diff --git a/src/browser/CdpCaptureEventHandlers.ts b/src/browser/CdpCaptureEventHandlers.ts
index b1b8b2ad67..493925d63f 100644
--- a/src/browser/CdpCaptureEventHandlers.ts
+++ b/src/browser/CdpCaptureEventHandlers.ts
@@ -25,7 +25,7 @@ import {
   initiatorLocation,
   integerOrNull,
   isJsonMediaType,
-  isMainFrameNavigation,
+  cdpFrameIdentity,
 } from "./CdpCaptureEventHelpers.js";
 import type { CdpCaptureEventsState } from "./CdpCaptureEventState.js";
 import {
@@ -607,25 +607,47 @@ export const handleWebSocketCreated = (
 
 export const handleFrameNavigated = (
   state: CdpCaptureEventsState,
-  params: UnknownRecord,
+  params: unknown,
 ): void => {
-  const frame = recordValue(params.frame);
   if (state.mainFrameId === undefined) return;
-  if (
-    !isMainFrameNavigation(
-      { method: "Page.frameNavigated", params },
-      state.mainFrameId,
+  const identity = cdpFrameIdentity(
+    { method: "Page.frameNavigated", params },
+    state.mainFrameId,
+  );
+  if (identity.identityIssue !== null || identity.parentIssue !== null) {
+    state.completeness.exclude("frames", "invalid_protocol_value");
+    if (
+      identity.selection === "unknown" &&
+      identity.parentRelation === "child" &&
+      identity.identityIssue !== null
     )
-  )
+      return;
+    state.unknownRootIdentity = true;
     return;
+  }
+  if (identity.selection === "other" && identity.parentRelation === "child")
+    return;
+  if (identity.selection === "unknown") {
+    state.completeness.exclude("frames", "invalid_protocol_value");
+    state.unknownRootIdentity = true;
+    return;
+  }
+  if (identity.selection === "other" && !identity.isRootReplacement) {
+    state.completeness.exclude("frames", "invalid_protocol_value");
+    state.unknownRootIdentity = true;
+    return;
+  }
+  const frame = recordValue(recordValue(params)?.frame);
   const rawUrl = cdpStringValue(frame?.url);
   const loaderId = cdpStringValue(frame?.loaderId);
   // `frameNavigated` re-fires for a document already reported (same URL and
   // loader). Treating that as a navigation aborted a valid capture with
   // `target_changed`, which is a false claim that the target moved.
-  const document = `${String(loaderId ?? "")}\0${String(rawUrl ?? "")}`;
+  const document = `${String(identity.frameId ?? "")}\0${String(loaderId ?? "")}\0${String(rawUrl ?? "")}`;
   if (state.committedDocument === document) return;
   state.committedDocument = document;
+  if (identity.isRootReplacement)
+    state.mainFrameId = identity.frameId ?? undefined;
   state.navigationDuringCapture = true;
   if (
     isHttpUrl(rawUrl) &&
diff --git a/src/browser/CdpCaptureEventHelpers.test.ts b/src/browser/CdpCaptureEventHelpers.test.ts
new file mode 100644
index 0000000000..ad34274e45
--- /dev/null
+++ b/src/browser/CdpCaptureEventHelpers.test.ts
@@ -0,0 +1,102 @@
+import { describe, expect, it } from "vitest";
+import { cdpFrameIdentity } from "./CdpCaptureEventHelpers.js";
+
+describe("CDP frame identity ownership", () => {
+  it.each([
+    ["missing", {}],
+    ["null", { frame: null }],
+    ["non-object", { frame: "frame" }],
+  ])("does not infer a root from a %s frame object", (_label, params) => {
+    expect(
+      cdpFrameIdentity(
+        { method: "Page.frameNavigated", params },
+        "selected-main",
+      ),
+    ).toMatchObject({
+      applicability: "applicable",
+      selection: "unknown",
+      isRootReplacement: false,
+      identityIssue: "missing_frame",
+      parentRelation: "unknown",
+      parentId: null,
+      parentIssue: null,
+    });
+  });
+
+  it("recognizes a valid frame with no parent property as a root", () => {
+    expect(
+      cdpFrameIdentity(
+        {
+          method: "Page.frameNavigated",
+          params: { frame: { id: "replacement-main" } },
+        },
+        "selected-main",
+      ),
+    ).toMatchObject({
+      applicability: "applicable",
+      selection: "other",
+      isRootReplacement: true,
+      identityIssue: null,
+      parentRelation: "root",
+      parentId: null,
+      parentIssue: null,
+    });
+  });
+
+  it("does not interpret a malformed parent value as root evidence", () => {
+    expect(
+      cdpFrameIdentity(
+        {
+          method: "Page.frameNavigated",
+          params: { frame: { id: "replacement-main", parentId: "" } },
+        },
+        "selected-main",
+      ),
+    ).toMatchObject({
+      selection: "other",
+      isRootReplacement: false,
+      identityIssue: null,
+      parentRelation: "unknown",
+      parentId: null,
+      parentIssue: "invalid_parent_id",
+    });
+  });
+
+  it("preserves a self-parented frame ID but leaves its parent relation unknown", () => {
+    expect(
+      cdpFrameIdentity(
+        {
+          method: "Page.frameNavigated",
+          params: {
+            frame: { id: "candidate-root", parentId: "candidate-root" },
+          },
+        },
+        "selected-main",
+      ),
+    ).toMatchObject({
+      selection: "other",
+      isRootReplacement: false,
+      frameId: "candidate-root",
+      identityIssue: null,
+      parentRelation: "unknown",
+      parentId: null,
+      parentIssue: "self_parent",
+    });
+  });
+
+  it("retains the selected-main parent issue for a self-parented selected frame", () => {
+    expect(
+      cdpFrameIdentity(
+        {
+          method: "Page.frameNavigated",
+          params: { frame: { id: "selected-main", parentId: "selected-main" } },
+        },
+        "selected-main",
+      ),
+    ).toMatchObject({
+      selection: "main",
+      parentRelation: "unknown",
+      parentIssue: "main_frame_has_parent",
+    });
+  });
+});
diff --git a/src/browser/CdpCaptureEventHelpers.ts b/src/browser/CdpCaptureEventHelpers.ts
index 6f718f0d6c..d3e60f2450 100644
--- a/src/browser/CdpCaptureEventHelpers.ts
+++ b/src/browser/CdpCaptureEventHelpers.ts
@@ -8,21 +8,132 @@ import {
 } from "./CdpCaptureValues.js";
 import type { CdpEvent } from "./CdpConnection.js";
 
