diff --git a/packages/agent/docs/harness-v2.md b/packages/agent/docs/harness-v2.md
index 9b8aa6bf3..b3a49cbc9 100644
--- a/packages/agent/docs/harness-v2.md
+++ b/packages/agent/docs/harness-v2.md
@@ -2982,12 +2982,13 @@ These packages merge R0 → R1 → R2 → R3. R1 and R2 add a reducer module ins
   - Prove that zero/one open operations are distinguishable from multiple-open-operation corruption, and that the latest run-kind start is an indexed query. Add the SQLite `(session_id, lane, run_id, type)` index.
   - Acceptance: memory and SQLite have identical query behavior, invalid query combinations reject, and no restore algorithm needs a full historical scan.
 
-**In progress and reserved: R1 by @vegarsti.** Other agents must not pick R1 while this ownership marker remains.
-
-- [ ] **R1 — pure record-log validity.** Dependencies: R0.
+- [x] **R1 — pure record-log validity.** Dependencies: R0.
   - Primary files: `packages/agent/src/harness/reducer.ts`, `packages/agent/test/harness/reducer.test.ts`.
   - Validate the section 5 corruption rules from discovered open starts, bounded records, and point-looked-up entries, with no writes or effects.
   - Acceptance: one focused rejection test per validity bullet, plus valid prefixes at every section 6 crash point.
+
+**In progress and reserved: R2 by @vegarsti.** Other agents must not pick R2 while this ownership marker remains.
+
 - [ ] **R2 — pure lane-state reduction.** Dependencies: R1.
   - Primary files: `packages/agent/src/harness/reducer.ts`, `packages/agent/test/harness/reducer.test.ts`.
   - Derive `LaneState`, pending queues/writes, attempts, tool batches, deferred handles, structural targets, terminal-failure state, idle next-run state, and effective configuration from the section 7 query inputs.
diff --git a/packages/agent/src/harness/reducer.ts b/packages/agent/src/harness/reducer.ts
new file mode 100644
index 000000000..c416bd7ca
--- /dev/null
+++ b/packages/agent/src/harness/reducer.ts
@@ -0,0 +1,297 @@
+import { Guard } from "typebox/guard";
+import type {
+	Entry,
+	LaneRecord,
+	OperationStartedRecord,
+	ProvisionedEntry,
+	StepAttemptRecord,
+} from "./session/types.ts";
+
+/**
+ * Machine-readable category for a contradiction in a lane's durable recovery
+ * slice. These indicate states the single-writer record protocol cannot
+ * produce, not ordinary operation failures or incomplete-but-recoverable
+ * intent/result prefixes. Restore must reject such states rather than repair or
+ * continue it; the accompanying error message supplies human-readable detail.
+ */
+export type RecordLogCorruptionReason =
+	| "multiple_open_operations"
+	| "unknown_operation"
+	| "record_after_finish"
+	| "non_consecutive_attempt"
+	| "invalid_compaction_reason"
+	| "queue_after_abort"
+	| "invalid_queue_cancellation"
+	| "inconsistent_step"
+	| "tool_call_mismatch"
+	| "duplicate_tool_invocation"
+	| "provisioned_entry_mismatch";
+
+export class RecordLogCorruption extends Error {
+	readonly reason: RecordLogCorruptionReason;
+
+	constructor(reason: RecordLogCorruptionReason, message: string) {
+		super(message);
+		this.name = "RecordLogCorruption";
+		this.reason = reason;
+	}
+}
+
+export interface RecordLogSlice {
+	lane: string;
+	openOperations: readonly OperationStartedRecord[];
+	records: readonly LaneRecord[];
+	/** Operation-owned entries plus entries fetched directly by provisioned or referenced ids. */
+	entries: readonly Entry[];
+}
+
+interface AttemptSeries {
+	record: StepAttemptRecord;
+}
+
+function corrupt(reason: RecordLogCorruptionReason, message: string): never {
+	throw new RecordLogCorruption(reason, message);
+}
+
+function hasRunId(record: LaneRecord): record is Exclude<LaneRecord, OperationStartedRecord> & { runId: string } {
+	return "runId" in record && typeof record.runId === "string";
+}
+
+function matchesProvisionedEntry(entry: Entry, target: ProvisionedEntry): boolean {
+	const { parentId: _parentId, seq: _seq, timestamp: _timestamp, ...payload } = entry;
+	return Guard.IsDeepEqual(payload, target);
+}
+
+function validateExactProvisionedEntry(entriesById: ReadonlyMap<string, Entry>, target: ProvisionedEntry): void {
+	const entry = entriesById.get(target.id);
+	if (entry && !matchesProvisionedEntry(entry, target)) {
+		corrupt(
+			"provisioned_entry_mismatch",
