diff --git a/src/resources/extensions/browser-tools/core.d.ts b/src/resources/extensions/browser-tools/core.d.ts
new file mode 100644
index 000000000..3f740e355
--- /dev/null
+++ b/src/resources/extensions/browser-tools/core.d.ts
@@ -0,0 +1,205 @@
+/**
+ * Type declarations for core.js — runtime-neutral helper logic for browser-tools.
+ */
+
+export interface ActionTimeline {
+	limit: number;
+	nextId: number;
+	entries: ActionEntry[];
+}
+
+export interface ActionEntry {
+	id: number;
+	tool: string;
+	paramsSummary: string;
+	startedAt: number;
+	finishedAt: number | null;
+	status: string;
+	beforeUrl: string;
+	afterUrl: string;
+	verificationSummary?: string;
+	warningSummary?: string;
+	diffSummary?: string;
+	changed?: boolean;
+	error?: string;
+}
+
+export interface ActionPartial {
+	tool: string;
+	paramsSummary?: string;
+	startedAt?: number;
+	beforeUrl?: string;
+	afterUrl?: string;
+	verificationSummary?: string;
+	warningSummary?: string;
+	diffSummary?: string;
+	changed?: boolean;
+	error?: string;
+}
+
+export interface ActionUpdates {
+	finishedAt?: number;
+	status?: string;
+	afterUrl?: string;
+	verificationSummary?: string;
+	warningSummary?: string;
+	diffSummary?: string;
+	changed?: boolean;
+	error?: string;
+}
+
+export interface DiffResult {
+	changed: boolean;
+	changes: Array<{ type: string; before: unknown; after: unknown }>;
+	summary: string;
+}
+
+export interface Threshold {
+	op: string;
+	n: number;
+}
+
+export interface PageRegistry {
+	pages: PageEntry[];
+	activePageId: number | null;
+	nextId: number;
+}
+
+export interface PageEntry {
+	id: number;
+	page: any;
+	title: string;
+	url: string;
+	opener: number | null;
+}
+
+export interface PageListEntry {
+	id: number;
+	title: string;
+	url: string;
+	opener: number | null;
+	isActive: boolean;
+}
+
+export interface SnapshotModeConfig {
+	tags: string[];
+	roles: string[];
+	selectors: string[];
+	ariaAttributes: string[];
+	useInteractiveFilter: boolean;
+	visibleOnly?: boolean;
+	containerExpand?: boolean;
+}
+
+export interface AssertionCheckResult {
+	name: string;
+	passed: boolean;
+	actual: unknown;
+	expected: unknown;
+	selector?: string;
+	text?: string;
+}
+
+export interface AssertionEvaluation {
+	verified: boolean;
+	checks: AssertionCheckResult[];
+	summary: string;
+	agentHint: string;
+}
+
+export interface WaitValidationError {
+	error: string;
+}
+
+export interface BatchStepResult {
+	ok: boolean;
+	stopReason: string | null;
+	failedStepIndex: number | null;
+	stepResults: unknown[];
+	summary: string;
+}
+
+export interface FormattedTimeline {
+	entries: Array<{
+		id: number | null;
+		tool: string;
+		status: string;
+		durationMs: number | null;
+		beforeUrl: string;
+		afterUrl: string;
+		line: string;
+	}>;
+	retained: number;
+	totalRecorded: number;
+	bounded: boolean;
+	summary: string;
+}
+
+export interface FailureHypothesis {
+	hasFailures: boolean;
+	categories: string[];
+	summary: string;
+	signals: Array<{ category: string; source: string; detail: string }>;
+}
+
+export interface SessionSummary {
+	counts: {
+		pages: number;
+		actions: { total: number; retained: number; success: number; error: number; running: number };
+		waits: { total: number; success: number; error: number; running: number };
+		assertions: { total: number; passed: number; failed: number; running: number };
+		consoleErrors: number;
+		failedRequests: number;
+		dialogs: number;
+	};
+	activePage: { id: number | null; title: string; url: string } | null;
+	caveats: string[];
+	failureHypothesis: FailureHypothesis;
+	summary: string;
+}
+
+export function createActionTimeline(limit?: number): ActionTimeline;
+export function beginAction(timeline: ActionTimeline, partial: ActionPartial): ActionEntry;
+export function finishAction(timeline: ActionTimeline, actionId: number, updates?: ActionUpdates): ActionEntry | null;
+export function findAction(timeline: ActionTimeline, actionId: number): ActionEntry | null;
+export function toActionParamsSummary(params: unknown): string;
+export function diffCompactStates(before: unknown, after: unknown): DiffResult;
+export function includesNeedle(haystack: string, needle: string): boolean;
+export function parseThreshold(value: string | null | undefined): Threshold | null;
+export function meetsThreshold(count: number, threshold: Threshold): boolean;
+export function getEntriesSince(
+	entries: Array<{ timestamp?: number }>,
+	sinceActionId: number | undefined,
+	timeline: ActionTimeline,
+): unknown[];
+export function evaluateAssertionChecks(args: { checks: unknown[]; state: unknown }): AssertionEvaluation;
+export function validateWaitParams(params: { condition: string; value?: string; threshold?: string }): WaitValidationError | null;
+export function createRegionStableScript(selector: string): string;
+export function createPageRegistry(): PageRegistry;
+export function registryAddPage(
+	registry: PageRegistry,