-/** Identify a top-level navigation event from its untrusted CDP envelope. */
-export const isMainFrameNavigation = (
+export type FrameSelection = "main" | "other" | "unknown";
+export type FrameParentRelation =
+  | "root"
+  | "child"
+  | "unknown"
+  | "not_applicable";
+
+/** Normalized frame identity facts shared by all selected-frame consumers. */
+export interface CdpFrameIdentity {
+  readonly applicability: "applicable" | "not_applicable";
+  readonly selection: FrameSelection;
+  readonly isRootReplacement: boolean;
+  readonly frameId: string | null;
+  readonly identityIssue:
+    | "missing_params"
+    | "invalid_params"
+    | "missing_frame"
+    | "missing_id"
+    | "invalid_id"
+    | null;
+  readonly parentRelation: FrameParentRelation;
+  readonly parentId: string | null;
+  readonly parentIssue:
+    | "invalid_parent_id"
+    | "self_parent"
+    | "main_frame_has_parent"
+    | null;
+}
+
+const nonEmptyString = (value: unknown): string | undefined =>
+  typeof value === "string" && value.length > 0 ? value : undefined;
+
+const identityIssueFor = (
+  navigated: boolean,
+  frame: UnknownRecord | undefined,
+  rawId: unknown,
+): CdpFrameIdentity["identityIssue"] => {
+  if (navigated && frame === undefined) return "missing_frame";
+  if (rawId === undefined) return "missing_id";
+  return nonEmptyString(rawId) === undefined ? "invalid_id" : null;
+};
+
+const parentFacts = (
+  navigated: boolean,
+  frame: UnknownRecord | undefined,
+): Pick<CdpFrameIdentity, "parentRelation" | "parentId" | "parentIssue"> => {
+  if (!navigated)
+    return {
+      parentRelation: "not_applicable",
+      parentId: null,
+      parentIssue: null,
+    };
+  if (frame === undefined)
+    return { parentRelation: "unknown", parentId: null, parentIssue: null };
+  if (!("parentId" in frame))
+    return { parentRelation: "root", parentId: null, parentIssue: null };
+  const parentId = nonEmptyString(frame.parentId);
+  return parentId === undefined || parentId === nonEmptyString(frame.id)
+    ? {
+        parentRelation: "unknown",
+        parentId: null,
+        parentIssue:
+          parentId === undefined ? "invalid_parent_id" : "self_parent",
+      }
+    : { parentRelation: "child", parentId, parentIssue: null };
+};
+
+/** Parse required frame identity separately from the optional parent fact. */
+export const cdpFrameIdentity = (
   event: CdpEvent,
   mainFrameId: string,
-): boolean => {
+): CdpFrameIdentity => {
+  const navigated = event.method === "Page.frameNavigated";
+  const requested = event.method === "Page.frameRequestedNavigation";
+  const sameDocument = event.method === "Page.navigatedWithinDocument";
+  if (!navigated && !requested && !sameDocument)
+    return {
+      applicability: "not_applicable",
+      selection: "unknown",
+      isRootReplacement: false,
+      frameId: null,
+      identityIssue: null,
+      parentRelation: "not_applicable",
+      parentId: null,
+      parentIssue: null,
+    };
   const params = recordValue(event.params);
-  if (params === undefined) return false;
-  const frameId =
-    event.method === "Page.frameNavigated"
-      ? cdpStringValue(recordValue(params.frame)?.id)
-      : event.method === "Page.frameRequestedNavigation" ||
-          event.method === "Page.navigatedWithinDocument"
-        ? cdpStringValue(params.frameId)
-        : undefined;
-  return frameId === mainFrameId;
+  if (params === undefined)
+    return {
+      applicability: "applicable",
+      selection: "unknown",
+      isRootReplacement: false,
+      frameId: null,
+      identityIssue:
+        event.params === undefined ? "missing_params" : "invalid_params",
+      parentRelation: "not_applicable",
+      parentId: null,
+      parentIssue: null,
+    };
+  const frame = navigated ? recordValue(params.frame) : undefined;
+  const rawId = navigated ? frame?.id : params.frameId;
+  const frameId = nonEmptyString(rawId);
+  const selection =
+    frameId === undefined
+      ? "unknown"
+      : frameId === mainFrameId
+        ? "main"
+        : "other";
+  const identityIssue = identityIssueFor(navigated, frame, rawId);
+  const parent = parentFacts(navigated, frame);
+  return {
+    applicability: "applicable",
+    selection,
+    isRootReplacement:
+      selection === "other" && parent.parentRelation === "root",
+    frameId: frameId ?? null,
+    identityIssue,
+    parentRelation: parent.parentRelation,
+    parentId: parent.parentId,
+    parentIssue:
+      selection === "main" &&
+      (parent.parentRelation === "child" ||
+        parent.parentIssue === "self_parent")
+        ? "main_frame_has_parent"
+        : parent.parentIssue,
+  };
 };
 
 export const firstCallFrame = (
diff --git a/src/browser/CdpCaptureEventState.ts b/src/browser/CdpCaptureEventState.ts
index 9883a3a3c1..2e72bd37ce 100644
--- a/src/browser/CdpCaptureEventState.ts
+++ b/src/browser/CdpCaptureEventState.ts
@@ -26,6 +26,7 @@ export interface CdpCaptureEventsState {
   readonly completeness: CdpCaptureCompleteness;
   originViolation: boolean;
   navigationDuringCapture: boolean;
+  unknownRootIdentity: boolean;
   mainFrameId: string | undefined;
   /**
    * Identity of the document last observed in the main frame. `frameNavigated`
diff --git a/src/browser/CdpCaptureEvents.ts b/src/browser/CdpCaptureEvents.ts
index b3f1ce41b8..96b8bba7b7 100644
--- a/src/browser/CdpCaptureEvents.ts
+++ b/src/browser/CdpCaptureEvents.ts
@@ -45,6 +45,7 @@ export class CdpCaptureEvents implements CdpCaptureEventsState {
   ]);
   originViolation = false;
   navigationDuringCapture = false;
+  unknownRootIdentity = false;
   mainFrameId: string | undefined = undefined;
   committedDocument: string | undefined = undefined;
 
@@ -60,6 +61,7 @@ export class CdpCaptureEvents implements CdpCaptureEventsState {
     this.mainFrameId = frameId;
     this.originViolation = false;
     this.navigationDuringCapture = false;
+    this.unknownRootIdentity = false;
     this.committedDocument = undefined;
     this.scripts.clear();
     this.scriptMetadataBudgetOmissionsById.clear();
@@ -122,6 +124,10 @@ export class CdpCaptureEvents implements CdpCaptureEventsState {
   }
 
   ingest(event: CdpEvent): void {
+    if (event.method === "Page.frameNavigated") {
+      handlers.handleFrameNavigated(this, event.params);
+      return;
+    }
     const params = recordValue(event.params);
     if (params === undefined) return;
     switch (event.method) {
@@ -158,9 +164,6 @@ export class CdpCaptureEvents implements CdpCaptureEventsState {
       case "Runtime.consoleAPICalled":
         handlers.handleConsoleAPICalled(this, params);
         break;
-      case "Page.frameNavigated":
-        handlers.handleFrameNavigated(this, params);
-        break;
       case "Network.webSocketCreated":
         handlers.handleWebSocketCreated(this, params);
         break;
diff --git a/src/browser/CdpObservationSession.ts b/src/browser/CdpObservationSession.ts
index 3500a3444d..51adfe5612 100644
--- a/src/browser/CdpObservationSession.ts
+++ b/src/browser/CdpObservationSession.ts
@@ -25,7 +25,7 @@ import {
   type UnknownRecord,
 } from "./CdpCaptureValues.js";
 import { mainFrameUrl } from "./CdpCaptureDocuments.js";
-import { isMainFrameNavigation } from "./CdpCaptureEventHelpers.js";
+import { cdpFrameIdentity } from "./CdpCaptureEventHelpers.js";
 import { cdpTargetEventMatches } from "./CdpTargetEvents.js";
 
