diff --git a/docs/public-demo.md b/docs/public-demo.md
index dd9efad..b4266cd 100644
--- a/docs/public-demo.md
+++ b/docs/public-demo.md
@@ -27,6 +27,9 @@ browser / tress attach / tress connect
    for the model calls. This is separate from the
    Harness key: Harness stores and shares the thread; the Tress host runs the
    agent and pays for inference. The current host expects `ANTHROPIC_API_KEY`.
+   If the key is not scoped to a workspace, also configure
+   `ANTHROPIC_WORKSPACE_ID` with the `wrkspc_...` ID from Anthropic's Workspaces
+   settings. This is separate from `HARNESS_WORKSPACE`.
 3. Provision a PostgreSQL database that the site can reach. The site uses it
    for visitor/session ownership, access-token hashes, usage limits, hosted
    virtual files, and connected-device jobs. Harness—not PostgreSQL—stores the
diff --git a/site/README.md b/site/README.md
index 5a7fc34..0d5960e 100644
--- a/site/README.md
+++ b/site/README.md
@@ -29,6 +29,12 @@ a model; the default is `claude-sonnet-5`. Keys stay on the server. Without a ke
 the playground explains the setup and disables sending; it does not simulate a
 live agent.
 
+For an Anthropic personal or service-account key that is not scoped to a
+workspace, also set `ANTHROPIC_WORKSPACE_ID=wrkspc_...` on the host. Use the ID
+from the Anthropic Console's Workspaces page. The host sends it on model and
+diagnostic requests. Workspace-scoped keys can omit it. For Vercel, configure
+both variables in the intended environment and redeploy to activate them.
+
 Install the Rust `wasm32-unknown-unknown` target and `wasm-bindgen-cli` matching
 the `wasm-bindgen` version in `Cargo.lock`. Starting the site automatically builds
 the Rust engine and generates its bundler bindings:
diff --git a/site/scripts/health-policy.test.mjs b/site/scripts/health-policy.test.mjs
index 316f3b8..0ea8f45 100644
--- a/site/scripts/health-policy.test.mjs
+++ b/site/scripts/health-policy.test.mjs
@@ -40,6 +40,7 @@ const {
 } = await import(output.href);
 const keys = [
   "ANTHROPIC_API_KEY",
+  "ANTHROPIC_WORKSPACE_ID",
   "TRESS_MODEL",
   "TRESS_API_URL",
   "HARNESS_API_KEY",
@@ -80,6 +81,7 @@ const store = (name) => createFileThreadStore(join(root, name));
 const request = () => new Request("https://host.example/api/health");
 