+	info: { page: unknown; title?: string; url?: string; opener?: number | null },
+): PageEntry;
+export function registryRemovePage(registry: PageRegistry, pageId: number): { removed: PageEntry; newActiveId: number | null };
+export function registrySetActive(registry: PageRegistry, pageId: number): void;
+export function registryGetActive(registry: PageRegistry): PageEntry;
+export function registryGetPage(registry: PageRegistry, pageId: number): PageEntry | null;
+export function registryListPages(registry: PageRegistry): PageListEntry[];
+export function createBoundedLogPusher(maxSize: number): (array: unknown[], entry: unknown) => void;
+export function runBatchSteps(args: {
+	steps: unknown[];
+	executeStep: (step: unknown, index: number) => Promise<{ ok: boolean; [key: string]: unknown }>;
+	stopOnFailure?: boolean;
+}): Promise<BatchStepResult>;
+
+export declare const SNAPSHOT_MODES: Record<string, SnapshotModeConfig>;
+export function getSnapshotModeConfig(mode: string): SnapshotModeConfig | null;
+export function computeContentHash(text: string): string;
+export function computeStructuralSignature(tag: string, role: string, childTags: string[]): string;
+export function matchFingerprint(
+	stored: { contentHash?: string; structuralSignature?: string },
+	candidate: { contentHash?: string; structuralSignature?: string },
+): boolean;
+export function formatTimelineEntries(entries?: unknown[], options?: Record<string, unknown>): FormattedTimeline;
+export function buildFailureHypothesis(session?: Record<string, unknown>): FailureHypothesis;
+export function summarizeBrowserSession(session?: Record<string, unknown>): SessionSummary;
diff --git a/src/resources/extensions/browser-tools/index.ts b/src/resources/extensions/browser-tools/index.ts
index b9753ba0e..11c062584 100644
--- a/src/resources/extensions/browser-tools/index.ts
+++ b/src/resources/extensions/browser-tools/index.ts
@@ -28,11 +28,11 @@ export default function (pi: ExtensionAPI) {
 		parseRef: u.parseRef, formatVersionedRef: u.formatVersionedRef, staleRefGuidance: u.staleRefGuidance,
 		beginTrackedAction: u.beginTrackedAction, finishTrackedAction: u.finishTrackedAction,
 		truncateText: u.truncateText, verificationFromChecks: u.verificationFromChecks,
-		verificationLine: u.verificationLine, collectAssertionState: u.collectAssertionState,
+		verificationLine: u.verificationLine, collectAssertionState: (p, checks, target?) => u.collectAssertionState(p, checks, captureCompactPageState, target),
 		formatAssertionText: u.formatAssertionText, formatDiffText: u.formatDiffText,
 		getUrlHash: u.getUrlHash,
 		captureClickTargetState: u.captureClickTargetState, readInputLikeValue: u.readInputLikeValue,
-		firstErrorLine: u.firstErrorLine, captureAccessibilityMarkdown: u.captureAccessibilityMarkdown,
+		firstErrorLine: u.firstErrorLine, captureAccessibilityMarkdown: (selector?) => u.captureAccessibilityMarkdown(getActiveTarget(), selector),
 		resolveAccessibilityScope: u.resolveAccessibilityScope,
 		getLivePagesSnapshot: u.createGetLivePagesSnapshot(ensureBrowser),
 		getSinceTimestamp: u.getSinceTimestamp, getConsoleEntriesSince: u.getConsoleEntriesSince,
diff --git a/src/resources/extensions/browser-tools/refs.ts b/src/resources/extensions/browser-tools/refs.ts
index 18339482c..09d80095c 100644
--- a/src/resources/extensions/browser-tools/refs.ts
+++ b/src/resources/extensions/browser-tools/refs.ts
@@ -124,7 +124,7 @@ export async function buildRefSnapshot(
 					sib = sib.previousElementSibling;
 				}
 				// Check if the parent itself is a heading (unlikely but possible)
-				const parent = current.parentElement;
+				const parent: Element | null = current.parentElement;
 				if (parent && (headingTags.has(parent.tagName) || parent.getAttribute("role") === "heading")) {
 					return (parent.textContent || "").trim().replace(/\s+/g, " ").slice(0, 80);
 				}
diff --git a/src/resources/extensions/browser-tools/tools/session.ts b/src/resources/extensions/browser-tools/tools/session.ts
index 43fb5b996..19ef2cb20 100644
--- a/src/resources/extensions/browser-tools/tools/session.ts
+++ b/src/resources/extensions/browser-tools/tools/session.ts
@@ -379,7 +379,7 @@ export function registerSessionTools(pi: ExtensionAPI, deps: ToolDeps): void {
 							console: consoleLogs.length,
 							network: networkLogs.length,
 							dialog: dialogLogs.length,
-							actions: timeline.count,
+							actions: timeline.retained,
 							pages: pages.length,
 						},
 						elapsedMs: Date.now() - startedAt,
diff --git a/tsconfig.extensions.json b/tsconfig.extensions.json
index b2f249376..07b44e356 100644
--- a/tsconfig.extensions.json
+++ b/tsconfig.extensions.json
@@ -3,8 +3,10 @@
   "compilerOptions": {
     "noEmit": true,
     "allowImportingTsExtensions": true,
+    "allowJs": true,
+    "checkJs": false,
     "rootDir": "."
   },
-  "include": ["src/resources/extensions/gsd"],
+  "include": ["src/resources/extensions"],
   "exclude": []
 }