diff --git a/canvas/src/components/tabs/ConfigTab.tsx b/canvas/src/components/tabs/ConfigTab.tsx
index d8a2d4f1e..199434eb0 100644
--- a/canvas/src/components/tabs/ConfigTab.tsx
+++ b/canvas/src/components/tabs/ConfigTab.tsx
@@ -288,6 +288,40 @@ export function deriveProvidersFromModels(models: ModelSpec[]): string[] {
   return out;
 }
 
+// billingModeForProvider — maps a selected PROVIDER (vendor key) to the
+// LLM billing_mode it implies (internal#703 Gap 2).
+//
+// Today, picking a non-Platform provider in the Config tab writes the
+// credential env (CLAUDE_CODE_OAUTH_TOKEN / vendor key) but leaves
+// llm_billing_mode at its resolved default (`platform_managed`). The CP
+// tenant_config endpoint then keeps injecting the platform proxy base
+// URLs, so the OAuth token / vendor key is never actually used — BYOK
+// silently no-ops (the live SEO-Agent symptom in #703). The workspace-
+// server even hard-blocks vendor-key writes on platform_managed
+// workspaces (secrets.go:87), pointing the user at this exact billing-
+// mode switch. Wiring the provider change to also set billing_mode is
+// the UI half that makes BYOK take (the CP/workspace-server backend half
+// is being fixed in parallel — internal#703 Gap 1).
+//
+// Mapping:
+//   - "platform" (the Platform-managed proxy) OR "" (no explicit
+//     provider override → inherit, defaults to platform) → "platform_managed".
+//   - any other vendor key ("anthropic-oauth" = Claude Code subscription
+//     OAuth, "anthropic" = Anthropic API key, "minimax", "openrouter",
+//     etc.) → "byok".
+//
+// Returns the billing_mode string the PUT body should carry. The valid
+// set is fixed by workspace-server's recognizer (platform_managed | byok
+// | disabled); "disabled" is never auto-selected by a provider choice —
+// it's an explicit operator action via the LLM Billing section.
+export type LLMBillingMode = "platform_managed" | "byok";
+
+export function billingModeForProvider(provider: string): LLMBillingMode {
+  const v = provider.trim().toLowerCase();
+  if (v === "" || v === "platform") return "platform_managed";
+  return "byok";
+}
+
 // Fallback used when /templates can't be fetched (offline, older backend).
 // Keep in sync with manifest.json workspace_templates as a defensive default.
 // Model + env suggestions only flow when the backend is reachable.
@@ -702,6 +736,36 @@ export function ConfigTab({ workspaceId }: Props) {
         }
       }
 
+      // Provider → billing_mode linkage (internal#703 Gap 2). When the
+      // provider actually changed AND its implied billing_mode differs
+      // from the previously-selected provider's, push the new mode to
+      // the per-tenant llm-billing-mode endpoint (same path the LLM
+      // Billing section uses). Without this, selecting a non-Platform
+      // provider leaves billing_mode=platform_managed → CP keeps
+      // injecting the platform proxy → BYOK never takes.
+      //
+      // Gated on (a) the provider PUT having succeeded — no point setting
+      // byok if the credential write failed — and (b) the mode actually
+      // changing, so an unrelated provider tweak between two BYOK vendors
+      // (e.g. minimax → openrouter) doesn't re-issue a redundant
+      // platform_managed→byok PUT and trigger a needless restart.
+      let billingModeSaveError: string | null = null;
+      if (providerChanged && !providerSaveError) {
+        const nextMode = billingModeForProvider(provider);
+        const prevMode = billingModeForProvider(originalProvider);
+        if (nextMode !== prevMode) {
+          try {
+            await api.put(
+              `/admin/workspaces/${workspaceId}/llm-billing-mode`,
+              { mode: nextMode },
+            );
+          } catch (e) {
+            billingModeSaveError =
+              e instanceof Error ? e.message : "Billing mode update was rejected";
+          }
+        }
+      }
+
       setOriginalYaml(content);
       if (rawMode) {
         const parsed = parseYaml(content);
@@ -721,16 +785,22 @@ export function ConfigTab({ workspaceId }: Props) {
       } else if (!restart) {
         useCanvasStore.getState().updateNodeData(workspaceId, { needsRestart: !providerWillAutoRestart });
       }
-      // Aggregate partial-save errors. Both modelSaveError and
-      // providerSaveError describe rejected updates from independent
-      // endpoints — show whichever fired so the user knows which
-      // field reverts on next reload (otherwise they'd see "Saved" and
-      // be confused why Provider snapped back).
