diff --git a/src/lib/ai/__tests__/provider-admin-host.test.ts b/src/lib/ai/__tests__/provider-admin-host.test.ts
new file mode 100644
index 000000000..d3ee44003
--- /dev/null
+++ b/src/lib/ai/__tests__/provider-admin-host.test.ts
@@ -0,0 +1,220 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+/**
+ * Client selection for the operator's global AI provider.
+ *
+ * The claim this file keeps true: the admin base URL decides which wire the
+ * operator's key rides. An Anthropic host gets the Anthropic Messages wire,
+ * everything else keeps the OpenAI chat-completions wire — including an
+ * OpenAI-compatible endpoint the operator allowlisted, an unset base URL and
+ * a value that does not parse as a URL. Every case is asserted at the wire:
+ * the test resolves a provider and runs a completion through a stubbed
+ * transport, so what is checked is the request that would leave the process.
+ */
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    user: { findUnique: vi.fn(), update: vi.fn() },
+    appSettings: { findUnique: vi.fn() },
+  },
+}));
+vi.mock("@/lib/crypto", () => ({
+  decrypt: vi.fn((v: string) => `decrypted:${v}`),
+  encrypt: vi.fn((v: string) => `encrypted:${v}`),
+}));
+vi.mock("@/lib/ai/codex-oauth", () => ({
+  refreshDeviceTokens: vi.fn(),
+  encryptCodexCreds: vi.fn(),
+  decryptCodexCreds: vi.fn(),
+}));
+vi.mock("@/lib/logging/context", () => ({
+  annotate: vi.fn(),
+  getEvent: vi.fn(() => undefined),
+}));
+vi.mock("undici", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("undici")>();
+  return {
+    ...actual,
+    fetch: (input: unknown, init?: unknown) =>
+      (globalThis.fetch as unknown as (i: unknown, n?: unknown) => unknown)(
+        input,
+        init,
+      ),
+  };
+});
+
+import { resolveProvider } from "../provider";
+import { singleUserTurn } from "../types";
+import { prisma } from "@/lib/db";
+
+/** A user with nothing configured, so resolution falls through to the admin key. */
+function emptyUserRow() {
+  return {
+    aiProvider: null,
+    aiModel: null,
+    aiBaseUrl: null,
+    aiAnthropicKeyEncrypted: null,
+    aiLocalKeyEncrypted: null,
+    aiOpenaiKeyEncrypted: null,
+    aiCompatBaseUrl: null,
+    aiCompatKeyEncrypted: null,
+    aiCompatModel: null,
+    aiProviderChain: null,
+    useCentralCodex: false,
+    codexConnectionStatus: null,
+    codexAccessTokenEncrypted: null,
+    codexRefreshTokenEncrypted: null,
+  } as never;
+}
+
+function adminSettings(overrides: Record<string, unknown> = {}) {
+  return {
+    id: "singleton",
+    adminAiKeyEncrypted: "enc-admin-key",
+    adminAiModel: null,
+    adminAiBaseUrl: null,
+    ...overrides,
+  } as never;
+}
+
+/**
+ * One transport stub for both wires: the OpenAI body reads `choices`, the
+ * Anthropic body reads `content`, and each client ignores the other's field.