 interface ObservationContext {
@@ -184,7 +184,7 @@ class TimelineCapture {
 
   constructor(
     private readonly allowedOrigins: ReadonlySet<string>,
-    private readonly mainFrameId: string,
+    private mainFrameId: string,
     initialUrl: string,
   ) {
     this.finalUrl =
@@ -204,7 +204,15 @@ class TimelineCapture {
   #ingestArmed(event: CdpTransportEvent, bufferedBeforeArm = false): void {
     if (this.#endReason !== undefined || this.#scopeExcluded) return;
     const params = recordValue(event.params);
-    if (params === undefined) return;
+    if (params === undefined) {
+      if (
+        event.method === "Page.frameRequestedNavigation" ||
+        event.method === "Page.frameNavigated" ||
+        event.method === "Page.navigatedWithinDocument"
+      )
+        this.completeness.exclude("timeline", "invalid_protocol_value");
+      return;
+    }
     const contextOnly =
       bufferedBeforeArm && hasNoComparableProducerTimestamp(event.method);
     if (contextOnly) this.#hasUnknownEventTime = true;
@@ -326,7 +334,10 @@ class TimelineCapture {
     params: UnknownRecord,
     contextOnly: boolean,
   ): void {
-    if (!isMainFrameNavigation(event, this.mainFrameId)) return;
+    const identity = cdpFrameIdentity(event, this.mainFrameId);
+    if (identity.identityIssue !== null || identity.parentIssue !== null)
+      this.completeness.exclude("timeline", "invalid_protocol_value");
+    if (identity.selection !== "main") return;
     const reason = safeDetail(params.reason);
     const rawUrl = cdpStringValue(params.url);
     const destination =
@@ -349,12 +360,23 @@ class TimelineCapture {
     contextOnly: boolean,
   ): void {
     const frame = recordValue(params.frame);
-    if (!isMainFrameNavigation(event, this.mainFrameId)) return;
+    const identity = cdpFrameIdentity(event, this.mainFrameId);
+    if (identity.identityIssue !== null || identity.parentIssue !== null)
+      this.completeness.exclude("timeline", "invalid_protocol_value");
+    if (identity.selection !== "main" && !identity.isRootReplacement) return;
     const rawUrl = cdpStringValue(frame?.url);
     const destination = scopedUrl(rawUrl, this.allowedOrigins);
+    if (identity.isRootReplacement) {
+      this.mainFrameId = identity.frameId ?? this.mainFrameId;
+      this.#pendingReload = false;
+      if (contextOnly) {
+        this.#stopIfOutOfScope(destination);
+        return;
+      }
+    }
     this.#add({
       type:
-        !contextOnly && this.#pendingReload
+        !identity.isRootReplacement && !contextOnly && this.#pendingReload
           ? "same_origin_reload"
           : "navigation_committed",
       params: { ...params, frameId: frame?.id, loaderId: frame?.loaderId },
@@ -374,7 +396,10 @@ class TimelineCapture {
     params: UnknownRecord,
     contextOnly: boolean,
   ): void {
-    if (!isMainFrameNavigation(event, this.mainFrameId)) return;
+    const identity = cdpFrameIdentity(event, this.mainFrameId);
+    if (identity.identityIssue !== null || identity.parentIssue !== null)
+      this.completeness.exclude("timeline", "invalid_protocol_value");
+    if (identity.selection !== "main") return;
     const rawUrl = cdpStringValue(params.url);
     const destination = scopedUrl(rawUrl, this.allowedOrigins);
     this.#add({
diff --git a/src/browser/CdpPageCapture.ts b/src/browser/CdpPageCapture.ts
index a986366cb0..ba3eb00ef5 100644
--- a/src/browser/CdpPageCapture.ts
+++ b/src/browser/CdpPageCapture.ts
@@ -170,6 +170,8 @@ const captureAuthorizedPage = async (
     operation: context.operation,
   });
   const completedUrl = mainFrameUrl(completedFrameResult) ?? "";
+  if (state.events.unknownRootIdentity)
+    throw new BrowserObservationError("inspect_web_page", "protocol_error");
   if (state.events.originViolation)
     throw new BrowserObservationError("inspect_web_page", "target_not_allowed");
   if (state.events.navigationDuringCapture || completedUrl !== attachedUrl)
@@ -264,6 +266,8 @@ const authorizeObservationWindow = async (
     context.operation,
     signal,
   );
+  if (events.unknownRootIdentity)
+    throw new BrowserObservationError("inspect_web_page", "protocol_error");
   if (events.originViolation)
     throw new BrowserObservationError("inspect_web_page", "target_not_allowed");
 };
diff --git a/src/browser/CdpScreenshot.ts b/src/browser/CdpScreenshot.ts
index e4093479b1..59a9e7768d 100644
--- a/src/browser/CdpScreenshot.ts
+++ b/src/browser/CdpScreenshot.ts
@@ -5,7 +5,7 @@ import type {
 } from "../domain/webScreenshot.js";
 import { createWebScreenshotArtifact } from "../domain/webScreenshot.js";
 import { BrowserObservationError } from "../domain/browserObservationError.js";
-import { isMainFrameNavigation } from "./CdpCaptureEventHelpers.js";
+import { cdpFrameIdentity } from "./CdpCaptureEventHelpers.js";
 import type { CdpEndpointDiscovery, CdpEndpointTarget } from "./CdpEndpoint.js";
 import type { CdpConnection, CdpEvent } from "./CdpConnection.js";
 import {
@@ -63,12 +63,15 @@ export const captureCdpScreenshot = async (
   const navigation = {
     changed: false,
     leftScope: false,
+    invalidIdentity: false,
+    frameId: mainFrame.frame_id,
     url: beforeUrl,
     loaderId: cdpStringValue(initialFrame?.loaderId),
+    completeness: new CdpCaptureCompleteness(),
   };
   const removeListener = context.connection.onEvent((event) => {
     if (event.sessionId !== context.sessionId) return;
-    observeScreenshotNavigation(event, mainFrame.frame_id, origins, navigation);
+    observeScreenshotNavigation(event, origins, navigation);
   });
   let result: ReturnType<typeof recordValue>;
   let after: unknown;
@@ -90,13 +93,10 @@ export const captureCdpScreenshot = async (
   } finally {
     removeListener();
   }
-  const encoded = cdpStringValue(result?.data);
-  const bytes =
-    encoded === undefined ? undefined : decodeCanonicalBase64(encoded);
-  if (bytes === undefined)
+  const screenshot = decodeScreenshot(result);
+  if (screenshot === undefined)
     throw new BrowserObservationError("inspect_web_page", "protocol_error");
-  const dimensions = readPngHeaderDimensions(bytes);
-  if (dimensions === undefined)
+  if (navigation.invalidIdentity)
     throw new BrowserObservationError("inspect_web_page", "protocol_error");
   if (navigation.leftScope)
     throw new BrowserObservationError("inspect_web_page", "target_not_allowed");
@@ -122,9 +122,9 @@ export const captureCdpScreenshot = async (
       origin: authorized.origin ?? "",
     },
     captured_at: new Date().toISOString(),
-    viewport: dimensions,
-    artifact: createWebScreenshotArtifact(bytes),
-    completeness: new CdpCaptureCompleteness().snapshot(),
+    viewport: screenshot.dimensions,
+    artifact: createWebScreenshotArtifact(screenshot.bytes),
+    completeness: navigation.completeness.snapshot(),
     limitations: [
       "The screenshot contains the visible viewport, including on-screen content.",
       "REA does not scroll, evaluate JavaScript, or capture beyond the current viewport.",
@@ -132,24 +132,62 @@ export const captureCdpScreenshot = async (
   };
 };
 
