fix: add pre-send validation for tool_use/tool_result pairing (#11777)

Adds validateMessageHistoryBeforeSend() as a safety net that runs on the
final message array right before the API call. This catches any
tool_use blocks that are missing corresponding tool_result blocks -
a mismatch that causes Anthropic API rejection.

The validation:
- Iterates through the final messages looking for assistant messages
  with tool_use blocks
- Checks the following user message has matching tool_result blocks
- Injects placeholder tool_results for any missing pairings
- Inserts synthetic user messages when none follow an assistant message
- Reports mismatches to telemetry via MissingToolResultError

This addresses cases where post-processing steps (getEffectiveApiHistory,
mergeConsecutiveApiMessages, buildCleanConversationHistory) may
introduce mismatches after the existing insert-time validation.

src/core/task/Task.ts

@@ -130,7 +130,7 @@ import { getMessagesSinceLastSummary, summarizeConversation, getEffectiveApiHist
 import { MessageQueueService } from "../message-queue/MessageQueueService"
 import { AutoApprovalHandler, checkAutoApproval } from "../auto-approval"
 import { MessageManager } from "../message-manager"
-import { validateAndFixToolResultIds } from "./validateToolResultIds"
+import { validateAndFixToolResultIds, validateMessageHistoryBeforeSend } from "./validateToolResultIds"
 import { mergeConsecutiveApiMessages } from "./mergeConsecutiveApiMessages"
 
 const MAX_EXPONENTIAL_BACKOFF_SECONDS = 600 // 10 minutes
@@ -4274,10 +4274,16 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 		// Reset the flag after using it
 		this.skipPrevResponseIdOnce = false
 
+		// Final safety-net: ensure every tool_use has a matching tool_result before sending.
+		// This catches mismatches introduced by post-processing (condensing, merging, cleaning).
+		const validatedHistory = validateMessageHistoryBeforeSend(
+			cleanConversationHistory as unknown as Anthropic.Messages.MessageParam[],
+		)
+
 		// The provider accepts reasoning items alongside standard messages; cast to the expected parameter type.
 		const stream = this.api.createMessage(
 			systemPrompt,
-			cleanConversationHistory as unknown as Anthropic.Messages.MessageParam[],
+			validatedHistory as unknown as Anthropic.Messages.MessageParam[],
 			metadata,
 		)
 		const iterator = stream[Symbol.asyncIterator]()