+			`Provisioned entry ${target.id} exists with content different from its intent`,
+		);
+	}
+}
+
+function validateResultEntry(
+	entriesById: ReadonlyMap<string, Entry>,
+	resultEntryId: string,
+	matches: (entry: Entry) => boolean,
+	description: string,
+): void {
+	const entry = entriesById.get(resultEntryId);
+	if (entry && !matches(entry)) {
+		corrupt(
+			"provisioned_entry_mismatch",
+			`Provisioned ${description} entry ${resultEntryId} exists with different content`,
+		);
+	}
+}
+
+function validateAttemptReason(record: StepAttemptRecord): void {
+	const reason = (record as { compactionReason?: unknown }).compactionReason;
+	if (record.step === "compaction") {
+		if (reason !== "manual" && reason !== "threshold" && reason !== "overflow") {
+			corrupt("invalid_compaction_reason", `Compaction attempt ${record.id} has no valid compaction reason`);
+		}
+	} else if (reason !== undefined) {
+		corrupt("invalid_compaction_reason", `${record.step} attempt ${record.id} has a compaction reason`);
+	}
+}
+
+function validateAttemptSequence(
+	record: StepAttemptRecord,
+	previous: AttemptSeries | undefined,
+	entriesById: ReadonlyMap<string, Entry>,
+): void {
+	const previousRecord = previous?.record;
+	const previousResult = previousRecord ? entriesById.get(previousRecord.resultEntryId) : undefined;
+	const continuesSeries =
+		previousRecord !== undefined &&
+		previousRecord.step === record.step &&
+		(previousResult === undefined || previousResult.seq >= record.seq);
+	const expectedAttempt = continuesSeries ? previousRecord.attempt + 1 : 1;
+	if (record.attempt !== expectedAttempt) {
+		corrupt(
+			"non_consecutive_attempt",
+			`${record.step} attempt ${record.id} is ${record.attempt}; expected ${expectedAttempt}`,
+		);
+	}
+	if (!continuesSeries || record.step === "assistant" || previousRecord === undefined) return;
+	if (record.resultEntryId !== previousRecord.resultEntryId) {
+		corrupt("inconsistent_step", `${record.step} attempts disagree on their result entry id`);
+	}
+	if (record.compactionReason !== previousRecord.compactionReason) {
+		corrupt("inconsistent_step", `${record.step} attempts disagree on their compaction reason`);
+	}
+}
+
+function validateAttemptResult(entriesById: ReadonlyMap<string, Entry>, record: StepAttemptRecord): void {
+	switch (record.step) {
+		case "assistant":
+			validateResultEntry(
+				entriesById,
+				record.resultEntryId,
+				(entry) => entry.type === "message" && entry.message.role === "assistant",
+				"assistant result",
+			);
+			break;
+		case "compaction":
+			validateResultEntry(
+				entriesById,
+				record.resultEntryId,
+				(entry) => entry.type === "compaction",
+				"compaction result",
+			);
+			break;
+		case "branch_summary":
+			validateResultEntry(
+				entriesById,
+				record.resultEntryId,
+				(entry) => entry.type === "branch_summary",
+				"branch-summary result",
+			);
+			break;
+	}
+}
+
+function validateToolStart(
+	record: Extract<LaneRecord, { type: "tool_started" }>,
+	entriesById: ReadonlyMap<string, Entry>,
+	invocations: Set<string>,
+): void {
+	const invocation = `${record.assistantEntryId}\u0000${record.toolIndex}`;
+	if (invocations.has(invocation)) {
+		corrupt(
+			"duplicate_tool_invocation",
+			`Tool invocation ${record.assistantEntryId}:${record.toolIndex} is duplicated`,
+		);
+	}
+	invocations.add(invocation);
+
+	const assistantEntry = entriesById.get(record.assistantEntryId);
+	if (!assistantEntry || assistantEntry.type !== "message" || assistantEntry.message.role !== "assistant") {
+		corrupt("tool_call_mismatch", `Tool start ${record.id} does not reference an assistant entry`);
+	}
+	const toolCalls = assistantEntry.message.content.filter((content) => content.type === "toolCall");
+	const toolCall = toolCalls[record.toolIndex];
+	if (!toolCall || toolCall.id !== record.toolCallId || toolCall.name !== record.toolName) {
+		corrupt("tool_call_mismatch", `Tool start ${record.id} does not match its assistant tool-call ordinal`);
+	}
+
+	validateResultEntry(
+		entriesById,
+		record.resultEntryId,
+		(entry) =>
+			entry.type === "message" &&
+			entry.message.role === "toolResult" &&
+			entry.message.toolCallId === record.toolCallId &&
+			entry.message.toolName === record.toolName,
+		"tool result",
+	);
+}
+