+const decodeScreenshot = (
+  result: ReturnType<typeof recordValue>,
+):
+  | {
+      readonly bytes: Buffer;
+      readonly dimensions: { width: number; height: number };
+    }
+  | undefined => {
+  const encoded = cdpStringValue(result?.data);
+  const bytes =
+    encoded === undefined ? undefined : decodeCanonicalBase64(encoded);
+  if (bytes === undefined) return undefined;
+  const dimensions = readPngHeaderDimensions(bytes);
+  return dimensions === undefined ? undefined : { bytes, dimensions };
+};
+
 const observeScreenshotNavigation = (
   event: CdpEvent,
-  mainFrameId: string,
   origins: ReadonlySet<string>,
   state: {
     changed: boolean;
     leftScope: boolean;
-    readonly url: string | undefined;
-    readonly loaderId: string | undefined;
+    invalidIdentity: boolean;
+    frameId: string;
+    url: string | undefined;
+    loaderId: string | undefined;
+    readonly completeness: CdpCaptureCompleteness;
   },
 ): void => {
-  if (!isMainFrameNavigation(event, mainFrameId)) return;
+  const identity = cdpFrameIdentity(event, state.frameId);
+  if (identity.applicability === "not_applicable") return;
+  if (identity.identityIssue !== null || identity.parentIssue !== null) {
+    state.completeness.exclude("screenshot", "invalid_protocol_value");
+    if (
+      identity.selection === "unknown" &&
+      identity.parentRelation === "child" &&
+      identity.identityIssue !== null
+    )
+      return;
+    state.invalidIdentity = true;
+    return;
+  }
+  if (identity.selection === "unknown") {
+    if (identity.parentRelation === "child") return;
+    state.completeness.exclude("screenshot", "invalid_protocol_value");
+    state.invalidIdentity = true;
+    return;
+  }
+  if (identity.selection === "other" && !identity.isRootReplacement) return;
   const params = recordValue(event.params);
   if (params === undefined) return;
   const frame =
     event.method === "Page.frameNavigated" ? recordValue(params.frame) : params;
   if (
     event.method === "Page.frameNavigated" &&
+    !identity.isRootReplacement &&
     state.loaderId !== undefined &&
     state.loaderId !== "" &&
     cdpStringValue(frame?.loaderId) === state.loaderId &&
@@ -160,4 +198,12 @@ const observeScreenshotNavigation = (
   state.changed = true;
   if (allowedSanitizedUrl(frame?.url, origins) === undefined)
     state.leftScope = true;
+  if (event.method === "Page.frameNavigated") {
+    if (identity.isRootReplacement)
+      state.frameId = identity.frameId ?? state.frameId;
+    state.url = cdpStringValue(frame?.url) ?? state.url;
+    state.loaderId = cdpStringValue(frame?.loaderId);
+  } else if (event.method === "Page.navigatedWithinDocument") {
+    state.url = cdpStringValue(frame?.url) ?? state.url;
+  }
 };
diff --git a/src/browser/PlaywrightScenarioSession.test.ts b/src/browser/PlaywrightScenarioSession.test.ts
index 7615c27dd6..62c3f3e179 100644
--- a/src/browser/PlaywrightScenarioSession.test.ts
+++ b/src/browser/PlaywrightScenarioSession.test.ts
@@ -216,7 +216,7 @@ const policyResponse = (method: string): unknown =>
       ? { result: { value: null } }
       : {};
 
-const prepareLateStopRetry = async () => {
+const prepareHeldPolicyRestoration = async () => {
   const { cleanup, page, storage } = fixture();
   const cancellation = new AbortController();
   storage?.close.mockResolvedValue({ loadingStopped: false });
@@ -241,6 +241,28 @@ const prepareLateStopRetry = async () => {
     return policyResponse(method);
   });
 
+  return {
+    cleanup,
+    page,
+    storage,
+    cancellation,
+    session,
+    finishRestoration,
+    started,
+    transportClosed,
+  };
+};
+
+const prepareLateStopRetry = async () => {
+  const {
+    cleanup,
+    page,
+    cancellation,
+    session,
+    finishRestoration,
+    started,
+    transportClosed,
+  } = await prepareHeldPolicyRestoration();
   vi.useFakeTimers();
   try {
     const firstClose = session.close(false).then(
@@ -417,29 +439,16 @@ it("transfers a late stop request after storage has already released", async ()
 it.each(["pending", "settled"] as const)(
   "honors a late stop after policy restoration is %s",
   async (phase) => {
-    const { cleanup, page, storage } = fixture();
-    const cancellation = new AbortController();
-    storage?.close.mockResolvedValue({ loadingStopped: false });
-    const attached = connectedScenario("block");
-    const session = await PlaywrightScenarioSession.open(
-      attached,
-      {},
-      { signal: cancellation.signal },
-    );
-    const { promise: held, resolve: finishRestoration } = deferred();
-    const { promise: started, resolve: restorationStarted } = deferred();
-    const { promise: transportClosed, resolve: signalTransportClosed } =
-      deferred();
-    mocks.policyTransportClose.mockImplementation(async () => {
-      signalTransportClosed();
-    });
-    mocks.policySend.mockImplementation(async (method: string) => {
-      if (method === "Runtime.evaluate") {
-        restorationStarted();
-        await held;
-      }
-      return policyResponse(method);
-    });
+    const {
+      cleanup,
+      page,
+      storage,
+      cancellation,
+      session,
+      finishRestoration,
+      started,
+      transportClosed,
+    } = await prepareHeldPolicyRestoration();
 
     vi.useFakeTimers();
     try {
diff --git a/src/browser/execution/CdpExecutionWindow.test.ts b/src/browser/execution/CdpExecutionWindow.test.ts
index e906be92ab..4a6268148d 100644
--- a/src/browser/execution/CdpExecutionWindow.test.ts
+++ b/src/browser/execution/CdpExecutionWindow.test.ts
@@ -63,3 +63,57 @@ it("settles caller cancellation and removes its local timer", async () => {
     vi.useRealTimers();
   }
 });
+
+it("keeps a root document transition distinct from a child frame event", async () => {
+  const root = new CdpExecutionWindow({
+    target: { frame_id: "selected-main" },
+    operation: "observe_web_execution",
+    options: {},
+  });
+  const rootResult = root.start(30_000);
+  root.ingest({
+    method: "Page.frameNavigated",
+    params: { frame: { id: "new-main", url: "https://example.test/" } },
+  });
+  await expect(rootResult).resolves.toBe("document_changed");
+
+  const child = new CdpExecutionWindow({
+    target: { frame_id: "selected-main" },
+    operation: "observe_web_execution",
+    options: {},
+  });
+  const childResult = child.start(30_000);
+  child.ingest({
+    method: "Page.frameNavigated",
+    params: {
+      frame: {
+        id: "child-frame",
+        parentId: "selected-main",
+        url: "https://example.test/child",
+      },
+    },
+  });
+  expect(child.active).toBe(true);
+  child.end("window_elapsed");
+  await expect(childResult).resolves.toBe("window_elapsed");
+});
+
+it("surfaces malformed frame identity as a protocol failure without guessing a transition", async () => {
+  const window = new CdpExecutionWindow({
+    target: { frame_id: "selected-main" },
+    operation: "observe_web_execution",
+    options: {},
+  });
+  const result = window.start(30_000);
+  window.ingest({
+    method: "Page.frameNavigated",
+    params: {
+      frame: { id: "child-frame", parentId: 7, url: "https://example.test/" },
+    },
+  });
+  await expect(result).rejects.toMatchObject({
+    _tag: "BrowserObservationError",
+    reason: "protocol_error",
+  });
+  expect(window.reason).toBeUndefined();
+});
diff --git a/src/browser/execution/CdpExecutionWindow.ts b/src/browser/execution/CdpExecutionWindow.ts
index f8fa4b7eff..fddfe10329 100644
--- a/src/browser/execution/CdpExecutionWindow.ts
+++ b/src/browser/execution/CdpExecutionWindow.ts
@@ -3,6 +3,8 @@ import type { AnalysisError } from "../../domain/analysisErrorBase.js";
 import type { WebExecution } from "../../domain/webExecution.js";
 import type { CdpEvent } from "../CdpConnection.js";
 import type { CdpRuntimeSession } from "./CdpRuntimeSession.js";
+import { BrowserObservationError } from "../../domain/browserObservationError.js";
+import { cdpFrameIdentity } from "../CdpCaptureEventHelpers.js";
 