 test("health uses host credentials, closes streams, and never generates a completion", async () => {
+  process.env.ANTHROPIC_WORKSPACE_ID = "wrkspc_health_fixture";
   const calls = [];
   let closed = 0;
   const result = await checkHost(
@@ -91,13 +93,16 @@ test("health uses host credentials, closes streams, and never generates a comple
       assert.equal(options.method, undefined);
       assert.equal(options.redirect, "error");
       assert(options.signal);
-      if (url.hostname === "provider.example")
+      if (url.hostname === "provider.example") {
         assert.equal(options.headers["x-api-key"], "fake-model-secret");
-      else
+        assert.equal(options.headers["anthropic-workspace-id"], "wrkspc_health_fixture");
+      } else {
+        assert.equal(options.headers["anthropic-workspace-id"], undefined);
         assert.equal(
           options.headers.Authorization,
           "Bearer fake-harness-secret",
         );
+      }
       return new Response(
         new ReadableStream({
           cancel() {
@@ -112,6 +117,45 @@ test("health uses host credentials, closes streams, and never generates a comple
   assert(calls.some((url) => url.endsWith("/v1/models/fixture-model")));
   assert(calls.some((url) => url.includes("/threads/fixture~")));
   assert(!JSON.stringify(result).includes("secret"));
+  assert(!JSON.stringify(result).includes("wrkspc_health_fixture"));
+});
+
+test("browser model requests and probes use only the host's optional Anthropic workspace", async (t) => {
+  const storeCache = globalThis[Symbol.for("tress.demo.thread-store.v4")];
+  const previousStore = storeCache.store;
+  t.after(() => { storeCache.store = previousStore; });
+  const routeOutput = new URL(`../.tress/anthropic-route-${process.pid}.mjs`, import.meta.url);
+  await build({
+    entryPoints: [new URL("../src/app/api/messages/route.ts", import.meta.url).pathname],
+    outfile: routeOutput.pathname, bundle: true, packages: "external", platform: "node", format: "esm",
+  });
+  t.after(() => rm(routeOutput, { force: true }));
+  const { POST } = await import(routeOutput.href);
+  process.env.TRESS_DEMO_SHARED = "1";
+  const calls = [];
+  t.mock.method(globalThis, "fetch", async (url, options) => {
+    calls.push({ url, headers: options.headers });
+    return new Response("data: fixture\n\n", { headers: { "content-type": "text/event-stream" } });
+  });
+  for (const workspace of [undefined, "  wrkspc_server_fixture  ", "   "]) {
+    if (workspace === undefined) delete process.env.ANTHROPIC_WORKSPACE_ID;
+    else process.env.ANTHROPIC_WORKSPACE_ID = workspace;
+    for (const probe of [false, true]) {
+      const response = await POST(new Request("https://host.example/api/messages", {
+        method: "POST",
+        headers: { "content-type": "application/json", "anthropic-workspace-id": "wrkspc_client_must_not_override" },
+        body: JSON.stringify({ probe, messages: [{ role: "user", content: "fixture" }] }),
+      }));
+      assert.equal(response.status, 200);
+      const call = calls.at(-1);
+      assert.equal(call.url, "https://api.anthropic.com/v1/messages");
+      assert.equal(call.headers["x-api-key"], "fake-model-secret");
+      assert.equal(call.headers["anthropic-workspace-id"], workspace?.trim() || undefined);
+      assert.equal(response.headers.get("anthropic-workspace-id"), null);
+      assert(!((await response.text()).includes("fake-model-secret")));
+    }
+  }
+  assert.equal(calls.length, 6);
 });
 
 test("health distinguishes database failure, rejected Harness access, and unsupported model metadata", async () => {
diff --git a/site/scripts/verify-health-policy.mjs b/site/scripts/verify-health-policy.mjs
index c75bb69..74f146e 100644
--- a/site/scripts/verify-health-policy.mjs
+++ b/site/scripts/verify-health-policy.mjs
@@ -23,6 +23,7 @@ let generations = 0;
 let metadataChecks = 0;
 const model = createServer(async (request, response) => {
   assert.equal(request.headers["x-api-key"], "mock-host-key");
+  assert.equal(request.headers["anthropic-workspace-id"], "wrkspc_host_fixture");
   if (request.method === "GET" && request.url === "/v1/models/test-model") {
     metadataChecks++;
     response.writeHead(200, { "Content-Type": "application/json" });
@@ -73,6 +74,7 @@ try {
       PORT: String(port),
       HOST: "127.0.0.1",
       ANTHROPIC_API_KEY: "mock-host-key",
+      ANTHROPIC_WORKSPACE_ID: "wrkspc_host_fixture",
       TRESS_API_URL: `http://127.0.0.1:${model.address().port}/v1/messages`,
       TRESS_MODEL: "test-model",
       TRESS_THREAD_MODE: "local",
diff --git a/site/src/app/api/messages/route.ts b/site/src/app/api/messages/route.ts
index d196081..42f1e82 100644
--- a/site/src/app/api/messages/route.ts
+++ b/site/src/app/api/messages/route.ts
@@ -10,6 +10,7 @@ import {
   sessionResponse,
 } from "../../../server/demo-session";
 import { admitRun } from "../../../server/demo-policy";
+import { anthropicHeaders } from "../../../server/anthropic";
 