+function validateOperationResult(entriesById: ReadonlyMap<string, Entry>, record: OperationStartedRecord): void {
+	switch (record.intent.kind) {
+		case "run":
+			for (const target of record.intent.initialMessages) validateExactProvisionedEntry(entriesById, target);
+			break;
+		case "compaction":
+			validateResultEntry(
+				entriesById,
+				record.intent.resultEntryId,
+				(entry) => entry.type === "compaction",
+				"manual compaction",
+			);
+			break;
+		case "navigation":
+			if (record.intent.summaryEntryId) {
+				validateResultEntry(
+					entriesById,
+					record.intent.summaryEntryId,
+					(entry) => entry.type === "branch_summary",
+					"navigation summary",
+				);
+			}
+			break;
+	}
+}
+
+/** Validates a bounded lane recovery slice without reading or mutating session state. */
+export function validateRecordLog(input: RecordLogSlice): void {
+	if (input.openOperations.length > 1) {
+		corrupt("multiple_open_operations", `Lane ${input.lane} has at least two open operations`);
+	}
+
+	const entriesById = new Map(input.entries.map((entry) => [entry.id, entry]));
+	const starts = new Map<string, OperationStartedRecord>();
+	const finishedAt = new Map<string, number>();
+	const abortedAt = new Map<string, number>();
+	const queueEnqueues = new Map<string, Extract<LaneRecord, { type: "queue_enqueued" }>>();
+	const latestAttempt = new Map<string, AttemptSeries>();
+	const toolInvocations = new Set<string>();
+	const records = [...input.records].sort((left, right) => left.seq - right.seq);
+
+	for (const record of records) {
+		if (record.type === "operation_started") {
+			starts.set(record.id, record);
+			validateOperationResult(entriesById, record);
+			continue;
+		}
+
+		if (hasRunId(record)) {
+			if (!starts.has(record.runId)) {
+				corrupt("unknown_operation", `Record ${record.id} references unknown operation ${record.runId}`);
+			}
+			const finishSeq = finishedAt.get(record.runId);
+			if (finishSeq !== undefined && record.seq > finishSeq) {
+				corrupt("record_after_finish", `Record ${record.id} follows the finish of operation ${record.runId}`);
+			}
+		}
+
+		switch (record.type) {
+			case "operation_finished":
+				finishedAt.set(record.runId, record.seq);
+				break;
+			case "abort_requested":
+				abortedAt.set(record.runId, record.seq);
+				break;
+			case "step_attempt":
+				validateAttemptReason(record);
+				validateAttemptSequence(record, latestAttempt.get(record.runId), entriesById);
+				validateAttemptResult(entriesById, record);
+				latestAttempt.set(record.runId, { record });
+				break;
+			case "tool_started":
+				validateToolStart(record, entriesById, toolInvocations);
+				break;
+			case "queue_enqueued":
+				if (
+					record.queue !== "nextRun" &&
+					abortedAt.get(record.runId) !== undefined &&
+					record.seq > abortedAt.get(record.runId)!
+				) {
+					corrupt("queue_after_abort", `${record.queue} item ${record.target.id} was enqueued after abort`);
+				}
+				queueEnqueues.set(record.target.id, record);
+				validateExactProvisionedEntry(entriesById, record.target);
+				break;
+			case "queue_cancelled": {
+				const enqueue = queueEnqueues.get(record.entryId);
+				if (
+					!enqueue ||
+					enqueue.seq >= record.seq ||
+					enqueue.runId !== record.runId ||
+					entriesById.has(record.entryId)
+				) {
+					corrupt("invalid_queue_cancellation", `Queue cancellation ${record.id} has no pending matching enqueue`);
+				}
+				break;
+			}
+			case "write_deferred":
+				validateExactProvisionedEntry(entriesById, record.target);
+				break;
+			case "usage":
+				break;
+		}
+	}
+}
diff --git a/packages/agent/src/harness/session/types.ts b/packages/agent/src/harness/session/types.ts
index d92d15c00..77b05ad24 100644
--- a/packages/agent/src/harness/session/types.ts
+++ b/packages/agent/src/harness/session/types.ts
@@ -297,6 +297,12 @@ export interface SessionStorage<TMetadata extends SessionMetadata = SessionMetad
 		query: RecordQuery & { type: K },
 	): Promise<Extract<LaneRecord, { type: K }>[]>;
 	findRecords(query?: RecordQuery): Promise<LaneRecord[]>;
+	/**
+	 * Returns unfinished operation starts newest first. Recovery uses `limit: 2`:
+	 * zero results mean the lane is idle, one means it is suspended, and two
+	 * mean at least two operations are open, which is corruption. Further
+	 * results provide no additional recovery state.