+ */
+function stubTransport() {
+  const fetchMock = vi.fn().mockResolvedValue({
+    ok: true,
+    json: () =>
+      Promise.resolve({
+        choices: [{ message: { content: '{"ok":true}' } }],
+        usage: { total_tokens: 1 },
+        content: [{ type: "text", text: '{"ok":true}' }],
+      }),
+  });
+  vi.stubGlobal("fetch", fetchMock);
+  return fetchMock;
+}
+
+const probe = () =>
+  singleUserTurn({ system: "s", user: "u", responseFormat: "json" });
+
+beforeEach(() => {
+  vi.resetAllMocks();
+  vi.mocked(prisma.user.findUnique).mockResolvedValue(emptyUserRow());
+});
+
+describe("the admin base URL picks the client", () => {
+  it("sends an Anthropic base URL down the Anthropic wire", async () => {
+    const fetchMock = stubTransport();
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue(
+      adminSettings({
+        adminAiBaseUrl: "https://api.anthropic.com/v1",
+        adminAiModel: "claude-sonnet-4-6",
+      }),
+    );
+
+    const provider = await resolveProvider("u-1");
+    expect(provider.type).toBe("anthropic");
+
+    const result = await provider.generateCompletion(probe());
+    const [url, init] = fetchMock.mock.calls[0];
+    expect(url).toBe("https://api.anthropic.com/v1/messages");
+    expect(init.headers["x-api-key"]).toBe("decrypted:enc-admin-key");
+    expect(JSON.parse(init.body).model).toBe("claude-sonnet-4-6");
+    expect(result.providerType).toBe("anthropic");
+  });
+
+  it("treats a subdomain of anthropic.com as Anthropic, case-insensitively", async () => {
+    const fetchMock = stubTransport();
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue(
+      adminSettings({
+        adminAiBaseUrl: "https://API.EU.Anthropic.COM/v1",
+        adminAiModel: "claude-sonnet-4-6",
+      }),
+    );
+
+    const provider = await resolveProvider("u-1");
+    expect(provider.type).toBe("anthropic");
+
+    await provider.generateCompletion(probe());
+    // The base URL rides through verbatim; only the host MATCH folds case.
+    expect(fetchMock.mock.calls[0][0]).toBe(
+      "https://API.EU.Anthropic.COM/v1/messages",
+    );
+  });
+
+  it("keeps the OpenAI wire for the OpenAI host", async () => {
+    const fetchMock = stubTransport();
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue(
+      adminSettings({
+        adminAiBaseUrl: "https://api.openai.com/v1",
+        adminAiModel: "gpt-4o-mini",
+      }),
+    );
+
+    const provider = await resolveProvider("u-1");
+    expect(provider.type).toBe("admin-key");
+
+    await provider.generateCompletion(probe());
+    const [url, init] = fetchMock.mock.calls[0];
+    expect(url).toBe("https://api.openai.com/v1/chat/completions");
+    expect(init.headers.Authorization).toBe("Bearer decrypted:enc-admin-key");
+    expect(JSON.parse(init.body).model).toBe("gpt-4o-mini");
+  });
+
+  it("keeps the OpenAI wire for an OpenAI-compatible endpoint", async () => {
+    const fetchMock = stubTransport();
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue(
+      adminSettings({
+        adminAiBaseUrl: "https://llm.example.com/v1",
+        adminAiModel: "llama-3.1-70b",
+      }),
+    );
+
+    const provider = await resolveProvider("u-1");
+    expect(provider.type).toBe("admin-key");
+
+    await provider.generateCompletion(probe());
+    expect(fetchMock.mock.calls[0][0]).toBe(
+      "https://llm.example.com/v1/chat/completions",
+    );
+    expect(JSON.parse(fetchMock.mock.calls[0][1].body).model).toBe(
+      "llama-3.1-70b",
+    );
+  });
+
+  it("keeps the OpenAI default when no base URL is set", async () => {
+    const fetchMock = stubTransport();
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue(adminSettings());
+
+    const provider = await resolveProvider("u-1");
+    expect(provider.type).toBe("admin-key");
+
+    await provider.generateCompletion(probe());
+    const [url, init] = fetchMock.mock.calls[0];