 type Block =
   | { kind: "text"; text: string }
@@ -93,8 +94,7 @@ async function keyState(key: string): Promise<"live" | "rejected"> {
       method: "POST",
       headers: {
         "content-type": "application/json",
-        "x-api-key": key,
-        "anthropic-version": "2023-06-01",
+        ...anthropicHeaders(key),
       },
       body: JSON.stringify({
         model: "claude-sonnet-5",
@@ -204,8 +204,7 @@ export async function POST(request: Request) {
       method: "POST",
       headers: {
         "content-type": "application/json",
-        "x-api-key": key,
-        "anthropic-version": "2023-06-01",
+        ...anthropicHeaders(key),
       },
       body,
     });
diff --git a/site/src/server/agent.ts b/site/src/server/agent.ts
index 0a08f8a..71f7aab 100644
--- a/site/src/server/agent.ts
+++ b/site/src/server/agent.ts
@@ -6,6 +6,7 @@ import {
   type WorkspaceToolsOptions,
 } from "@tress/workspaces";
 import { workspaceConfig } from "./workspace";
+import { anthropicHeaders } from "./anthropic";
 
 export const openSession = async (
   workspace: Workspace,
@@ -37,7 +38,7 @@ export const openSession = async (
     workspace,
     url: process.env.TRESS_API_URL ?? "https://api.anthropic.com/v1/messages",
     model: process.env.TRESS_MODEL ?? "claude-sonnet-5",
-    headers: { "x-api-key": key, "anthropic-version": "2023-06-01" },
+    headers: anthropicHeaders(key),
     system: `You are tress, a coding agent working in a shared workspace. Browser and terminal clients share this conversation and its files.
 Answer questions directly. Only inspect or change files when needed for the user's request. ${workspace.filesystemRoot ? `Paths may be absolute under filesystem root ${JSON.stringify(workspace.filesystemRoot)} or relative to that root.` : "Paths are relative to the workspace root."}
 ${connected ? "Files are served by a user-approved native folder. No shell or project test runner is available in this connection." : nativeSandboxShell ? "Commands run inside a sandbox, with a timeout. Installed runtimes and dependencies belong to that sandbox." : nativeDeviceShell ? "Commands run on the host device under its configured tool policy. Only claim a command ran when the tool confirms it." : "The shell is just-bash: simulated file and text commands, not a native operating system. Node, npm, git, and project test runners are not available. Never claim tests ran unless a tool actually ran them."}
diff --git a/site/src/server/anthropic.ts b/site/src/server/anthropic.ts
new file mode 100644
index 0000000..d645497
--- /dev/null
+++ b/site/src/server/anthropic.ts
@@ -0,0 +1,9 @@
+/** Server-owned authentication for both workspace keys and personal keys. */
+export const anthropicHeaders = (key: string): Record<string, string> => {
+  const workspace = process.env.ANTHROPIC_WORKSPACE_ID?.trim();
+  return {
+    "x-api-key": key,
+    "anthropic-version": "2023-06-01",
+    ...(workspace ? { "anthropic-workspace-id": workspace } : {}),
+  };
+};
diff --git a/site/src/server/health.ts b/site/src/server/health.ts
index 9317bbd..562ad4e 100644
--- a/site/src/server/health.ts
+++ b/site/src/server/health.ts
@@ -7,6 +7,7 @@ import {
 import { threadStore, type ThreadStore } from "./thread-store";
 import { managedBackendUrl, managedStreamHeaders } from "./managed-backend";
 import { demoLimits } from "./demo-policy";
+import { anthropicHeaders } from "./anthropic";
 
 type Check = {
   name: string;
@@ -112,7 +113,7 @@ export const checkHost = async (
       return await probe(
         "Model",
         url,
-        { "x-api-key": key, "anthropic-version": "2023-06-01" },
+        anthropicHeaders(key),
         fetcher,
       );
     } catch {