 type EndReason = WebExecution["window"]["end_reason"];
 /** One finite document-scoped window, with an actual armed point and event-driven termination. */
@@ -33,15 +35,30 @@ export class CdpExecutionWindow {
     if (!this.active) return;
     if (event.method === "Runtime.executionContextsCleared")
       this.end("document_changed");
-    if (
-      event.method === "Page.frameNavigated" &&
-      typeof event.params === "object" &&
-      event.params !== null &&
-      "frame" in event.params
-    ) {
-      const frame = event.params.frame;
-      if (typeof frame === "object" && frame !== null && !("parentId" in frame))
+    if (event.method === "Page.frameNavigated") {
+      const identity = cdpFrameIdentity(event, this.session.target.frame_id);
+      if (identity.identityIssue !== null || identity.parentIssue !== null) {
+        this.fail(
+          new BrowserObservationError(this.session.operation, "protocol_error"),
+        );
+        return;
+      }
+      if (identity.selection === "main") {
         this.end("document_changed");
+        return;
+      }
+      if (identity.parentRelation === "child") return;
+      if (
+        identity.selection === "other" &&
+        identity.parentRelation === "root"
+      ) {
+        this.end("document_changed");
+        return;
+      }
+      this.fail(
+        new BrowserObservationError(this.session.operation, "protocol_error"),
+      );
+      return;
     }
     if (
       event.method === "Page.frameDetached" &&
diff --git a/tests/boundary/browser/cdpBrowserProvider.discovery_authorization.test.ts b/tests/boundary/browser/cdpBrowserProvider.discovery_authorization.test.ts
index 053cd673ea..3f17c1d813 100644
--- a/tests/boundary/browser/cdpBrowserProvider.discovery_authorization.test.ts
+++ b/tests/boundary/browser/cdpBrowserProvider.discovery_authorization.test.ts
@@ -5,8 +5,30 @@ import {
   inspectWebPageInputSchema,
   listBrowserTargetsInputSchema,
 } from "../../../src/domain/browserObservation.js";
-import { startFakeCdpBrowser } from "../../fixtures/fakeCdpBrowser.js";
-import { trackBrowser } from "./cdpBrowserProvider.support.js";
+import {
+  startFakeCdpBrowser,
+  type FakeOptions,
+} from "../../fixtures/fakeCdpBrowser.js";
+import { frameNavigated, trackBrowser } from "./cdpBrowserProvider.support.js";
+
+const inspectWithNetworkEvents = async (
+  commandEvents: NonNullable<FakeOptions["commandEvents"]>,
+) => {
+  const browser = await startFakeCdpBrowser({
+    eventsBeforeReply: ["Network.enable"],
+    commandEvents,
+  });
+  trackBrowser(browser);
+  const result = await new CdpBrowserProvider().inspectPage(
+    inspectWebPageInputSchema.parse({
+      cdp_endpoint: browser.endpoint,
+      allowed_origins: [browser.allowedOrigin],
+      target_id: "allowed-page",
+      observation_ms: 1,
+    }),
+  );
+  return { browser, result };
+};
 
 describe("CdpBrowserProvider: discovery authorization 1", () => {
   it("discovers pages and captures a selected target without an origin list", async () => {
@@ -158,6 +180,107 @@ describe("CdpBrowserProvider: discovery authorization 1", () => {
   });
 });
 
+it("fails with a protocol error after an unidentifiable frame event and detaches", async () => {
+  const { browser, result } = await inspectWithNetworkEvents((command) =>
+    command.method === "Network.enable"
+      ? [
+          {
+            method: "Page.frameNavigated",
+            ...(command.sessionId === undefined
+              ? {}
+              : { sessionId: command.sessionId }),
+            params: undefined,
+          },
+        ]
+      : undefined,
+  );
+  expect(result).toMatchObject({
+    ok: false,
+    error: { _tag: "BrowserObservationError", reason: "protocol_error" },
+  });
+  await expect.poll(() => browser.activeConnectionCount()).toBe(0);
+});
+
+it("retains malformed child-frame identity as frame coverage while completing capture", async () => {
+  const { browser, result } = await inspectWithNetworkEvents((command) =>
+    command.method === "Network.enable"
+      ? [
+          frameNavigated(command, {
+            id: 3,
+            parentId: "frame-main",
+            url: "https://outside.invalid/child",
+          }),
+        ]
+      : undefined,
+  );
+  if (!result.ok) throw result.error;
+  expect(result.value.completeness.unavailable_sections).toContain("frames");
+  expect(result.value.target.url).toBe(
+    `${browser.allowedOrigin}/app?token=frame-secret`,
+  );
+});
+
+it("fails capture on a contradictory self-parented frame instead of accepting it as a child", async () => {
+  const { browser, result } = await inspectWithNetworkEvents(
+    (command, origin) =>
+      command.method === "Network.enable"
+        ? [
+            frameNavigated(command, {
+              id: "candidate-root",
+              parentId: "candidate-root",
+              url: `${origin}/app?token=frame-secret`,
+            }),
+          ]
+        : undefined,
+  );
+  expect(result).toMatchObject({
+    ok: false,
+    error: { reason: "protocol_error" },
+  });
+  await expect.poll(() => browser.activeConnectionCount()).toBe(0);
+});
+
+it("tracks a known root replacement through later frame-scope events", async () => {
+  const { result } = await inspectWithNetworkEvents((command, origin) =>
+    command.method === "Network.enable"
+      ? [
+          frameNavigated(command, {
+            id: "replacement-main",
+            loaderId: "replacement-loader",
+            url: `${origin}/app?token=frame-secret`,
+          }),
+          frameNavigated(command, {
+            id: "replacement-main",
+            loaderId: "outside-loader",
+            url: "https://outside.invalid/path",
+          }),
+        ]
+      : undefined,
+  );
+  expect(result).toMatchObject({
+    ok: false,
+    error: { _tag: "BrowserObservationError", reason: "target_not_allowed" },
+  });
+});
+
+it("treats a same-URL and same-loader root frame ID change as a capture change", async () => {
+  const { result } = await inspectWithNetworkEvents((command, origin) =>
+    command.method === "Network.enable"
+      ? [
+          frameNavigated(command, {
+            id: "replacement-main",
+            loaderId: "loader-main",
+            url: `${origin}/app?token=frame-secret`,
+          }),
+        ]
+      : undefined,
+  );
+  expect(result).toMatchObject({
+    ok: false,
+    error: { _tag: "BrowserObservationError", reason: "target_changed" },
+  });
+});
+
 describe("CdpBrowserProvider: discovery authorization 2", () => {
   it("rejects empty browser attachment session identifiers", async () => {
     const browser = await startFakeCdpBrowser({ invalidAttachedSession: true });
diff --git a/tests/boundary/browser/cdpBrowserProvider.support.ts b/tests/boundary/browser/cdpBrowserProvider.support.ts
index ebb688df55..d1d2357191 100644
--- a/tests/boundary/browser/cdpBrowserProvider.support.ts
+++ b/tests/boundary/browser/cdpBrowserProvider.support.ts
@@ -1,6 +1,9 @@
 import { afterEach } from "vitest";
 
