diff --git a/apps/brunch-agent/README.md b/apps/brunch-agent/README.md
new file mode 100644
index 00000000000..e2ca70e74cb
--- /dev/null
+++ b/apps/brunch-agent/README.md
@@ -0,0 +1,17 @@
+# Brunch agent application
+
+## Run the process-model panel locally
+
+From the repository root, make `ANTHROPIC_API_KEY` available in the environment and run:
+
+```sh
+yarn dev:brunch
+```
+
+The command starts the Brunch server at `http://127.0.0.1:4321` and the real Petrinaut website at
+`http://127.0.0.1:4915`. The website proxies `/api/chat` to Brunch, where the panel runs the SDCPN
+process-model elicitor.
+
+Conversations persist in `apps/brunch-agent/.data-wipe-me/conversations.db`. Owned target documents
+persist as per-document JSON files under `apps/brunch-agent/.data-wipe-me/target-documents/`.
+`BRUNCH_DEV_DB_PATH` and `BRUNCH_DEV_TARGET_DOCUMENT_DIR` override those local paths.
diff --git a/apps/brunch-agent/petrinaut-local.vite.config.ts b/apps/brunch-agent/petrinaut-local.vite.config.ts
index 249686c8498..58b2973bda7 100644
--- a/apps/brunch-agent/petrinaut-local.vite.config.ts
+++ b/apps/brunch-agent/petrinaut-local.vite.config.ts
@@ -41,9 +41,7 @@ const withoutIncumbentChatHandler = (
 export default defineConfig(async (environment) => {
   const websiteRoot = process.env.PETRINAUT_WEBSITE_ROOT;
   if (!websiteRoot) {
-    throw new Error(
-      "PETRINAUT_WEBSITE_ROOT must point at hash/apps/petrinaut-website for the real-panel run.",
-    );
+    throw new Error("PETRINAUT_WEBSITE_ROOT is required.");
   }
   const root = resolve(websiteRoot);
   // Babel resolves the React compiler plugin from the launched project's cwd,
diff --git a/apps/brunch-agent/src/agents/gherkin-elicitor.ts b/apps/brunch-agent/src/agents/gherkin-elicitor.ts
index 9aaf8b1e493..d61da9094df 100644
--- a/apps/brunch-agent/src/agents/gherkin-elicitor.ts
+++ b/apps/brunch-agent/src/agents/gherkin-elicitor.ts
@@ -35,6 +35,7 @@ import { createGherkinElicitationSession } from "../elicitation-session.ts";
 export const GHERKIN_MODEL_ID = "claude-haiku-4-5";
 
 const gherkinElicitorInitialData = v.object({
+  ownerKey: v.optional(v.pipe(v.string(), v.nonEmpty())),
   targetDocumentId: v.pipe(v.string(), v.nonEmpty()),
 });
 
@@ -44,7 +45,11 @@ export function GherkinElicitor(props: AgentProps) {
     useInitialData<v.InferOutput<typeof gherkinElicitorInitialData>>();
   return useElicitation(
     gherkin,
-    createGherkinElicitationSession(props.id, initialData.targetDocumentId),
+    createGherkinElicitationSession(
+      props.id,
+      initialData.targetDocumentId,
+      initialData.ownerKey,
+    ),
   );
 }
 
diff --git a/apps/brunch-agent/src/agents/sdcpn-elicitor.ts b/apps/brunch-agent/src/agents/sdcpn-elicitor.ts
index 9609c98e7ae..fadcbe0cc07 100644
--- a/apps/brunch-agent/src/agents/sdcpn-elicitor.ts
+++ b/apps/brunch-agent/src/agents/sdcpn-elicitor.ts
@@ -33,6 +33,7 @@ export const SDCPN_MODEL_ID =
   process.env["BRUNCH_SDCPN_MODEL"] || "claude-haiku-4-5";
 
 const sdcpnElicitorInitialData = v.object({
+  ownerKey: v.optional(v.pipe(v.string(), v.nonEmpty())),
   targetDocumentId: v.pipe(v.string(), v.nonEmpty()),
 });
 
@@ -42,7 +43,11 @@ export function SdcpnElicitor(props: AgentProps) {
     useInitialData<v.InferOutput<typeof sdcpnElicitorInitialData>>();
   return useElicitation(
     sdcpn,
-    createSdcpnElicitationSession(props.id, initialData.targetDocumentId),
+    createSdcpnElicitationSession(
+      props.id,
+      initialData.targetDocumentId,
+      initialData.ownerKey,
+    ),
   );
 }
 
diff --git a/apps/brunch-agent/src/elicitation-session.ts b/apps/brunch-agent/src/elicitation-session.ts
index be16fd92e6c..f99c159f325 100644
--- a/apps/brunch-agent/src/elicitation-session.ts
+++ b/apps/brunch-agent/src/elicitation-session.ts
@@ -1,4 +1,5 @@
 /** Host-owned wiring for the local Flue binding's history transport and store. */
+import { createHash } from "node:crypto";
 
 import {
   createFlueHistoryReader,
@@ -10,6 +11,20 @@ import {
 import { AGENT_ROUTES, type AgentTarget } from "./routes.ts";
 import { targetDocumentPath } from "./target-document-path.ts";
 