+	 */
 	findOpenOperations(lane: string, options?: { limit?: number }): Promise<OperationStartedRecord[]>;
 	getLog(options?: { afterSeq?: number; limit?: number }): Promise<LogItem[]>;
 
diff --git a/packages/agent/test/harness/reducer.test.ts b/packages/agent/test/harness/reducer.test.ts
new file mode 100644
index 000000000..e824deae4
--- /dev/null
+++ b/packages/agent/test/harness/reducer.test.ts
@@ -0,0 +1,619 @@
+import type { AssistantMessage, ToolResultMessage, Usage, UserMessage } from "@earendil-works/pi-ai";
+import { describe, expect, it } from "vitest";
+import {
+	RecordLogCorruption,
+	type RecordLogCorruptionReason,
+	type RecordLogSlice,
+	validateRecordLog,
+} from "../../src/harness/reducer.ts";
+import type {
+	AbortRequestedRecord,
+	BranchSummaryEntry,
+	CompactionEntry,
+	Entry,
+	LaneRecord,
+	MessageEntry,
+	OperationFinishedRecord,
+	OperationStartedRecord,
+	ProvisionedEntry,
+	QueueCancelledRecord,
+	QueueEnqueuedRecord,
+	SessionStopReason,
+	StepAttemptRecord,
+	ToolStartedRecord,
+	UsageRecord,
+	WriteDeferredRecord,
+} from "../../src/harness/session/types.ts";
+
+const usage: Usage = {
+	input: 1,
+	output: 1,
+	cacheRead: 0,
+	cacheWrite: 0,
+	totalTokens: 2,
+	cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+};
+
+function userMessage(text: string): UserMessage {
+	return { role: "user", content: text, timestamp: 1 };
+}
+
+function assistantMessage(
+	content: AssistantMessage["content"],
+	stopReason: AssistantMessage["stopReason"] = "stop",
+): AssistantMessage {
+	return {
+		role: "assistant",
+		content,
+		api: "openai-responses",
+		provider: "openai",
+		model: "test-model",
+		usage,
+		stopReason,
+		timestamp: 1,
+		...(stopReason === "deferred"
+			? { deferred: { provider: "openai", modelId: "test-model", api: "openai-responses", id: "deferred-1" } }
+			: {}),
+	};
+}
+
+function toolResultMessage(toolCallId = "call-1", toolName = "tool-1"): ToolResultMessage {
+	return {
+		role: "toolResult",
+		toolCallId,
+		toolName,
+		content: [{ type: "text", text: "result" }],
+		isError: false,
+		timestamp: 1,
+	};
+}
+
+function messageTarget(
+	id: string,
+	message: UserMessage | AssistantMessage | ToolResultMessage,
+): ProvisionedEntry<MessageEntry> {
+	return { type: "message", id, message };
+}
+
+function persistedEntry<TEntry extends Entry>(
+	target: ProvisionedEntry<TEntry>,
+	seq: number,
+	parentId: string | null = null,
+): TEntry {
+	return { ...target, parentId, seq, timestamp: seq } as unknown as TEntry;
+}
+
+function runStarted(
+	seq = 1,
+	options: { id?: string; initialMessages?: ProvisionedEntry[] } = {},
+): OperationStartedRecord {
+	return {
+		type: "operation_started",
+		id: options.id ?? "run-1",
+		lane: "main",
+		seq,
+		timestamp: seq,
+		sourceLeafId: null,
+		intent: { kind: "run", originalPrompt: [], initialMessages: options.initialMessages ?? [] },
+	};
+}
+
+function compactionStarted(seq: number, resultEntryId = "compaction-1"): OperationStartedRecord {
+	return {
+		type: "operation_started",
+		id: "compact-1",
+		lane: "main",
+		seq,
+		timestamp: seq,
+		sourceLeafId: "source",
+		intent: { kind: "compaction", resultEntryId },
+	};
+}
+
+function navigationStarted(seq: number, summaryEntryId = "summary-1"): OperationStartedRecord {
+	return {
+		type: "operation_started",
+		id: "navigate-1",
+		lane: "main",
+		seq,
+		timestamp: seq,
+		sourceLeafId: "source",
+		intent: { kind: "navigation", targetId: "target", summarize: true, summaryEntryId },
+	};
+}
+
+function attempt(
+	seq: number,
+	runId: string,
+	step: StepAttemptRecord["step"],
+	attemptNumber: number,
+	resultEntryId: string,
+	compactionReason?: "manual" | "threshold" | "overflow",
+): StepAttemptRecord {
+	const base = {
+		type: "step_attempt" as const,
+		id: `attempt-${seq}`,
+		lane: "main",
+		seq,
+		timestamp: seq,
+		runId,
+		attempt: attemptNumber,
+		resultEntryId,
+	};
+	return step === "compaction" ? { ...base, step, compactionReason: compactionReason ?? "manual" } : { ...base, step };