-import type { FakeCdpBrowser } from "../../fixtures/fakeCdpBrowser.js";
+import type {
+  FakeCdpBrowser,
+  FakeOptions,
+} from "../../fixtures/fakeCdpBrowser.js";
 
 const browsers: FakeCdpBrowser[] = [];
 
@@ -12,6 +15,15 @@ export const trackBrowser = (browser: FakeCdpBrowser): void => {
   browsers.push(browser);
 };
 
+export const frameNavigated = (
+  command: Parameters<NonNullable<FakeOptions["commandEvents"]>>[0],
+  frame: unknown,
+) => ({
+  method: "Page.frameNavigated",
+  ...(command.sessionId === undefined ? {} : { sessionId: command.sessionId }),
+  params: { frame },
+});
+
 export const waitFor = async (ready: () => boolean, description: string) => {
   const deadline = Date.now() + 5_000;
   while (!ready()) {
diff --git a/tests/boundary/browser/cdpObservationWindowScope.test.ts b/tests/boundary/browser/cdpObservationWindowScope.test.ts
index c3b8e5f10c..7e76b967c8 100644
--- a/tests/boundary/browser/cdpObservationWindowScope.test.ts
+++ b/tests/boundary/browser/cdpObservationWindowScope.test.ts
@@ -6,12 +6,12 @@ import {
   startFakeCdpBrowser,
   type FakeOptions,
 } from "../../fixtures/fakeCdpBrowser.js";
-import { trackBrowser } from "./cdpBrowserProvider.support.js";
+import { frameNavigated, trackBrowser } from "./cdpBrowserProvider.support.js";
 
 const observeDuringClockSampling = async (
   commandEvents: NonNullable<FakeOptions["commandEvents"]>,
 ) => {
-  const browser = await startFakeCdpBrowser({
+  return observeWithFake({
     eventsBeforeReply: ["Performance.getMetrics"],
     commandResult: (command) =>
       command.method === "Performance.getMetrics"
@@ -19,6 +19,10 @@ const observeDuringClockSampling = async (
         : undefined,
     commandEvents,
   });
+};
+
+const observeWithFake = async (options: FakeOptions) => {
+  const browser = await startFakeCdpBrowser(options);
   trackBrowser(browser);
   const result = await new CdpBrowserProvider().observeSession(
     observeWebSessionInputSchema.parse({
@@ -32,34 +36,47 @@ const observeDuringClockSampling = async (
   return { browser, observation: result.value };
 };
 
+const laterInWindowRequest = (
+  command: Parameters<NonNullable<FakeOptions["commandEvents"]>>[0],
+  origin: string,
+) => ({
+  method: "Network.requestWillBeSent",
+  sessionId: command.sessionId,
+  params: {
+    requestId: "later-in-window-request",
+    timestamp: 21,
+    type: "Document",
+    frameId: "frame-main",
+    redirectResponse: { url: `${origin}/prior` },
+    request: { url: `${origin}/later` },
+  },
+});
+
+const expectNoCommittedFrameWithInvalidTimeline = (
+  observation: Awaited<ReturnType<typeof observeWithFake>>["observation"],
+) => {
+  expect(observation.timeline).not.toContainEqual(
+    expect.objectContaining({ type: "navigation_committed" }),
+  );
+  expect(observation.completeness.excluded).toContainEqual(
+    expect.objectContaining({
+      section: "timeline",
+      reason: "invalid_protocol_value",
+    }),
+  );
+};
+
 it("fails closed on an untimestamped cross-origin main-frame commit during clock sampling", async () => {
   const { observation } = await observeDuringClockSampling(
     (command, origin) => {
       if (command.method !== "Performance.getMetrics") return undefined;
       return [
-        {
-          method: "Page.frameNavigated",
-          sessionId: command.sessionId,
-          params: {
-            frame: {
-              id: "frame-main",
-              loaderId: "outside-loader",
-              url: "https://outside.invalid/path",
-            },
-          },
-        },
-        {
-          method: "Network.requestWillBeSent",
-          sessionId: command.sessionId,
-          params: {
-            requestId: "later-in-window-request",
-            timestamp: 21,
-            type: "Document",
-            frameId: "frame-main",
-            redirectResponse: { url: `${origin}/prior` },
-            request: { url: `${origin}/later` },
-          },
-        },
+        frameNavigated(command, {
+          id: "frame-main",
+          loaderId: "outside-loader",
+          url: "https://outside.invalid/path",
+        }),
+        laterInWindowRequest(command, origin),
       ];
     },
   );
@@ -80,6 +97,93 @@ it("fails closed on an untimestamped cross-origin main-frame commit during clock
   );
 });
 