+export const resolvePetrinautSessionIdentity = (
+  principalKey: string,
+  conversationId: string,
+) => {
+  const sessionDigest = createHash("sha256")
+    .update(JSON.stringify([principalKey, conversationId]))
+    .digest("hex");
+  return {
+    ownerKey: principalKey,
+    sessionId: `petrinaut-local:${sessionDigest}`,
+    targetDocumentId: `petrinaut-local:${principalKey}`,
+  } as const;
+};
+
 const appTransport: FlueHistoryReaderOptions["transport"] = async (
   input,
   init,
@@ -27,9 +42,11 @@ const createElicitationSession = (
   target: AgentTarget,
   sessionId: string,
   targetDocumentId: string,
+  ownerKey?: string,
 ): ElicitationSession => {
   const captureStore = createLocalCaptureStore(
     targetDocumentPath(targetDocumentId),
+    ownerKey === undefined ? {} : { ownerKey },
   );
   return {
     sessionId,
@@ -46,11 +63,13 @@ const createElicitationSession = (
 export const createGherkinElicitationSession = (
   sessionId: string,
   targetDocumentId: string,
+  ownerKey?: string,
 ): ElicitationSession =>
-  createElicitationSession("gherkin", sessionId, targetDocumentId);
+  createElicitationSession("gherkin", sessionId, targetDocumentId, ownerKey);
 
 export const createSdcpnElicitationSession = (
   sessionId: string,
   targetDocumentId: string,
+  ownerKey?: string,
 ): ElicitationSession =>
-  createElicitationSession("sdcpn", sessionId, targetDocumentId);
+  createElicitationSession("sdcpn", sessionId, targetDocumentId, ownerKey);
diff --git a/apps/brunch-agent/src/petrinaut-chat.ts b/apps/brunch-agent/src/petrinaut-chat.ts
index 280fed99ad8..498049d22a2 100644
--- a/apps/brunch-agent/src/petrinaut-chat.ts
+++ b/apps/brunch-agent/src/petrinaut-chat.ts
@@ -16,8 +16,11 @@ import {
   type TransportInspectionEvent,
 } from "@hashintel/brunch-agent-transport-aisdk";
 
-import { GherkinElicitor } from "./agents/gherkin-elicitor.ts";
-import { createGherkinElicitationSession } from "./elicitation-session.ts";
+import { SdcpnElicitor } from "./agents/sdcpn-elicitor.ts";
+import {
+  createSdcpnElicitationSession,
+  resolvePetrinautSessionIdentity,
+} from "./elicitation-session.ts";
 import { defaultPanelOrigins } from "./local-dev-origins.ts";
 
 const inspect =
@@ -29,20 +32,23 @@ const inspect =
       }
     : undefined;
 
-// FE-1439 replaces this local one-conversation/one-document identity
-// with principal-owned private session lookup. Keep it opaque here.
-const targetDocumentIdFor = (conversationId: string): string =>
-  `petrinaut-local:${conversationId}`;
-
 const streamElicitorTurn = async (
+  principalKey: string,
   conversationId: string,
   dispatch: { readonly message: string; readonly idempotencyKey: string },
   emit: (event: HarnessReplyEvent) => void,
 ): Promise<void> => {
-  const agent = init(GherkinElicitor, { id: conversationId });
+  const identity = resolvePetrinautSessionIdentity(
+    principalKey,
+    conversationId,
+  );
+  const agent = init(SdcpnElicitor, { id: identity.sessionId });
   const receipt = await agent.dispatch({
     ...dispatch,
-    initialData: { targetDocumentId: targetDocumentIdFor(conversationId) },
+    initialData: {
+      ownerKey: identity.ownerKey,
+      targetDocumentId: identity.targetDocumentId,
+    },
   });
   const projector = createFlueReplyProjector({
     submissionId: receipt.submissionId,
@@ -61,6 +67,7 @@ export const petrinautChatHandler = createAiSdkChatHandler({
   inspect,
   runTurn: (input, emit) =>
     streamElicitorTurn(
+      input.principalKey,
       input.conversationId,
       { message: input.userMessage.text, idempotencyKey: input.idempotencyKey },
       emit,
@@ -70,12 +77,17 @@ export const petrinautChatHandler = createAiSdkChatHandler({
     // submission resumes the conversation only when its tool-call id
     // correlates with the one ask still awaiting a reply.
     async admit(input) {
-      const session = createGherkinElicitationSession(
+      const identity = resolvePetrinautSessionIdentity(
+        input.principalKey,
         input.conversationId,
-        targetDocumentIdFor(input.conversationId),
+      );
+      const session = createSdcpnElicitationSession(
+        identity.sessionId,
+        identity.targetDocumentId,
+        identity.ownerKey,
       );
       const entries = projectFlueHistoryForSweep(
-        await session.historyReader.peek(input.conversationId),
+        await session.historyReader.peek(identity.sessionId),
       );
       return decideAskReplyAdmission(
         pendingAskAffordanceId(entries),
@@ -86,6 +98,7 @@ export const petrinautChatHandler = createAiSdkChatHandler({
     // binds it to the pending affordance, making it the user-affordance reply.
     run: (input, emit) =>
       streamElicitorTurn(
+        input.principalKey,
         input.conversationId,
         { message: input.ask.answer, idempotencyKey: input.idempotencyKey },
         emit,
diff --git a/apps/brunch-agent/test/elicitation-session.test.ts b/apps/brunch-agent/test/elicitation-session.test.ts
new file mode 100644
index 00000000000..946c3d9596c
--- /dev/null
+++ b/apps/brunch-agent/test/elicitation-session.test.ts
@@ -0,0 +1,25 @@
+import { expect, test } from "vitest";
+
+import { resolvePetrinautSessionIdentity } from "../src/elicitation-session.ts";
+
+test("namespaces panel sessions by principal while keeping one document per principal", () => {
+  const firstSession = resolvePetrinautSessionIdentity(
+    "principal-a",
+    "conversation-shared",
+  );
+  const reloadedSession = resolvePetrinautSessionIdentity(
+    "principal-a",
+    "conversation-after-reload",
+  );
+  const otherPrincipal = resolvePetrinautSessionIdentity(
+    "principal-b",
+    "conversation-shared",
+  );
+
+  expect(reloadedSession.targetDocumentId).toBe(firstSession.targetDocumentId);
+  expect(reloadedSession.sessionId).not.toBe(firstSession.sessionId);
+  expect(otherPrincipal.sessionId).not.toBe(firstSession.sessionId);
+  expect(otherPrincipal.targetDocumentId).not.toBe(
+    firstSession.targetDocumentId,
+  );
+});
diff --git a/apps/brunch-agent/test/local-dev-origins.test.ts b/apps/brunch-agent/test/local-dev-origins.test.ts
index 26eec099d35..342c794f530 100644
--- a/apps/brunch-agent/test/local-dev-origins.test.ts
+++ b/apps/brunch-agent/test/local-dev-origins.test.ts
@@ -13,6 +13,26 @@ import {
 const readAppFile = (relativePath: string): string =>
   readFileSync(new URL(`../${relativePath}`, import.meta.url), "utf8");
 