+}
+
+function abortRequested(seq: number, runId = "run-1"): AbortRequestedRecord {
+	return { type: "abort_requested", id: `abort-${seq}`, lane: "main", seq, timestamp: seq, runId };
+}
+
+function operationFinished(
+	seq: number,
+	runId = "run-1",
+	outcome: OperationFinishedRecord["outcome"] = "completed",
+): OperationFinishedRecord {
+	return { type: "operation_finished", id: `finish-${seq}`, lane: "main", seq, timestamp: seq, runId, outcome };
+}
+
+function toolStarted(
+	seq: number,
+	overrides: Partial<
+		Pick<ToolStartedRecord, "assistantEntryId" | "toolIndex" | "toolCallId" | "toolName" | "resultEntryId">
+	> = {},
+): ToolStartedRecord {
+	return {
+		type: "tool_started",
+		id: `tool-start-${seq}`,
+		lane: "main",
+		seq,
+		timestamp: seq,
+		runId: "run-1",
+		assistantEntryId: overrides.assistantEntryId ?? "assistant-tools",
+		toolIndex: overrides.toolIndex ?? 0,
+		toolCallId: overrides.toolCallId ?? "call-1",
+		toolName: overrides.toolName ?? "tool-1",
+		effectiveArgs: {},
+		resultEntryId: overrides.resultEntryId ?? "tool-result-1",
+		replay: "never",
+	};
+}
+
+function queueEnqueued(
+	seq: number,
+	target: ProvisionedEntry = messageTarget("queue-1", userMessage("queued")),
+	queue: QueueEnqueuedRecord["queue"] = "steer",
+): QueueEnqueuedRecord {
+	const base = { type: "queue_enqueued" as const, id: `queue-${seq}`, lane: "main", seq, timestamp: seq, target };
+	return queue === "nextRun" ? { ...base, queue } : { ...base, queue, runId: "run-1" };
+}
+
+function queueCancelled(seq: number, entryId = "queue-1", runId: string | null = "run-1"): QueueCancelledRecord {
+	return {
+		type: "queue_cancelled",
+		id: `cancel-${seq}`,
+		lane: "main",
+		seq,
+		timestamp: seq,
+		entryId,
+		...(runId === null ? {} : { runId }),
+	};
+}
+
+function writeDeferred(
+	seq: number,
+	target: ProvisionedEntry = messageTarget("write-1", userMessage("deferred write")),
+): WriteDeferredRecord {
+	return { type: "write_deferred", id: `write-${seq}`, lane: "main", seq, timestamp: seq, runId: "run-1", target };
+}
+
+function usageRecord(
+	seq: number,
+	resultEntryId: string,
+	stopReason: SessionStopReason = "error",
+	attemptNumber = 1,
+): UsageRecord {
+	return {
+		type: "usage",
+		id: `usage-${seq}`,
+		lane: "main",
+		seq,
+		timestamp: seq,
+		cause: "assistant",
+		runId: "run-1",
+		entryId: resultEntryId,
+		attempt: attemptNumber,
+		stopReason,
+		usage,
+	};
+}
+
+function compactionEntry(id: string, seq: number): CompactionEntry {
+	return {
+		type: "compaction",
+		id,
+		parentId: null,
+		seq,
+		timestamp: seq,
+		summary: "summary",
+		retainedTail: [],
+		tokensBefore: 10,
+	};
+}
+
+function branchSummaryEntry(id: string, seq: number): BranchSummaryEntry {
+	return {
+		type: "branch_summary",
+		id,
+		parentId: "target",
+		seq,
+		timestamp: seq,
+		fromId: "source",
+		summary: "summary",
+	};
+}
+
+function recoverySlice(records: readonly LaneRecord[], entries: readonly Entry[] = []): RecordLogSlice {
+	const finished = new Set(
+		records
+			.filter((record): record is OperationFinishedRecord => record.type === "operation_finished")
+			.map((record) => record.runId),
+	);
+	const openOperations = records
+		.filter(
+			(record): record is OperationStartedRecord => record.type === "operation_started" && !finished.has(record.id),
+		)
+		.sort((left, right) => right.seq - left.seq);
+	return { lane: "main", openOperations, records, entries };
+}
+
+function expectCorruption(input: RecordLogSlice, reason: RecordLogCorruptionReason): void {
+	try {
+		validateRecordLog(input);
+		expect.fail(`Expected ${reason}`);
+	} catch (error) {
+		expect(error).toBeInstanceOf(RecordLogCorruption);
+		expect(error).toMatchObject({ reason });
+	}
+}
+
+const assistantToolsEntry = persistedEntry(
+	messageTarget(
+		"assistant-tools",
+		assistantMessage([{ type: "toolCall", id: "call-1", name: "tool-1", arguments: {} }], "toolUse"),
+	),
+	3,
+);
+
+interface CorruptionCase {
+	name: string;
+	reason: RecordLogCorruptionReason;