+it("keeps an unidentifiable frame commit unknown without inventing a main-frame event", async () => {
+  const { observation } = await observeDuringClockSampling((command) =>
+    command.method === "Performance.getMetrics"
+      ? [frameNavigated(command, { url: "https://outside.invalid/unknown" })]
+      : undefined,
+  );
+
+  expect(observation.window.end_reason).toBe("window_elapsed");
+  expectNoCommittedFrameWithInvalidTimeline(observation);
+});
+
+it("marks a frame event with absent parameters as unknown timeline evidence", async () => {
+  const { observation } = await observeDuringClockSampling((command) =>
+    command.method === "Performance.getMetrics"
+      ? [
+          {
+            method: "Page.frameNavigated",
+            sessionId: command.sessionId,
+            params: undefined,
+          },
+        ]
+      : undefined,
+  );
+
+  expectNoCommittedFrameWithInvalidTimeline(observation);
+  expect(observation.completeness.unavailable_sections).toContain("timeline");
+});
+
+it("records a malformed child identity without treating it as the main frame", async () => {
+  const { observation } = await observeDuringClockSampling((command) =>
+    command.method === "Performance.getMetrics"
+      ? [
+          frameNavigated(command, {
+            id: 3,
+            parentId: "frame-main",
+            url: "https://outside.invalid/child",
+          }),
+        ]
+      : undefined,
+  );
+
+  expect(observation.window.end_reason).toBe("window_elapsed");
+  expectNoCommittedFrameWithInvalidTimeline(observation);
+});
+
+it("updates the main frame from a pre-arm root replacement without inventing a navigation row", async () => {
+  const { observation } = await observeDuringClockSampling((command, origin) =>
+    command.method === "Performance.getMetrics"
+      ? [
+          frameNavigated(command, {
+            id: "replacement-main",
+            loaderId: "replacement-loader",
+            url: `${origin}/app?token=frame-secret`,
+          }),
+          {
+            method: "Page.lifecycleEvent",
+            sessionId: command.sessionId,
+            params: {
+              frameId: "replacement-main",
+              loaderId: "replacement-loader",
+              name: "DOMContentLoaded",
+              timestamp: 21,
+            },
+          },
+        ]
+      : undefined,
+  );
+
+  expect(observation.timeline).not.toContainEqual(
+    expect.objectContaining({ type: "navigation_committed" }),
+  );
+  expect(observation.timeline).toContainEqual(
+    expect.objectContaining({
+      type: "lifecycle",
+      frame_id: "replacement-main",
+      loader_id: "replacement-loader",
+      timestamp: 21,
+      detail: "domcontentloaded",
+    }),
+  );
+  expect(observation.limitations).toEqual(
+    expect.arrayContaining([
+      expect.stringContaining("Buffered events with unknown timing"),
+    ]),
+  );
+});
+
 it("fails closed on an untimestamped out-of-scope main-frame request during clock sampling", async () => {
   const { observation } = await observeDuringClockSampling(
     (command, origin) => {
@@ -94,18 +198,7 @@ it("fails closed on an untimestamped out-of-scope main-frame request during cloc
             reason: "scriptInitiated",
           },
         },
-        {
-          method: "Network.requestWillBeSent",
-          sessionId: command.sessionId,
-          params: {
-            requestId: "later-in-window-request",
-            timestamp: 21,
-            type: "Document",
-            frameId: "frame-main",
-            redirectResponse: { url: `${origin}/prior` },
-            request: { url: `${origin}/later` },
-          },
-        },
+        laterInWindowRequest(command, origin),
       ];
     },
   );
@@ -191,7 +284,7 @@ it.each([
 ] as const)(
   "ends on buffered untimestamped %s before collecting later events",
   async (_method, terminalEvent) => {
-    const browser = await startFakeCdpBrowser({
+    const { observation } = await observeWithFake({
       eventsBeforeReply: ["Performance.getMetrics"],
       commandResult: (command) =>
         command.method === "Performance.getMetrics"
@@ -216,35 +309,25 @@ it.each([
         ];
       },
     });
-    trackBrowser(browser);
-    const result = await new CdpBrowserProvider().observeSession(
-      observeWebSessionInputSchema.parse({
-        cdp_endpoint: browser.endpoint,
-        allowed_origins: [browser.allowedOrigin],
-        target_id: "allowed-page",
-        observation_ms: 20,
-      }),
-    );
 
-    if (!result.ok) throw result.error;
-    expect(result.value.window.end_reason).toBe("target_terminated");
-    expect(result.value.timeline).toContainEqual(
+    expect(observation.window.end_reason).toBe("target_terminated");
+    expect(observation.timeline).toContainEqual(
       expect.objectContaining({
         type: "target_terminated",
         timestamp: null,
       }),
     );
-    expect(result.value.timeline).not.toContainEqual(
+    expect(observation.timeline).not.toContainEqual(
       expect.objectContaining({ request_id: "after-terminal-event" }),
     );
-    expect(result.value.limitations).toContain(
+    expect(observation.limitations).toContain(
       "Timeline rows without a comparable producer timestamp have a null timestamp. Buffered events with unknown timing are retained as context only; they do not establish activity inside the selected observation window.",
     );
   },
 );
 
 it("represents absent timeline timestamps as unknown and preserves an observed zero", async () => {
-  const browser = await startFakeCdpBrowser({
+  const { browser, observation } = await observeWithFake({
     eventsBeforeReply: ["Page.setLifecycleEventsEnabled"],
     commandEvents: (command, origin) => {
       if (command.method !== "Page.setLifecycleEventsEnabled") return undefined;
@@ -271,18 +354,8 @@ it("represents absent timeline timestamps as unknown and preserves an observed z
       ];
     },
   });
-  trackBrowser(browser);
-  const result = await new CdpBrowserProvider().observeSession(
-    observeWebSessionInputSchema.parse({
-      cdp_endpoint: browser.endpoint,
-      allowed_origins: [browser.allowedOrigin],
-      target_id: "allowed-page",
-      observation_ms: 20,
-    }),
-  );
 