src/core/task/__tests__/validateToolResultIds.spec.ts

@@ -2,6 +2,7 @@ import { Anthropic } from "@anthropic-ai/sdk"
 import { TelemetryService } from "@roo-code/telemetry"
 import {
 	validateAndFixToolResultIds,
+	validateMessageHistoryBeforeSend,
 	ToolResultIdMismatchError,
 	MissingToolResultError,
 } from "../validateToolResultIds"
@@ -995,3 +996,225 @@ describe("validateAndFixToolResultIds", () => {
 		})
 	})
 })
+
+describe("validateMessageHistoryBeforeSend", () => {
+	beforeEach(() => {
+		vi.clearAllMocks()
+	})
+
+	it("should return the same array reference when all tool_use blocks have matching tool_results", () => {
+		const messages: Anthropic.Messages.MessageParam[] = [
+			{
+				role: "user",
+				content: [{ type: "text", text: "Hello" }],
+			},
+			{
+				role: "assistant",
+				content: [
+					{ type: "tool_use", id: "tool_1", name: "read_file", input: { path: "foo.ts" } },
+					{ type: "tool_use", id: "tool_2", name: "read_file", input: { path: "bar.ts" } },
+				],
+			},
+			{
+				role: "user",
+				content: [
+					{ type: "tool_result", tool_use_id: "tool_1", content: "file contents 1" },
+					{ type: "tool_result", tool_use_id: "tool_2", content: "file contents 2" },
+				],
+			},
+		]
+
+		const result = validateMessageHistoryBeforeSend(messages)
+		expect(result).toBe(messages) // Same reference = no modification
+		expect(TelemetryService.instance.captureException).not.toHaveBeenCalled()
+	})
+
+	it("should inject placeholder tool_results for missing tool_use IDs", () => {
+		const messages: Anthropic.Messages.MessageParam[] = [
+			{
+				role: "user",
+				content: [{ type: "text", text: "Hello" }],
+			},
+			{
+				role: "assistant",
+				content: [
+					{ type: "tool_use", id: "tool_1", name: "read_file", input: { path: "foo.ts" } },
+					{ type: "tool_use", id: "tool_2", name: "read_file", input: { path: "bar.ts" } },
+					{ type: "tool_use", id: "tool_3", name: "read_file", input: { path: "baz.ts" } },
+					{ type: "tool_use", id: "tool_4", name: "read_file", input: { path: "qux.ts" } },
+				],
+			},
+			{
+				role: "user",
+				content: [
+					{ type: "tool_result", tool_use_id: "tool_1", content: "result 1" },
+					// tool_2, tool_3, tool_4 are missing
+				],
+			},
+		]
+
+		const result = validateMessageHistoryBeforeSend(messages)
+
+		// Should have the same number of messages
+		expect(result.length).toBe(3)
+
+		// The patched user message should contain placeholders for tool_2, tool_3, tool_4
+		const patchedUser = result[2]
+		expect(patchedUser.role).toBe("user")
+		const content = patchedUser.content as Anthropic.Messages.ContentBlockParam[]
+
+		// 3 placeholders + 1 existing tool_result = 4
+		expect(content.length).toBe(4)
+
+		const toolResults = content.filter((b): b is Anthropic.ToolResultBlockParam => b.type === "tool_result")
+		expect(toolResults.length).toBe(4)
+
+		const toolResultIds = toolResults.map((r) => r.tool_use_id)
+		expect(toolResultIds).toContain("tool_1")
+		expect(toolResultIds).toContain("tool_2")
+		expect(toolResultIds).toContain("tool_3")
+		expect(toolResultIds).toContain("tool_4")
+
+		// Should report to telemetry
+		expect(TelemetryService.instance.captureException).toHaveBeenCalledTimes(1)
+		const capturedError = (TelemetryService.instance.captureException as ReturnType<typeof vi.fn>).mock.calls[0][0]
+		expect(capturedError).toBeInstanceOf(MissingToolResultError)
+		expect(capturedError.missingToolUseIds).toEqual(["tool_2", "tool_3", "tool_4"])
+	})
+
+	it("should insert a synthetic user message when no following user message exists", () => {
+		const messages: Anthropic.Messages.MessageParam[] = [
+			{
+				role: "user",
+				content: [{ type: "text", text: "Hello" }],
+			},
+			{
+				role: "assistant",
+				content: [{ type: "tool_use", id: "tool_1", name: "read_file", input: { path: "foo.ts" } }],
+			},