+	input: RecordLogSlice;
+}
+
+const corruptionCases: CorruptionCase[] = [
+	{
+		name: "multiple operations are open",
+		reason: "multiple_open_operations",
+		input: recoverySlice([runStarted(1), runStarted(2, { id: "run-2" })]),
+	},
+	{
+		name: "a record references an operation that does not exist",
+		reason: "unknown_operation",
+		input: recoverySlice([abortRequested(1, "missing")]),
+	},
+	{
+		name: "a record follows its operation finish",
+		reason: "record_after_finish",
+		input: recoverySlice([runStarted(1), operationFinished(2), abortRequested(3)]),
+	},
+	{
+		name: "attempt numbers skip within one assistant step",
+		reason: "non_consecutive_attempt",
+		input: recoverySlice([
+			runStarted(1),
+			attempt(2, "run-1", "assistant", 1, "assistant-1"),
+			attempt(3, "run-1", "assistant", 3, "assistant-2"),
+		]),
+	},
+	{
+		name: "a non-compaction attempt carries compactionReason",
+		reason: "invalid_compaction_reason",
+		input: recoverySlice([
+			runStarted(1),
+			{ ...attempt(2, "run-1", "assistant", 1, "assistant-1"), compactionReason: "manual" } as unknown as LaneRecord,
+		]),
+	},
+	{
+		name: "a compaction attempt omits compactionReason",
+		reason: "invalid_compaction_reason",
+		input: recoverySlice([
+			runStarted(1),
+			{
+				...attempt(2, "run-1", "compaction", 1, "compaction-1"),
+				compactionReason: undefined,
+			} as unknown as LaneRecord,
+		]),
+	},
+	{
+		name: "steering is enqueued after abort",
+		reason: "queue_after_abort",
+		input: recoverySlice([runStarted(1), abortRequested(2), queueEnqueued(3)]),
+	},
+	{
+		name: "a queue cancellation has no enqueue",
+		reason: "invalid_queue_cancellation",
+		input: recoverySlice([runStarted(1), queueCancelled(2)]),
+	},
+	{
+		name: "a queue cancellation targets an entry that exists",
+		reason: "invalid_queue_cancellation",
+		input: recoverySlice(
+			[runStarted(1), queueEnqueued(2), queueCancelled(4)],
+			[persistedEntry(messageTarget("queue-1", userMessage("queued")), 3)],
+		),
+	},
+	{
+		name: "structural attempts disagree on resultEntryId",
+		reason: "inconsistent_step",
+		input: recoverySlice([
+			runStarted(1),
+			attempt(2, "run-1", "compaction", 1, "compaction-1", "threshold"),
+			attempt(3, "run-1", "compaction", 2, "compaction-2", "threshold"),
+		]),
+	},
+	{
+		name: "structural attempts disagree on compactionReason",
+		reason: "inconsistent_step",
+		input: recoverySlice([
+			runStarted(1),
+			attempt(2, "run-1", "compaction", 1, "compaction-1", "threshold"),
+			attempt(3, "run-1", "compaction", 2, "compaction-1", "overflow"),
+		]),
+	},
+	{
+		name: "tool_started does not match the assistant tool call",
+		reason: "tool_call_mismatch",
+		input: recoverySlice([runStarted(1), toolStarted(4, { toolCallId: "different-call" })], [assistantToolsEntry]),
+	},
+	{
+		name: "two tool_started records share an invocation identity",
+		reason: "duplicate_tool_invocation",
+		input: recoverySlice(
+			[
+				runStarted(1),
+				toolStarted(4),
+				{ ...toolStarted(5, { resultEntryId: "tool-result-2" }), id: "tool-start-duplicate" },
+			],
+			[assistantToolsEntry],
+		),
+	},
+	{
+		name: "a provisioned id exists with different content",
+		reason: "provisioned_entry_mismatch",
+		input: recoverySlice(
+			[runStarted(1, { initialMessages: [messageTarget("prompt-1", userMessage("expected"))] })],
+			[persistedEntry(messageTarget("prompt-1", userMessage("different")), 2)],
+		),
+	},
+];
+
+describe("record-log validity", () => {
+	it.each(corruptionCases)("rejects $name", ({ input, reason }) => {
+		expectCorruption(input, reason);
+	});
+
+	it("does not mutate its bounded recovery inputs", () => {
+		const target = messageTarget("prompt-1", userMessage("hello"));
+		const start = Object.freeze(runStarted(1, { initialMessages: [target] }));
+		const entry = Object.freeze(persistedEntry(target, 2));
+		const input = Object.freeze({
+			lane: "main",
+			openOperations: Object.freeze([start]),
+			records: Object.freeze([start]),