+const readRepoFile = (relativePath: string): string =>
+  readFileSync(new URL(`../../../${relativePath}`, import.meta.url), "utf8");
+
+test("one documented root command starts the Brunch server and Petrinaut panel", () => {
+  const rootPackage = JSON.parse(readRepoFile("package.json")) as {
+    scripts: Record<string, string>;
+  };
+
+  expect(rootPackage.scripts["dev:brunch"]).toBe(
+    "npm-run-all --parallel dev:brunch:server dev:brunch:panel",
+  );
+  expect(rootPackage.scripts["dev:brunch:server"]).toBe(
+    "yarn workspace @apps/brunch-agent dev",
+  );
+  expect(rootPackage.scripts["dev:brunch:panel"]).toBe(
+    'PETRINAUT_WEBSITE_ROOT="$PWD/apps/petrinaut-website" yarn workspace @apps/brunch-agent petrinaut:dev',
+  );
+  expect(readAppFile("README.md")).toContain("yarn dev:brunch");
+});
+
 test("dev listens on the chat origin the panel proxy already assumes", () => {
   expect(defaultChatOrigin).toBe("http://127.0.0.1:4321");
   expect(localChatListen).toEqual({
diff --git a/apps/brunch-agent/test/petrinaut-ask.integration.ts b/apps/brunch-agent/test/petrinaut-ask.integration.ts
index ae6033c43ec..88c22867e4b 100644
--- a/apps/brunch-agent/test/petrinaut-ask.integration.ts
+++ b/apps/brunch-agent/test/petrinaut-ask.integration.ts
@@ -19,10 +19,7 @@ import {
 } from "@earendil-works/pi-ai";
 import { start } from "@flue/runtime/node";
 
-import {
-  GHERKIN_MODEL_ID,
-  GherkinElicitor,
-} from "../src/agents/gherkin-elicitor.ts";
+import { SDCPN_MODEL_ID, SdcpnElicitor } from "../src/agents/sdcpn-elicitor.ts";
 
 import type { PetrinautAskResult } from "./petrinaut-ask-result";
 import type { UIMessageChunk } from "ai";
@@ -33,7 +30,7 @@ process.env.BRUNCH_TRANSPORT_AISDK_INSPECT = "1";
 
 const faux = fauxProvider({
   provider: "anthropic",
-  models: [{ id: GHERKIN_MODEL_ID, reasoning: true }],
+  models: [{ id: SDCPN_MODEL_ID, reasoning: true }],
 });
 faux.setResponses([
   fauxAssistantMessage(
@@ -58,7 +55,7 @@ faux.setResponses([
 ]);
 
 const flue = await start({
-  agents: [GherkinElicitor],
+  agents: [SdcpnElicitor],
   providers: [faux.provider],
 });
 
@@ -80,6 +77,7 @@ try {
         method: "POST",
         headers: {
           "content-type": "application/json",
+          "x-brunch-principal": "principal-fe1449",
           "x-request-id": requestId,
         },
         body: JSON.stringify(body),
diff --git a/apps/brunch-agent/test/petrinaut-chat.integration.ts b/apps/brunch-agent/test/petrinaut-chat.integration.ts
index 3325a33e151..fc71c397a50 100644
--- a/apps/brunch-agent/test/petrinaut-chat.integration.ts
+++ b/apps/brunch-agent/test/petrinaut-chat.integration.ts
@@ -11,10 +11,7 @@ import {
 } from "@earendil-works/pi-ai";
 import { start } from "@flue/runtime/node";
 
-import {
-  GHERKIN_MODEL_ID,
-  GherkinElicitor,
-} from "../src/agents/gherkin-elicitor.ts";
+import { SDCPN_MODEL_ID, SdcpnElicitor } from "../src/agents/sdcpn-elicitor.ts";
 
 import type { PetrinautChatResult } from "./petrinaut-chat-result";
 import type { UIMessageChunk } from "ai";
@@ -25,7 +22,7 @@ process.env.BRUNCH_TRANSPORT_AISDK_INSPECT = "1";
 
 const faux = fauxProvider({
   provider: "anthropic",
-  models: [{ id: GHERKIN_MODEL_ID, reasoning: true }],
+  models: [{ id: SDCPN_MODEL_ID, reasoning: true }],
 });
 faux.setResponses([
   fauxAssistantMessage([
@@ -43,7 +40,7 @@ faux.setResponses([
 ]);
 
 const flue = await start({
-  agents: [GherkinElicitor],
+  agents: [SdcpnElicitor],
   providers: [faux.provider],
 });
 