-  if (!result.ok) throw result.error;
-  expect(result.value.timeline).toEqual(
+  expect(observation.timeline).toEqual(
     expect.arrayContaining([
       expect.objectContaining({
         type: "same_document_navigation",
diff --git a/tests/boundary/browser/cdpScreenshotDocumentIdentity.test.ts b/tests/boundary/browser/cdpScreenshotDocumentIdentity.test.ts
index b768485989..6657769292 100644
--- a/tests/boundary/browser/cdpScreenshotDocumentIdentity.test.ts
+++ b/tests/boundary/browser/cdpScreenshotDocumentIdentity.test.ts
@@ -1,26 +1,33 @@
 import { describe, expect, it, onTestFinished } from "vitest";
 import { CdpBrowserProvider } from "../../../src/browser/CdpBrowserProvider.js";
 import { captureWebScreenshotInputSchema } from "../../../src/domain/webScreenshot.js";
-import { startFakeCdpBrowser } from "../../fixtures/fakeCdpBrowser.js";
+import {
+  startFakeCdpBrowser,
+  type FakeOptions,
+} from "../../fixtures/fakeCdpBrowser.js";
+import { frameNavigated } from "./cdpBrowserProvider.support.js";
+
+const captureWithFake = async (options: FakeOptions) => {
+  const browser = await startFakeCdpBrowser(options);
+  onTestFinished(() => browser.close());
+  const result = await new CdpBrowserProvider().captureScreenshot(
+    captureWebScreenshotInputSchema.parse({
+      cdp_endpoint: browser.endpoint,
+      allowed_origins: [browser.allowedOrigin],
+      target_id: "allowed-page",
+    }),
+  );
+  return { browser, result };
+};
 describe("screenshot committed document identity", () => {
   it.each([
     ["missing screenshot data", {}],
     ["invalid base64 screenshot data", { data: "not base64%%%" }],
   ])("reports %s as a protocol error", async (_label, result) => {
-    const browser = await startFakeCdpBrowser({
+    const { result: observation } = await captureWithFake({
       commandResult: ({ method }) =>
         method === "Page.captureScreenshot" ? result : undefined,
     });
-    onTestFinished(async () => {
-      await browser.close();
-    });
-    const observation = await new CdpBrowserProvider().captureScreenshot(
-      captureWebScreenshotInputSchema.parse({
-        cdp_endpoint: browser.endpoint,
-        allowed_origins: [browser.allowedOrigin],
-        target_id: "allowed-page",
-      }),
-    );
     expect(observation).toMatchObject({
       ok: false,
       error: { reason: "protocol_error" },
@@ -37,22 +44,12 @@ describe("screenshot committed document identity", () => {
       "base64",
     );
     bytes.writeUInt32BE(value, offset);
-    const browser = await startFakeCdpBrowser({
+    const { result } = await captureWithFake({
       commandResult: ({ method }) =>
         method === "Page.captureScreenshot"
           ? { data: bytes.toString("base64") }
           : undefined,
     });
-    onTestFinished(async () => {
-      await browser.close();
-    });
-    const result = await new CdpBrowserProvider().captureScreenshot(
-      captureWebScreenshotInputSchema.parse({
-        cdp_endpoint: browser.endpoint,
-        allowed_origins: [browser.allowedOrigin],
-        target_id: "allowed-page",
-      }),
-    );
     expect(result).toMatchObject({
       ok: false,
       error: { reason: "protocol_error" },
@@ -61,17 +58,7 @@ describe("screenshot committed document identity", () => {
   it.each(["loader-main", "loader-reloaded"])(
     "distinguishes %s for the same URL",
     async (screenshotDocumentLoader) => {
-      const browser = await startFakeCdpBrowser({ screenshotDocumentLoader });
-      onTestFinished(async () => {
-        await browser.close();
-      });
-      const result = await new CdpBrowserProvider().captureScreenshot(
-        captureWebScreenshotInputSchema.parse({
-          cdp_endpoint: browser.endpoint,
-          allowed_origins: [browser.allowedOrigin],
-          target_id: "allowed-page",
-        }),
-      );
+      const { result } = await captureWithFake({ screenshotDocumentLoader });
       if (screenshotDocumentLoader === "loader-main")
         expect(result.ok).toBe(true);
       else
@@ -81,4 +68,159 @@ describe("screenshot committed document identity", () => {
         });
     },
   );
+
+  it("defers an unidentifiable navigation failure until screenshot cleanup completes", async () => {
+    const { browser, result } = await captureWithFake({
+      commandEvents: (command) =>
+        command.method === "Page.captureScreenshot"
+          ? [
+              {
+                method: "Page.frameNavigated",
+                ...(command.sessionId === undefined
+                  ? {}
+                  : { sessionId: command.sessionId }),
+                params: undefined,
+              },
+            ]
+          : undefined,
+    });
+    expect(result).toMatchObject({
+      ok: false,
+      error: { reason: "protocol_error" },
+    });
+    await expect.poll(() => browser.activeConnectionCount()).toBe(0);
+  });
+});
+
+describe("screenshot frame identity transitions", () => {
+  it("rejects a self-parented root candidate as malformed identity", async () => {
+    const { browser, result } = await captureWithFake({
+      commandEvents: (command, origin) =>
+        command.method === "Page.captureScreenshot"
+          ? [
+              frameNavigated(command, {
+                id: "candidate-root",
+                parentId: "candidate-root",
+                url: `${origin}/app?token=frame-secret`,
+              }),
+            ]
+          : undefined,
+    });
+    expect(result).toMatchObject({
+      ok: false,
+      error: { reason: "protocol_error" },
+    });
+    await expect.poll(() => browser.activeConnectionCount()).toBe(0);
+  });
+
+  it("keeps a malformed known child as screenshot coverage", async () => {
+    const { result } = await captureWithFake({
+      commandEvents: (command) =>
+        command.method === "Page.captureScreenshot"
+          ? [
+              frameNavigated(command, {
+                id: 7,
+                parentId: "frame-main",
+                url: "https://outside.invalid/child",
+              }),
+            ]
+          : undefined,
+    });
+    if (!result.ok) throw result.error;
+    expect(result.value.completeness.excluded).toContainEqual(
+      expect.objectContaining({
+        section: "screenshot",
+        reason: "invalid_protocol_value",
+      }),
+    );
+  });
+
+  it("detects a same-URL root replacement and scopes later events to its new identity", async () => {
+    const { result } = await captureWithFake({
+      commandEvents: (command, origin) =>
+        command.method === "Page.captureScreenshot"
+          ? [
+              frameNavigated(command, {
+                id: "replacement-main",
+                loaderId: "loader-main",
+                url: `${origin}/app?token=frame-secret`,
+              }),
+              {
+                method: "Page.navigatedWithinDocument",
+                ...(command.sessionId === undefined
+                  ? {}
+                  : { sessionId: command.sessionId }),
+                params: {
+                  frameId: "replacement-main",
+                  url: "https://outside.invalid/path",
+                },
+              },
+            ]
+          : undefined,
+    });
+    expect(result).toMatchObject({
+      ok: false,
+      error: { reason: "target_not_allowed" },
+    });
+  });
+});
+
+describe("screenshot navigation event applicability", () => {
+  it("ignores non-navigation events with selected-frame or malformed parameters", async () => {
+    const { result } = await captureWithFake({
+      commandEvents: (command) =>
+        command.method === "Page.captureScreenshot"
+          ? [
+              {
+                method: "Runtime.consoleAPICalled",
+                sessionId: command.sessionId,
+                params: {
+                  type: "log",
+                  args: [],
+                  executionContextId: 1,
+                  timestamp: 1,
+                },
+              },
+              {
+                method: "Page.lifecycleEvent",
+                sessionId: command.sessionId,
+                params: {
+                  frameId: "frame-main",
+                  loaderId: "loader-main",
+                  name: "DOMContentLoaded",
+                  timestamp: 1,
+                },
+              },
+              {
+                method: "Runtime.consoleAPICalled",
+                sessionId: command.sessionId,
+                params: { frameId: "frame-main", args: "not-an-array" },
+              },
+            ]
+          : undefined,
+    });
+    expect(result.ok).toBe(true);
+  });
+
+  it.each([
+    "Page.frameRequestedNavigation",
+    "Page.navigatedWithinDocument",
+  ] as const)("ignores a known non-main frame's %s event", async (method) => {
+    const { result } = await captureWithFake({
+      commandEvents: (command) =>
+        command.method === "Page.captureScreenshot"
+          ? [
+              {
+                method,
+                sessionId: command.sessionId,
+                params: {
+                  frameId: "frame-child",
+                  url: "https://outside.invalid/child",
+                },
+              },
+            ]
+          : undefined,
+    });
+    expect(result.ok).toBe(true);
+  });
 });
diff --git a/tests/boundary/browser/cdpWebRuntime.test.ts b/tests/boundary/browser/cdpWebRuntime.test.ts
index 7b5c772ca4..db44df1868 100644
--- a/tests/boundary/browser/cdpWebRuntime.test.ts
+++ b/tests/boundary/browser/cdpWebRuntime.test.ts
@@ -109,6 +109,45 @@ describe("native runtime producer boundary", () => {
 });
 
 describe("runtime window and producer granularity", () => {
+  it("fails malformed frame identity and completes execution cleanup", async () => {
+    const { browser, provider, execution } = await fixture();
+    const result = await provider.observeExecution(
+      { ...execution, observation_ms: 30_000 },
+      {
+        progress: {
+          report: async (update) => {
+            if (update.completed !== 1) return;
+            browser.emitEvent({
+              method: "Page.frameNavigated",
+              sessionId: "session-1",
+              params: {
+                frame: {
+                  id: "other-frame",
+                  parentId: 7,
+                  url: "https://example.test/child",
+                },
+              },
+            });
+          },
+        },
+      },
+    );
+    expect(result).toMatchObject({
+      ok: false,
+      error: { _tag: "BrowserObservationError", reason: "protocol_error" },
+    });
+    expect(
+      browser.commands.some(
+        (command) => command.method === "Profiler.stopPreciseCoverage",
+      ),
+    ).toBe(true);
+    expect(
+      browser.commands.some(
+        (command) => command.method === "Target.detachFromTarget",
+      ),
+    ).toBe(true);
+  });
+
   it("ends at document replacement and leaves coverage explicitly unavailable", async () => {
     const { browser, provider, execution } = await fixture();
     const result = await provider.observeExecution(