+    expect(url).toBe("https://api.openai.com/v1/chat/completions");
+    expect(JSON.parse(init.body).model).toBe("gpt-4o");
+  });
+
+  it("keeps the OpenAI path for a base URL that does not parse", async () => {
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue(
+      adminSettings({ adminAiBaseUrl: "api.anthropic.com" }),
+    );
+
+    const provider = await resolveProvider("u-1");
+    expect(provider.type).toBe("admin-key");
+  });
+
+  it("defaults the model to a Claude model on the Anthropic wire", async () => {
+    const fetchMock = stubTransport();
+    vi.mocked(prisma.appSettings.findUnique).mockResolvedValue(
+      adminSettings({ adminAiBaseUrl: "https://api.anthropic.com/v1" }),
+    );
+
+    const provider = await resolveProvider("u-1");
+    await provider.generateCompletion(probe());
+    expect(JSON.parse(fetchMock.mock.calls[0][1].body).model).toBe(
+      "claude-sonnet-4-6",
+    );
+  });
+});
diff --git a/src/lib/ai/provider.ts b/src/lib/ai/provider.ts
index 33e2228fb..017e4b4fe 100644
--- a/src/lib/ai/provider.ts
+++ b/src/lib/ai/provider.ts
@@ -142,16 +142,65 @@ function buildUserProvider(row: UserAIRow): AIProvider | null {
   }
 }
 
+/**
+ * True when a base URL addresses Anthropic's API: the host itself or any
+ * subdomain of `anthropic.com`, case-insensitively. Parsed rather than
+ * substring-matched, so `https://anthropic.com.attacker.example/v1` and
+ * `https://evil.example/?x=api.anthropic.com` both answer false.
+ *
+ * An unparseable value answers false and keeps the OpenAI path, which is the
+ * behaviour that value had before this function existed.
+ */
+function isAnthropicBaseUrl(baseUrl: string): boolean {
+  let host: string;
+  try {
+    host = new URL(baseUrl).hostname.toLowerCase();
+  } catch {
+    return false;
+  }
+  return host === "anthropic.com" || host.endsWith(".anthropic.com");
+}
+
+/**
+ * The operator's global provider — one key, one model, one base URL in the
+ * admin settings, serving every user who has configured nothing themselves.
+ *
+ * The base URL decides the wire. An operator who fills these fields with an
+ * Anthropic endpoint and an Anthropic key used to get OpenAI-shaped requests
+ * posted at Anthropic, which fails every call with nothing pointing at the
+ * cause; the host check routes them to the same `AnthropicClient` the
+ * per-user `ANTHROPIC` arm builds. Every other host stays on `OpenAIClient`,
+ * which is also the right client for an OpenAI-compatible endpoint (a local
+ * server, a gateway) — the admin base URL is host-allowlisted at write time
+ * in `/api/admin/ai-settings`.
+ *
+ * The chain still reports this entry as `admin-openai` whichever client it
+ * builds. That tag names the SLOT — "the operator's shared credential, last
+ * in the chain" — not the wire: it is persisted in `User.aiProviderChain`,
+ * offered in the chain editor, and pinned by the OpenAPI enum, so splitting
+ * it per wire would rewrite stored chains and widen a structural allowlist to
+ * express something the runtime provider type (`admin-key` / `anthropic` on
+ * the instance, which is what the wide events and the budget classifier read)
+ * already carries.
+ */
 async function resolveAdminProvider(): Promise<AIProvider> {
   const settings = await prisma.appSettings.findUnique({
     where: { id: "singleton" },
   });
 
   if (settings?.adminAiKeyEncrypted) {
+    const baseUrl = settings.adminAiBaseUrl ?? "https://api.openai.com/v1";
+    if (isAnthropicBaseUrl(baseUrl)) {
+      return new AnthropicClient({
+        apiKey: decrypt(settings.adminAiKeyEncrypted),
+        model: settings.adminAiModel ?? "claude-sonnet-4-6",
+        baseUrl,
+      });
+    }
     return new OpenAIClient({
       apiKey: decrypt(settings.adminAiKeyEncrypted),
       model: settings.adminAiModel ?? "gpt-4o",
-      baseUrl: settings.adminAiBaseUrl ?? "https://api.openai.com/v1",
+      baseUrl,
     });
   }