+			entries: Object.freeze([entry]),
+		});
+
+		expect(validateRecordLog(input)).toBeUndefined();
+		expect(input.records).toEqual([start]);
+		expect(input.entries).toEqual([entry]);
+	});
+});
+
+type DurableAction = { record: LaneRecord } | { entry: Entry };
+
+function validPrefixes(trace: string, actions: readonly DurableAction[]): { name: string; input: RecordLogSlice }[] {
+	return actions.map((_, index) => {
+		const prefix = actions.slice(0, index + 1);
+		return {
+			name: `${trace} after action ${index + 1}`,
+			input: recoverySlice(
+				prefix.flatMap((action) => ("record" in action ? [action.record] : [])),
+				prefix.flatMap((action) => ("entry" in action ? [action.entry] : [])),
+			),
+		};
+	});
+}
+
+const promptTarget = messageTarget("prompt-1", userMessage("fix the bug"));
+const assistantToolTarget = messageTarget(
+	"assistant-tools",
+	assistantMessage([{ type: "toolCall", id: "call-1", name: "tool-1", arguments: {} }], "toolUse"),
+);
+const toolResultTarget = messageTarget("tool-result-1", toolResultMessage());
+const assistantFinalTarget = messageTarget("assistant-final", assistantMessage([{ type: "text", text: "done" }]));
+
+const validPrefixCases = [
+	...validPrefixes("one-tool run X1-X5", [
+		{ record: runStarted(1, { initialMessages: [promptTarget] }) },
+		{ entry: persistedEntry(promptTarget, 2) },
+		{ record: attempt(3, "run-1", "assistant", 1, "assistant-tools") },
+		{ entry: persistedEntry(assistantToolTarget, 4, "prompt-1") },
+		{ record: toolStarted(5) },
+		{ entry: persistedEntry(toolResultTarget, 6, "assistant-tools") },
+		{ record: attempt(7, "run-1", "assistant", 1, "assistant-final") },
+		{ entry: persistedEntry(assistantFinalTarget, 8, "tool-result-1") },
+		{ record: operationFinished(9) },
+	]),
+	...validPrefixes("assistant retry", [
+		{ record: runStarted(1) },
+		{ record: attempt(2, "run-1", "assistant", 1, "assistant-attempt-1") },
+		{ record: usageRecord(3, "assistant-attempt-1") },
+		{ record: attempt(4, "run-1", "assistant", 2, "assistant-attempt-2") },
+		{ record: usageRecord(5, "assistant-attempt-2", "stop", 2) },
+		{
+			entry: persistedEntry(
+				messageTarget("assistant-attempt-2", assistantMessage([{ type: "text", text: "ok" }])),
+				6,
+			),
+		},
+	]),
+	...validPrefixes("terminal assistant failure", [
+		{ record: runStarted(1) },
+		{ record: attempt(2, "run-1", "assistant", 1, "assistant-error") },
+		{
+			entry: persistedEntry(
+				messageTarget("assistant-error", { ...assistantMessage([], "error"), errorMessage: "failed" }),
+				3,
+			),
+		},
+		{ record: operationFinished(4, "run-1", "failed") },
+	]),
+	...validPrefixes("overflow compaction and retry", [
+		{ record: runStarted(1) },
+		{ record: attempt(2, "run-1", "assistant", 1, "discarded-overflow") },
+		{ record: usageRecord(3, "discarded-overflow", "length") },
+		{ record: attempt(4, "run-1", "compaction", 1, "overflow-compaction", "overflow") },
+		{ entry: compactionEntry("overflow-compaction", 5) },
+		{ record: attempt(6, "run-1", "assistant", 1, "assistant-after-compaction") },
+		{
+			entry: persistedEntry(
+				messageTarget("assistant-after-compaction", assistantMessage([{ type: "text", text: "fits" }])),
+				7,
+			),
+		},
+	]),
+	...validPrefixes("steering acceptance and consumption", [
+		{ record: runStarted(1) },
+		{ record: queueEnqueued(2) },
+		{ entry: persistedEntry(messageTarget("queue-1", userMessage("queued")), 3) },
+	]),
+	...validPrefixes("queue cancellation", [
+		{ record: runStarted(1) },
+		{ record: queueEnqueued(2) },
+		{ record: queueCancelled(3) },
+	]),
+	...validPrefixes("deferred write acceptance and application", [
+		{ record: runStarted(1) },
+		{ record: writeDeferred(2) },
+		{ entry: persistedEntry(messageTarget("write-1", userMessage("deferred write")), 3) },
+	]),
+	...validPrefixes("abort during a tool", [
+		{ record: runStarted(1) },
+		{ record: attempt(2, "run-1", "assistant", 1, "assistant-tools") },
+		{ entry: persistedEntry(assistantToolTarget, 3) },