@@ -60,6 +57,7 @@ try {
       method: "POST",
       headers: {
         "content-type": "application/json",
+        "x-brunch-principal": "principal-fe1436-application",
         "x-request-id": "request-fe1436-application",
       },
       body: await readFile(fixturePath, "utf8"),
diff --git a/apps/brunch-agent/test/transport-aisdk-server.test.ts b/apps/brunch-agent/test/transport-aisdk-server.test.ts
index cecfb9042e9..e8cdee84f54 100644
--- a/apps/brunch-agent/test/transport-aisdk-server.test.ts
+++ b/apps/brunch-agent/test/transport-aisdk-server.test.ts
@@ -128,7 +128,10 @@ describe("FE-1436 Petrinaut wire server", () => {
       const response = await handler(
         new Request("http://brunch.test/api/petrinaut/chat", {
           method: "POST",
-          headers: { "content-type": "application/json" },
+          headers: {
+            "content-type": "application/json",
+            "x-brunch-principal": "principal-malformed-request",
+          },
           body: JSON.stringify(body),
         }),
       );
@@ -177,7 +180,10 @@ describe("FE-1436 Petrinaut wire server", () => {
       const response = await handler(
         new Request("http://brunch.test/api/petrinaut/chat", {
           method: "POST",
-          headers: { "content-type": "application/json" },
+          headers: {
+            "content-type": "application/json",
+            "x-brunch-principal": `principal-${terminalState}`,
+          },
           body: JSON.stringify({
             id: `conversation-${terminalState}`,
             trigger: "submit-message",
@@ -254,7 +260,10 @@ describe("FE-1436 Petrinaut wire server", () => {
     const response = await handler(
       new Request("http://brunch.test/api/petrinaut/chat", {
         method: "POST",
-        headers: { "content-type": "application/json" },
+        headers: {
+          "content-type": "application/json",
+          "x-brunch-principal": "principal-tool-failed",
+        },
         body: JSON.stringify({
           id: "conversation-tool-failed",
           trigger: "submit-message",
@@ -309,6 +318,9 @@ describe("FE-1436 Petrinaut wire server", () => {
     expect(allowed.headers.get("access-control-allow-methods")).toBe(
       "POST, OPTIONS",
     );
+    expect(allowed.headers.get("access-control-allow-headers")).toContain(
+      "x-brunch-principal",
+    );
 
     const refused = await handler(
       new Request("http://brunch.test/api/petrinaut/chat", {
@@ -328,6 +340,7 @@ describe("FE-1436 Petrinaut wire server", () => {
         expect(input).toEqual({
           conversationId: "m5z0GU9KJPzhOTlx",
           idempotencyKey: "m5z0GU9KJPzhOTlx:6ddgGkjhSxGjOtiv",
+          principalKey: "principal-fe1436",
           userMessage: {
             id: "6ddgGkjhSxGjOtiv",
             text: "Run the FE-1435 transport probe.",
@@ -342,6 +355,7 @@ describe("FE-1436 Petrinaut wire server", () => {
         method: "POST",
         headers: {
           "content-type": "application/json",
+          "x-brunch-principal": "principal-fe1436",
           "x-request-id": "request-fe1436-contract",
         },
         body: fixture("panel-initial.post.json"),
@@ -418,6 +432,7 @@ describe("FE-1436 Petrinaut wire server", () => {
         headers: {
           "content-type": "application/json",
           origin: "http://127.0.0.1:4915",
+          "x-brunch-principal": "principal-fe1436",
         },
         body: fixture("panel-tool-results.post.json"),
       }),
diff --git a/apps/petrinaut-website/src/main/app/local-storage-demo/brunch-principal.test.ts b/apps/petrinaut-website/src/main/app/local-storage-demo/brunch-principal.test.ts
new file mode 100644
index 00000000000..f71a3a82591
--- /dev/null
+++ b/apps/petrinaut-website/src/main/app/local-storage-demo/brunch-principal.test.ts
@@ -0,0 +1,21 @@
+import { expect, test, vi } from "vitest";
+
+import { getOrCreateBrunchPrincipal } from "./brunch-principal";
+
+test("reuses one UI-shell principal across transport requests and reloads", () => {
+  const storedPrincipalsByKey = new Map<string, string>();
+  const storage = {
+    getItem: (key: string) => storedPrincipalsByKey.get(key) ?? null,
+    setItem: (key: string, value: string) =>
+      storedPrincipalsByKey.set(key, value),
+  };
+  const createPrincipal = vi.fn(() => "principal-created-once");
+
+  expect(getOrCreateBrunchPrincipal(storage, createPrincipal)).toBe(
+    "principal-created-once",
+  );
+  expect(getOrCreateBrunchPrincipal(storage, createPrincipal)).toBe(
+    "principal-created-once",
+  );
+  expect(createPrincipal).toHaveBeenCalledOnce();
+});
diff --git a/apps/petrinaut-website/src/main/app/local-storage-demo/brunch-principal.ts b/apps/petrinaut-website/src/main/app/local-storage-demo/brunch-principal.ts
new file mode 100644
index 00000000000..7fc4861b0c2
--- /dev/null
+++ b/apps/petrinaut-website/src/main/app/local-storage-demo/brunch-principal.ts
@@ -0,0 +1,20 @@
+const principalStorageKey = "brunch-principal-v1";
+
+interface PrincipalStorage {
+  getItem(key: string): string | null;
+  setItem(key: string, value: string): void;
+}
+
+export const getOrCreateBrunchPrincipal = (
+  storage: PrincipalStorage = window.localStorage,
+  createPrincipal: () => string = () => crypto.randomUUID(),
+): string => {
+  const existingPrincipal = storage.getItem(principalStorageKey);
+  if (existingPrincipal) {
+    return existingPrincipal;
+  }
+
+  const principal = createPrincipal();
+  storage.setItem(principalStorageKey, principal);
+  return principal;
+};
diff --git a/apps/petrinaut-website/src/main/app/local-storage-demo/local-storage-demo-app.tsx b/apps/petrinaut-website/src/main/app/local-storage-demo/local-storage-demo-app.tsx
index 6f770fc9602..81d6b3bbf22 100644
--- a/apps/petrinaut-website/src/main/app/local-storage-demo/local-storage-demo-app.tsx
+++ b/apps/petrinaut-website/src/main/app/local-storage-demo/local-storage-demo-app.tsx
@@ -1,6 +1,7 @@
 import { produce } from "immer";
 import { useEffect, useMemo, useState } from "react";
 
