",
+				content: [{ type: "tool_use", id: "tool-orphan", name: "read_file", input: { path: "test.ts" } }],
+				truncationParent: truncationId,
+			},
+			// User message with both orphan tool_result AND text content
+			{
+				role: "user",
+				content: [
+					{ type: "text", text: "Here's some context" },
+					{ type: "tool_result", tool_use_id: "tool-orphan", content: "file contents" },
+				],
+			},
+			{
+				role: "assistant",
+				content: "Got it.",
+			},
+		]
+
+		const result = getEffectiveApiHistory(messages)
+
+		// Should keep the user message but strip the orphan tool_result
+		expect(result).toHaveLength(3)
+		expect(result[0].isTruncationMarker).toBe(true)
+		const userContent = result[1].content as any[]
+		expect(userContent).toHaveLength(1)
+		expect(userContent[0].type).toBe("text")
+		expect(userContent[0].text).toBe("Here's some context")
+	})
+})
+
 describe("cleanupAfterTruncation", () => {
 	it("should clear orphaned condenseParent references", () => {
 		const orphanedCondenseId = "deleted-summary"

src/core/condense/index.ts

@@ -489,7 +489,7 @@ 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) => {
+	const visibleMessages = messages.filter((msg) => {
 		// Filter out condensed messages if their summary exists
 		if (msg.condenseParent && existingSummaryIds.has(msg.condenseParent)) {
 			return false
@@ -500,6 +500,43 @@ export function getEffectiveApiHistory(messages: ApiMessage[]): ApiMessage[] {
 		}
 		return true
 	})
+
+	// Collect all tool_use IDs from visible assistant messages.
+	// This is needed to filter out orphan tool_result blocks that reference
+	// tool_use IDs from messages that were truncated away.
+	const toolUseIds = new Set<string>()
+	for (const msg of visibleMessages) {
+		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) {
+					toolUseIds.add((block as Anthropic.Messages.ToolUseBlockParam).id)
+				}
+			}
+		}
+	}
+
+	// Filter out orphan tool_result blocks from user messages
+	return visibleMessages
+		.map((msg) => {
+			if (msg.role === "user" && Array.isArray(msg.content)) {
+				const filteredContent = msg.content.filter((block) => {
+					if (block.type === "tool_result") {
+						return toolUseIds.has((block as Anthropic.Messages.ToolResultBlockParam).tool_use_id)
+					}
+					return true
+				})
+				// If all content was filtered out, mark for removal
+				if (filteredContent.length === 0) {
+					return null
+				}
+				// If some content was filtered, return updated message
+				if (filteredContent.length !== msg.content.length) {
+					return { ...msg, content: filteredContent }
+				}
+			}
+			return msg
+		})
+		.filter((msg): msg is ApiMessage => msg !== null)
 }
 
 /**