+		{ record: toolStarted(4) },
+		{ record: abortRequested(5) },
+		{
+			entry: persistedEntry(
+				messageTarget("tool-result-1", {
+					...toolResultMessage(),
+					content: [{ type: "text", text: "interrupted" }],
+					isError: true,
+				}),
+				6,
+			),
+		},
+	]),
+	...validPrefixes("threshold auto-compaction", [
+		{ record: runStarted(1) },
+		{ record: attempt(2, "run-1", "compaction", 1, "threshold-compaction", "threshold") },
+		{ entry: compactionEntry("threshold-compaction", 3) },
+		{ record: attempt(4, "run-1", "assistant", 1, "assistant-after-threshold") },
+	]),
+	...validPrefixes("manual compaction", [
+		{ record: compactionStarted(1) },
+		{ record: attempt(2, "compact-1", "compaction", 1, "compaction-1", "manual") },
+		{ entry: compactionEntry("compaction-1", 3) },
+		{ record: operationFinished(4, "compact-1") },
+	]),
+	...validPrefixes("move-first navigation summary", [
+		{ record: navigationStarted(1) },
+		{ record: attempt(2, "navigate-1", "branch_summary", 1, "summary-1") },
+		{ entry: branchSummaryEntry("summary-1", 3) },
+		{ record: operationFinished(4, "navigate-1") },
+	]),
+	...validPrefixes("blocked tool without an intent record", [
+		{ record: runStarted(1) },
+		{ record: attempt(2, "run-1", "assistant", 1, "assistant-tools") },
+		{ entry: persistedEntry(assistantToolTarget, 3) },
+		{
+			entry: persistedEntry(
+				messageTarget("blocked-result", {
+					...toolResultMessage(),
+					content: [{ type: "text", text: "blocked" }],
+					isError: true,
+				}),
+				4,
+			),
+		},
+	]),
+	...validPrefixes("idle next-run cancellation", [
+		{ record: queueEnqueued(1, messageTarget("next-1", userMessage("later")), "nextRun") },
+		{ record: queueCancelled(2, "next-1", null) },
+	]),
+	...validPrefixes("next-run enqueue after abort", [
+		{ record: runStarted(1) },
+		{ record: abortRequested(2) },
+		{ record: queueEnqueued(3, messageTarget("next-1", userMessage("later")), "nextRun") },
+	]),
+	...validPrefixes("deferred write applied during abort reconciliation", [
+		{ record: runStarted(1) },
+		{ record: writeDeferred(2) },
+		{ record: abortRequested(3) },
+		{ entry: persistedEntry(messageTarget("write-1", userMessage("deferred write")), 4) },
+	]),
+	...validPrefixes("accepted steering killed by abort", [
+		{ record: runStarted(1) },
+		{ record: queueEnqueued(2) },
+		{ record: abortRequested(3) },
+	]),
+	...validPrefixes("compaction retry", [
+		{ record: runStarted(1) },
+		{ record: attempt(2, "run-1", "compaction", 1, "threshold-compaction", "threshold") },
+		{ record: attempt(3, "run-1", "compaction", 2, "threshold-compaction", "threshold") },
+		{ entry: compactionEntry("threshold-compaction", 4) },
+	]),
+	...validPrefixes("hook-supplied manual compaction", [
+		{ record: compactionStarted(1) },
+		{ entry: compactionEntry("compaction-1", 2) },
+		{ record: operationFinished(3, "compact-1") },
+	]),
+	...validPrefixes("hook-supplied navigation summary", [
+		{ record: navigationStarted(1) },
+		{ entry: branchSummaryEntry("summary-1", 2) },
+		{ record: operationFinished(3, "navigate-1") },
+	]),
+	...validPrefixes("deferred provider suspension and redemption", [
+		{ record: runStarted(1) },
+		{ record: attempt(2, "run-1", "assistant", 1, "assistant-deferred") },
+		{ entry: persistedEntry(messageTarget("assistant-deferred", assistantMessage([], "deferred")), 3) },
+		{
+			entry: persistedEntry(
+				messageTarget("assistant-redeemed", assistantMessage([{ type: "text", text: "ready" }])),
+				4,
+			),
+		},
+	]),
+	...validPrefixes("abort of a deferred provider request", [
+		{ record: runStarted(1) },
+		{ record: attempt(2, "run-1", "assistant", 1, "assistant-deferred") },
+		{ entry: persistedEntry(messageTarget("assistant-deferred", assistantMessage([], "deferred")), 3) },
+		{ record: abortRequested(4) },
+	]),
+];
+
+describe("valid section 6 durable prefixes", () => {
+	it.each(validPrefixCases)("accepts $name", ({ input }) => {
+		expect(validateRecordLog(input)).toBeUndefined();
+	});
+});