+import { BRUNCH_PRINCIPAL_HEADER } from "@hashintel/brunch-agent-transport-aisdk/headers";
 import {
   createJsonDocHandle,
   type MinimalNetMetadata,
@@ -18,6 +19,7 @@ import {
 
 import { useSentryFeedbackAction } from "../sentry-feedback-button";
 import { brunchAskInteractiveTool } from "./brunch-ask-interactive-tool";
+import { getOrCreateBrunchPrincipal } from "./brunch-principal";
 import { useLocalStorageAiMessages } from "./use-local-storage-ai-messages";
 import {
   type SDCPNInLocalStorage,
@@ -83,6 +85,9 @@ const createHandle = (net: SDCPNInLocalStorage): PetrinautDocHandle =>
 const petrinautAiChatTransport: PetrinautAiChatTransport =
   new DefaultChatTransport({
     api: "/api/chat",
+    headers: () => ({
+      [BRUNCH_PRINCIPAL_HEADER]: getOrCreateBrunchPrincipal(),
+    }),
   });
 
 const getStoredSDCPNsForDisplay = (
diff --git a/libs/@hashintel/brunch-agent/packages/binding-flue/src/local-capture-store.ts b/libs/@hashintel/brunch-agent/packages/binding-flue/src/local-capture-store.ts
index 4e947830fc8..c9ef95860fd 100644
--- a/libs/@hashintel/brunch-agent/packages/binding-flue/src/local-capture-store.ts
+++ b/libs/@hashintel/brunch-agent/packages/binding-flue/src/local-capture-store.ts
@@ -25,18 +25,23 @@ import {
 
 import { registerArchiveWriter } from "./archive-capability";
 
-const FORMAT_VERSION = 1 as const;
+const FORMAT_VERSION = 2 as const;
+const LEGACY_FORMAT_VERSION = 1 as const;
 
 interface TargetDocumentRecord {
   readonly formatVersion: typeof FORMAT_VERSION;
+  readonly ownerKey: string | null;
   readonly captureStore: CaptureStoreSnapshot;
   readonly sessionLogArchive: SessionLogArchive;
 }
 
 const writesByPath = new Map<string, Promise<void>>();
 
-const createEmptyTargetDocument = (): TargetDocumentRecord => ({
+const createEmptyTargetDocument = (
+  ownerKey: string | null,
+): TargetDocumentRecord => ({
   formatVersion: FORMAT_VERSION,
+  ownerKey,
   captureStore: createEmptyCaptureStoreSnapshot(),
   sessionLogArchive: createEmptySessionLogArchive(),
 });
@@ -47,20 +52,41 @@ const isRecord = (value: unknown): value is Record<string, unknown> =>
 const parseTargetDocument = (input: unknown): TargetDocumentRecord => {
   if (isRecord(input) && "formatVersion" in input) {
     const fields = Object.keys(input).sort();
+    if (input.formatVersion === FORMAT_VERSION) {
+      if (
+        JSON.stringify(fields) !==
+          JSON.stringify([
+            "captureStore",
+            "formatVersion",
+            "ownerKey",
+            "sessionLogArchive",
+          ]) ||
+        (typeof input.ownerKey !== "string" && input.ownerKey !== null)
+      ) {
+        throw new TypeError("Invalid target-document ownership record.");
+      }
+      return {
+        formatVersion: FORMAT_VERSION,
+        ownerKey: input.ownerKey,
+        captureStore: parseCaptureStoreSnapshot(input.captureStore),
+        sessionLogArchive: parseSessionLogArchive(input.sessionLogArchive),
+      };
+    }
     if (
-      input.formatVersion !== FORMAT_VERSION ||
-      JSON.stringify(fields) !==
+      input.formatVersion === LEGACY_FORMAT_VERSION &&
+      JSON.stringify(fields) ===
         JSON.stringify(["captureStore", "formatVersion", "sessionLogArchive"])
     ) {
-      throw new TypeError(
-        `Unsupported target-document format version ${String(input.formatVersion)}.`,
-      );
+      return {
+        formatVersion: FORMAT_VERSION,
+        ownerKey: null,
+        captureStore: parseCaptureStoreSnapshot(input.captureStore),
+        sessionLogArchive: parseSessionLogArchive(input.sessionLogArchive),
+      };
     }
-    return {
-      formatVersion: FORMAT_VERSION,
-      captureStore: parseCaptureStoreSnapshot(input.captureStore),
-      sessionLogArchive: parseSessionLogArchive(input.sessionLogArchive),
-    };
+    throw new TypeError(
+      `Unsupported target-document format version ${String(input.formatVersion)}.`,
+    );
   }
 