+			// No following user message
+		]
+
+		const result = validateMessageHistoryBeforeSend(messages)
+
+		// Should now have 3 messages (synthetic user message added)
+		expect(result.length).toBe(3)
+		expect(result[2].role).toBe("user")
+
+		const content = result[2].content as Anthropic.ToolResultBlockParam[]
+		expect(content.length).toBe(1)
+		expect(content[0].type).toBe("tool_result")
+		expect(content[0].tool_use_id).toBe("tool_1")
+		expect(content[0].content).toBe("Tool execution was interrupted before completion.")
+	})
+
+	it("should handle multiple assistant messages with tool_use blocks", () => {
+		const messages: Anthropic.Messages.MessageParam[] = [
+			{
+				role: "user",
+				content: [{ type: "text", text: "Hello" }],
+			},
+			{
+				role: "assistant",
+				content: [{ type: "tool_use", id: "tool_1", name: "read_file", input: { path: "foo.ts" } }],
+			},
+			{
+				role: "user",
+				content: [{ type: "tool_result", tool_use_id: "tool_1", content: "result 1" }],
+			},
+			{
+				role: "assistant",
+				content: [
+					{ type: "tool_use", id: "tool_2", name: "read_file", input: { path: "bar.ts" } },
+					{ type: "tool_use", id: "tool_3", name: "read_file", input: { path: "baz.ts" } },
+				],
+			},
+			{
+				role: "user",
+				content: [
+					// Only tool_2 has a result, tool_3 is missing
+					{ type: "tool_result", tool_use_id: "tool_2", content: "result 2" },
+				],
+			},
+		]
+
+		const result = validateMessageHistoryBeforeSend(messages)
+
+		expect(result.length).toBe(5)
+
+		// First pair should be untouched
+		const firstUserContent = result[2].content as Anthropic.Messages.ContentBlockParam[]
+		expect(firstUserContent.length).toBe(1)
+
+		// Second pair should have placeholder for tool_3
+		const secondUserContent = result[4].content as Anthropic.Messages.ContentBlockParam[]
+		const toolResults = secondUserContent.filter(
+			(b): b is Anthropic.ToolResultBlockParam => b.type === "tool_result",
+		)
+		expect(toolResults.length).toBe(2)
+		expect(toolResults.map((r) => r.tool_use_id).sort()).toEqual(["tool_2", "tool_3"])
+	})
+
+	it("should not modify messages without tool_use blocks", () => {
+		const messages: Anthropic.Messages.MessageParam[] = [
+			{
+				role: "user",
+				content: [{ type: "text", text: "Hello" }],
+			},
+			{
+				role: "assistant",
+				content: [{ type: "text", text: "Hi there!" }],
+			},
+			{
+				role: "user",
+				content: [{ type: "text", text: "Thanks" }],
+			},
+		]
+
+		const result = validateMessageHistoryBeforeSend(messages)
+		expect(result).toBe(messages) // Same reference
+	})
+
+	it("should handle assistant messages with string content", () => {
+		const messages: Anthropic.Messages.MessageParam[] = [
+			{
+				role: "user",
+				content: "Hello",
+			},
+			{
+				role: "assistant",
+				content: "Hi there!",
+			},
+		]
+
+		const result = validateMessageHistoryBeforeSend(messages)
+		expect(result).toBe(messages)
+	})
+
+	it("should handle the next message being an assistant (not user)", () => {
+		const messages: Anthropic.Messages.MessageParam[] = [
+			{
+				role: "user",
+				content: [{ type: "text", text: "Hello" }],
+			},
+			{
+				role: "assistant",
+				content: [{ type: "tool_use", id: "tool_1", name: "read_file", input: { path: "foo.ts" } }],
+			},
+			{
+				role: "assistant",
+				content: [{ type: "text", text: "Continuing..." }],
+			},
+		]
+
+		const result = validateMessageHistoryBeforeSend(messages)
+
+		// Should insert a synthetic user message between the two assistant messages
+		expect(result.length).toBe(4)
+		expect(result[2].role).toBe("user")
+		const syntheticContent = result[2].content as Anthropic.ToolResultBlockParam[]
+		expect(syntheticContent[0].tool_use_id).toBe("tool_1")
+		// The original second assistant message should still be there
+		expect(result[3].role).toBe("assistant")
+	})
+})