   // FE-1390 files predate the archive slot. Reading that exact capture-store
@@ -68,16 +94,28 @@ const parseTargetDocument = (input: unknown): TargetDocumentRecord => {
   // rewrites it atomically in the current format.
   return {
     formatVersion: FORMAT_VERSION,
+    ownerKey: null,
     captureStore: parseCaptureStoreSnapshot(input),
     sessionLogArchive: createEmptySessionLogArchive(),
   };
 };
 
+class TargetDocumentOwnerMismatchError extends Error {
+  readonly code = "target-document-owner-mismatch";
+
+  constructor() {
+    super("The target document is owned by a different principal.");
+    this.name = "TargetDocumentOwnerMismatchError";
+  }
+}
+
 class LocalCaptureStore implements CaptureStore {
+  readonly #ownerKey: string | null;
   readonly #path: string;
 
-  constructor(path: string) {
+  constructor(path: string, ownerKey: string | null) {
     this.#path = resolve(path);
+    this.#ownerKey = ownerKey;
     registerArchiveWriter(this, (read) => this.#archiveSessionLog(read));
   }
 
@@ -163,16 +201,20 @@ class LocalCaptureStore implements CaptureStore {
 
   async #readFile(): Promise<TargetDocumentRecord> {
     try {
-      return parseTargetDocument(
+      const document = parseTargetDocument(
         JSON.parse(await readFile(this.#path, "utf8")),
       );
+      if (document.ownerKey !== this.#ownerKey) {
+        throw new TargetDocumentOwnerMismatchError();
+      }
+      return document;
     } catch (error) {
       if (
         error instanceof Error &&
         "code" in error &&
         (error as NodeJS.ErrnoException).code === "ENOENT"
       ) {
-        return createEmptyTargetDocument();
+        return createEmptyTargetDocument(this.#ownerKey);
       }
       throw error;
     }
@@ -193,5 +235,12 @@ class LocalCaptureStore implements CaptureStore {
   }
 }
 
-export const createLocalCaptureStore = (path: string): CaptureStore =>
-  new LocalCaptureStore(path);
+export const createLocalCaptureStore = (
+  path: string,
+  options: { readonly ownerKey?: string } = {},
+): CaptureStore => {
+  if (options.ownerKey !== undefined && options.ownerKey.length === 0) {
+    throw new TypeError("A target-document owner key cannot be empty.");
+  }
+  return new LocalCaptureStore(path, options.ownerKey ?? null);
+};
diff --git a/libs/@hashintel/brunch-agent/packages/binding-flue/test/history-reader.test.ts b/libs/@hashintel/brunch-agent/packages/binding-flue/test/history-reader.test.ts
index a46c48a5cba..294fc08da0e 100644
--- a/libs/@hashintel/brunch-agent/packages/binding-flue/test/history-reader.test.ts
+++ b/libs/@hashintel/brunch-agent/packages/binding-flue/test/history-reader.test.ts
@@ -298,11 +298,13 @@ describe("Flue materialized-history reader", () => {
 
     const persisted = JSON.parse(await readFile(path, "utf8")) as {
       formatVersion: number;
+      ownerKey: string | null;
       sessionLogArchive: {
         sessions: { reads: { substrateConversationId?: string }[] }[];
       };
     };
-    expect(persisted.formatVersion).toBe(1);
+    expect(persisted.formatVersion).toBe(2);
+    expect(persisted.ownerKey).toBeNull();
     expect(persisted.sessionLogArchive.sessions).toHaveLength(1);
     expect(
       persisted.sessionLogArchive.sessions[0]!.reads[0]!
diff --git a/libs/@hashintel/brunch-agent/packages/binding-flue/test/local-capture-store.test.ts b/libs/@hashintel/brunch-agent/packages/binding-flue/test/local-capture-store.test.ts
index 8e7ac59bafe..209025b69b7 100644
--- a/libs/@hashintel/brunch-agent/packages/binding-flue/test/local-capture-store.test.ts
+++ b/libs/@hashintel/brunch-agent/packages/binding-flue/test/local-capture-store.test.ts
@@ -77,6 +77,49 @@ const storePath = async (): Promise<string> => {
 };
 
 describe("local capture store", () => {
+  test("refuses a different opaque owner before reading or writing a target document", async () => {
+    const path = await storePath();
+    const ownerStore = createLocalCaptureStoreAdapter(path, {
+      ownerKey: "principal-a",
+    });
+    await archiveThroughBinding(ownerStore, {
+      sessionId: "session-a",
+      offset: "0",
+      entries: [],
+      settlements: [],
+    });
+    await archiveThroughBinding(ownerStore, {
+      sessionId: "session-c",
+      offset: "0",
+      entries: [],
+      settlements: [],
+    });
+
+    const intruderStore = createLocalCaptureStoreAdapter(path, {
+      ownerKey: "principal-b",
+    });
+    await expect(intruderStore.read()).rejects.toMatchObject({
+      code: "target-document-owner-mismatch",
+    });
+    await expect(
+      archiveThroughBinding(intruderStore, {
+        sessionId: "session-b",
+        offset: "0",
+        entries: [],
+        settlements: [],
+      }),
+    ).rejects.toMatchObject({
+      code: "target-document-owner-mismatch",
+    });
+
+    expect(JSON.parse(await readFile(path, "utf8"))).toMatchObject({
+      ownerKey: "principal-a",
+      sessionLogArchive: {
+        sessions: [{ sessionId: "session-a" }, { sessionId: "session-c" }],
+      },
+    });
+  });
+
   test("persists captures through JSON tmp-and-rename without stored statuses", async () => {
     const path = await storePath();
     const first = createLocalCaptureStore(path);
@@ -242,7 +285,8 @@ describe("local capture store", () => {
     });
 
     expect(JSON.parse(await readFile(path, "utf8"))).toEqual({
-      formatVersion: 1,
+      formatVersion: 2,
+      ownerKey: null,
       captureStore: legacy,
       sessionLogArchive: {
         sessions: [
diff --git a/libs/@hashintel/brunch-agent/packages/transport-aisdk/package.json b/libs/@hashintel/brunch-agent/packages/transport-aisdk/package.json
index 1c0f37880ff..9124c2db255 100644
--- a/libs/@hashintel/brunch-agent/packages/transport-aisdk/package.json
+++ b/libs/@hashintel/brunch-agent/packages/transport-aisdk/package.json
@@ -13,6 +13,10 @@
     "./client-tools": {
       "types": "./src/client-tools.ts",
       "import": "./dist/client-tools.js"
+    },
+    "./headers": {
+      "types": "./src/headers.ts",
+      "import": "./dist/headers.js"
     }
   },
   "scripts": {
diff --git a/libs/@hashintel/brunch-agent/packages/transport-aisdk/src/headers.ts b/libs/@hashintel/brunch-agent/packages/transport-aisdk/src/headers.ts
new file mode 100644
index 00000000000..965e5feef1c
--- /dev/null
+++ b/libs/@hashintel/brunch-agent/packages/transport-aisdk/src/headers.ts
@@ -0,0 +1,2 @@
+/** Stable browser-to-Brunch principal header owned by the HTTP transport. */
+export const BRUNCH_PRINCIPAL_HEADER = "x-brunch-principal";
diff --git a/libs/@hashintel/brunch-agent/packages/transport-aisdk/src/index.ts b/libs/@hashintel/brunch-agent/packages/transport-aisdk/src/index.ts
index 2fda5f55683..8060ec11089 100644
--- a/libs/@hashintel/brunch-agent/packages/transport-aisdk/src/index.ts
+++ b/libs/@hashintel/brunch-agent/packages/transport-aisdk/src/index.ts
@@ -19,11 +19,13 @@ import {
 } from "@hashintel/brunch-agent";
 
 import { ASK_TOOL_NAME, type BrunchAskOutput } from "./client-tools";
+import { BRUNCH_PRINCIPAL_HEADER } from "./headers";
 
 export {
   type AskReplyAdmission,
   type HarnessReplyEvent,
 } from "@hashintel/brunch-agent";
+export { BRUNCH_PRINCIPAL_HEADER } from "./headers";
 export {
   ASK_TOOL_NAME,
   type BrunchAskInput,
@@ -35,6 +37,7 @@ export {
 export interface HarnessTurnInput {
   readonly conversationId: string;
   readonly idempotencyKey: string;
+  readonly principalKey: string;
   readonly userMessage: {
     readonly id: string;
     readonly text: string;
@@ -65,6 +68,7 @@ export interface HarnessAskReplyInput {
   /** Existing assistant UI message whose pending tool call this continues. */
   readonly assistantMessageId: string;
   readonly idempotencyKey: string;
+  readonly principalKey: string;
   readonly ask: BrunchAskOutput & {
     readonly toolCallId: string;
   };
@@ -183,6 +187,11 @@ const transportRequestRefusals = {
     status: 400,
     error: "invalid_chat_request",
   },
+  invalidPrincipal: {
+    reason: "invalid-principal",
+    status: 400,
+    error: "invalid_principal",
+  },
   toolResultFollowUpNotSupported: {
     reason: "tool-result-follow-up-not-supported",
     status: 422,
@@ -213,7 +222,7 @@ const corsHeaders = (origin: string): Headers =>
   new Headers({
     "access-control-allow-origin": origin,
     "access-control-allow-methods": "POST, OPTIONS",
-    "access-control-allow-headers": "content-type, x-request-id",
+    "access-control-allow-headers": `content-type, x-request-id, ${BRUNCH_PRINCIPAL_HEADER}`,
     vary: "Origin",
   });
 
@@ -258,7 +267,10 @@ const isAnsweredAskPart = (
  * Petrinaut mutation outputs, the synthetic diagnostics message — remains
  * the machine-input protocol this transport still refuses (FE-1438 owns it).
  */
-const parseAskReplyTurn = (body: PanelPostBody): ParsedTransportRequest => {
+const parseAskReplyTurn = (
+  body: PanelPostBody,
+  principalKey: string,
+): ParsedTransportRequest => {
   if (
     typeof body.id !== "string" ||
     body.id.length === 0 ||
@@ -306,12 +318,16 @@ const parseAskReplyTurn = (body: PanelPostBody): ParsedTransportRequest => {
       // Keyed by the ask itself: concurrent duplicate submissions of the same
       // pending ask collapse to one dispatch at the substrate.
       idempotencyKey: `${body.id}:ask:${askPart.toolCallId}`,
+      principalKey,
       ask: { toolCallId: askPart.toolCallId, answer: submission.output.answer },
     },
   };
 };
 