src/core/task/validateToolResultIds.ts

@@ -232,3 +232,118 @@ export function validateAndFixToolResultIds(
 		content: finalContent,
 	}
 }
+
+/**
+ * Pre-send validation that ensures every tool_use block in the final message
+ * array has a corresponding tool_result block in the immediately following
+ * user message. This acts as a last-resort safety net right before the API
+ * call, catching any mismatches introduced by post-processing steps like
+ * getEffectiveApiHistory(), mergeConsecutiveApiMessages(), or
+ * buildCleanConversationHistory().
+ *
+ * For any missing tool_result, a placeholder is injected so the API request
+ * remains valid. Mismatches are reported to telemetry.
+ *
+ * @param messages - The final message array about to be sent to the API
+ * @returns A new array with any missing tool_result placeholders injected
+ */
+export function validateMessageHistoryBeforeSend(
+	messages: Anthropic.Messages.MessageParam[],
+): Anthropic.Messages.MessageParam[] {
+	// Work on a shallow copy so we don't mutate the caller's array.
+	const result: Anthropic.Messages.MessageParam[] = []
+	let modified = false
+
+	for (let i = 0; i < messages.length; i++) {
+		const current = messages[i]
+
+		// We only care about assistant messages that contain tool_use blocks.
+		if (current.role !== "assistant" || !Array.isArray(current.content)) {
+			result.push(current)
+			continue
+		}
+
+		const toolUseBlocks = (current.content as Anthropic.Messages.ContentBlockParam[]).filter(
+			(block): block is Anthropic.ToolUseBlock => block.type === "tool_use",
+		)
+
+		if (toolUseBlocks.length === 0) {
+			result.push(current)
+			continue
+		}
+
+		result.push(current)
+
+		// Collect tool_use IDs that need matching tool_results.
+		const toolUseIds = new Set(toolUseBlocks.map((b) => b.id))
+
+		// Look at the next message - it should be a user message with tool_results.
+		const next = messages[i + 1]
+
+		// Gather existing tool_result IDs from the next message (if it's a user message).
+		const existingToolResultIds = new Set<string>()
+		let nextContent: Anthropic.Messages.ContentBlockParam[] = []
+
+		if (next && next.role === "user" && Array.isArray(next.content)) {
+			nextContent = next.content as Anthropic.Messages.ContentBlockParam[]
+			for (const block of nextContent) {
+				if (block.type === "tool_result") {
+					existingToolResultIds.add((block as Anthropic.ToolResultBlockParam).tool_use_id)
+				}
+			}
+		}
+
+		// Determine which tool_use IDs are missing a tool_result.
+		const missingIds = [...toolUseIds].filter((id) => !existingToolResultIds.has(id))
+
+		if (missingIds.length === 0) {
+			continue // All good for this pair.
+		}
+
+		// Report to telemetry.
+		if (TelemetryService.hasInstance()) {
+			TelemetryService.instance.captureException(
+				new MissingToolResultError(
+					`Pre-send validation: missing tool_result blocks for tool_use IDs: [${missingIds.join(", ")}]`,
+					missingIds,
+					[...existingToolResultIds],
+				),
+				{
+					missingToolUseIds: missingIds,
+					existingToolResultIds: [...existingToolResultIds],
+					toolUseCount: toolUseBlocks.length,
+					existingToolResultCount: existingToolResultIds.size,
+					messageIndex: i,
+				},
+			)
+		}
+
+		modified = true
+
+		// Build placeholder tool_result blocks for the missing IDs.
+		const placeholders: Anthropic.ToolResultBlockParam[] = missingIds.map((id) => ({
+			type: "tool_result" as const,
+			tool_use_id: id,
+			content: "Tool execution was interrupted before completion.",
+		}))
+
+		if (next && next.role === "user") {
+			// Inject placeholders into the existing user message.
+			const patchedNext: Anthropic.Messages.MessageParam = {
+				...next,
+				content: [...placeholders, ...nextContent],
+			}
+			result.push(patchedNext)
+			i++ // Skip the next message since we already pushed the patched version.
+		} else {
+			// No following user message at all - insert a synthetic one.
+			result.push({
+				role: "user" as const,
+				content: placeholders,
+			})
+			// Don't skip - the next message (if any) still needs to be processed.
+		}
+	}
+
+	return modified ? result : messages
+}