-const parseInitialTurn = (body: PanelPostBody): ParsedTransportRequest => {
+const parseInitialTurn = (
+  body: PanelPostBody,
+  principalKey: string,
+): ParsedTransportRequest => {
   if (
     typeof body.id !== "string" ||
     body.id.length === 0 ||
@@ -354,6 +370,7 @@ const parseInitialTurn = (body: PanelPostBody): ParsedTransportRequest => {
     value: {
       conversationId: body.id,
       idempotencyKey: `${body.id}:${message.id}`,
+      principalKey,
       userMessage: { id: message.id, text },
     },
   };
@@ -512,17 +529,30 @@ export const createAiSdkChatHandler =
       );
     }
     const postBody = validatedBody.output;
+    const principalKey = request.headers.get(BRUNCH_PRINCIPAL_HEADER)?.trim();
+    if (
+      principalKey === undefined ||
+      principalKey.length === 0 ||
+      principalKey.length > 256
+    ) {
+      const refusal = transportRequestRefusals.invalidPrincipal;
+      return jsonResponse(
+        { error: refusal.error },
+        refusal.status,
+        crossOriginHeaders,
+      );
+    }
     // The follow-up admits exactly the pending ask's correlated human answer;
     // absent an application ask-reply seam, every follow-up stays refused.
     const parsed =
       postBody.messageId !== undefined && options.askReply !== undefined
-        ? parseAskReplyTurn(postBody)
+        ? parseAskReplyTurn(postBody, principalKey)
         : postBody.messageId !== undefined
           ? ({
               kind: "refused",
               refusal: transportRequestRefusals.toolResultFollowUpNotSupported,
             } as const)
-          : parseInitialTurn(postBody);
+          : parseInitialTurn(postBody, principalKey);
     if (parsed.kind === "refused") {
       return jsonResponse(
         { error: parsed.refusal.error },
diff --git a/libs/@hashintel/brunch-agent/packages/transport-aisdk/test/ask-reply.test.ts b/libs/@hashintel/brunch-agent/packages/transport-aisdk/test/ask-reply.test.ts
index e66c0674b51..cfa3e5ace5e 100644
--- a/libs/@hashintel/brunch-agent/packages/transport-aisdk/test/ask-reply.test.ts
+++ b/libs/@hashintel/brunch-agent/packages/transport-aisdk/test/ask-reply.test.ts
@@ -42,6 +42,7 @@ const post = (body: unknown): Request =>
     method: "POST",
     headers: {
       "content-type": "application/json",
+      "x-brunch-principal": "principal-fe1449",
       "x-request-id": "request-fe1449",
     },
     body: typeof body === "string" ? body : JSON.stringify(body),
@@ -135,6 +136,37 @@ const resumedTurn: AskReplyHandler["run"] = async (_input, emit) => {
   });
 };
 
+test("refuses a valid turn without the UI shell principal", async () => {
+  let dispatched = false;
+  const handler = createAiSdkChatHandler({
+    async runTurn() {
+      dispatched = true;
+    },
+  });
+
+  const response = await handler(
+    new Request("http://brunch.test/api/petrinaut/chat", {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({
+        id: "conversation-without-principal",
+        trigger: "submit-message",
+        messages: [
+          {
+            id: "user-without-principal",
+            role: "user",
+            parts: [{ type: "text", text: "Help me model checkout." }],
+          },
+        ],
+      }),
+    }),
+  );
+
+  expect(response.status).toBe(400);
+  expect(await response.json()).toEqual({ error: "invalid_principal" });
+  expect(dispatched).toBe(false);
+});
+
 describe("FE-1449 ask suspension on the wire", () => {
   test("translates the ask as an awaiting client tool and withholds the affordance output", async () => {
     const inspections: TransportInspectionEvent[] = [];
@@ -208,6 +240,7 @@ describe("FE-1449 ask return POST", () => {
     expect(admitted).toEqual([
       {
         conversationId: "conversation-fe1449",
+        principalKey: "principal-fe1449",
         assistantMessageId: "assistant-fe1449-1",
         idempotencyKey: "conversation-fe1449:ask:tool-ask-fe1449",
         ask: {
diff --git a/libs/@hashintel/brunch-agent/packages/transport-aisdk/vite.config.ts b/libs/@hashintel/brunch-agent/packages/transport-aisdk/vite.config.ts
index 77a7a9fe479..369dc937a5d 100644
--- a/libs/@hashintel/brunch-agent/packages/transport-aisdk/vite.config.ts
+++ b/libs/@hashintel/brunch-agent/packages/transport-aisdk/vite.config.ts
@@ -11,6 +11,7 @@ export default defineConfig({
         "client-tools": fileURLToPath(
           new URL("src/client-tools.ts", import.meta.url),
         ),
+        headers: fileURLToPath(new URL("src/headers.ts", import.meta.url)),
         index: fileURLToPath(new URL("src/index.ts", import.meta.url)),
       },
       fileName: (_format, entryName) => `${entryName}.js`,
diff --git a/package.json b/package.json
index 7c07af5d451..f1a9e5ffd88 100644
--- a/package.json
+++ b/package.json
@@ -33,6 +33,9 @@
     "dev": "CARGO_TERM_PROGRESS_WHEN=never turbo dev --log-order stream --filter '@apps/hash-api' --filter '@apps/hash-frontend' --",
     "dev:backend": "CARGO_TERM_PROGRESS_WHEN=never turbo dev --log-order stream --filter '@apps/hash-api' --",
     "dev:backend:api": "CARGO_TERM_PROGRESS_WHEN=never turbo dev --log-order stream --filter '@apps/hash-api' --",
+    "dev:brunch": "npm-run-all --parallel dev:brunch:server dev:brunch:panel",
+    "dev:brunch:panel": "PETRINAUT_WEBSITE_ROOT=\"$PWD/apps/petrinaut-website\" yarn workspace @apps/brunch-agent petrinaut:dev",
+    "dev:brunch:server": "yarn workspace @apps/brunch-agent dev",
     "dev:frontend": "CARGO_TERM_PROGRESS_WHEN=never turbo dev --log-order stream --filter '@apps/hash-frontend' --",
     "dev:petrinaut-optimization": "yarn workspace @apps/petrinaut-website dev:optimization",
     "fix": "npm-run-all --continue-on-error \"fix:*\"",