+      // Aggregate partial-save errors. modelSaveError, providerSaveError,
+      // and billingModeSaveError describe rejected updates from
+      // independent endpoints — show whichever fired so the user knows
+      // which field reverts on next reload (otherwise they'd see "Saved"
+      // and be confused why Provider snapped back). The billing-mode case
+      // is the most important to surface: the provider credential saved
+      // but BYOK won't actually take until billing_mode flips, so a
+      // silent failure here is exactly the #703 "selecting a provider has
+      // no effect" symptom.
       const partialError = providerSaveError
         ? `Other fields saved, but provider update failed: ${providerSaveError}`
-        : modelSaveError
-          ? `Other fields saved, but model update failed: ${modelSaveError}`
-          : null;
+        : billingModeSaveError
+          ? `Provider saved, but switching billing mode failed — your own provider key/OAuth may not take effect until billing mode is set: ${billingModeSaveError}`
+          : modelSaveError
+            ? `Other fields saved, but model update failed: ${modelSaveError}`
+            : null;
       if (partialError) {
         setError(partialError);
       } else {
diff --git a/canvas/src/components/tabs/__tests__/ConfigTab.billingMode.test.tsx b/canvas/src/components/tabs/__tests__/ConfigTab.billingMode.test.tsx
new file mode 100644
index 000000000..b37d26301
--- /dev/null
+++ b/canvas/src/components/tabs/__tests__/ConfigTab.billingMode.test.tsx
@@ -0,0 +1,255 @@
+// @vitest-environment jsdom
+//
+// Tests for the provider → llm_billing_mode linkage (internal#703 Gap 2).
+//
+// What this pins: when the operator changes the PROVIDER in the Config
+// tab, the workspace's llm_billing_mode must follow — a non-Platform
+// provider sets billing_mode=byok; Platform sets platform_managed. Before
+// this wiring, selecting "Claude Code subscription (OAuth)" or any vendor
+// key wrote the credential env but left billing_mode=platform_managed, so
+// CP kept injecting the platform proxy base URL and the OAuth token /
+// vendor key was never used — BYOK silently no-op'd (the live jrs-auto
+// SEO-Agent symptom in #703).
+//
+// The billing-mode PUT targets the same per-tenant endpoint the LLM
+// Billing section uses: PUT /admin/workspaces/:id/llm-billing-mode with
+// body {mode: "byok" | "platform_managed"}.
+
+import { describe, it, expect, vi, afterEach, beforeEach } from "vitest";
+import { render, screen, cleanup, waitFor, fireEvent } from "@testing-library/react";
+import React from "react";
+
+afterEach(cleanup);
+
+const apiGet = vi.fn();
+const apiPatch = vi.fn();
+const apiPut = vi.fn();
+vi.mock("@/lib/api", () => ({
+  api: {
+    get: (path: string) => apiGet(path),
+    patch: (path: string, body: unknown) => apiPatch(path, body),
+    put: (path: string, body: unknown) => apiPut(path, body),
+    post: vi.fn(),
+    del: vi.fn(),
+  },
+}));
+
+const storeUpdateNodeData = vi.fn();
+const storeRestartWorkspace = vi.fn();
+vi.mock("@/store/canvas", () => ({
+  useCanvasStore: Object.assign(
+    (selector: (s: unknown) => unknown) =>
+      selector({ restartWorkspace: storeRestartWorkspace, updateNodeData: storeUpdateNodeData }),
+    {
+      getState: () => ({
+        restartWorkspace: storeRestartWorkspace,
+        updateNodeData: storeUpdateNodeData,
+      }),
+    },
+  ),
+}));
+
+vi.mock("../AgentCardSection", () => ({
+  AgentCardSection: () => <div data-testid="agent-card-stub" />,
+}));
+
+import { ConfigTab, billingModeForProvider } from "../ConfigTab";
+
+function wireApi(opts: { providerValue?: string | "missing" }) {
+  apiGet.mockImplementation((path: string) => {
+    if (path === `/workspaces/ws-test`) {
+      return Promise.resolve({ runtime: "hermes" });
+    }
+    if (path === `/workspaces/ws-test/model`) {
+      return Promise.resolve({ model: "nousresearch/hermes-4-70b" });
+    }
+    if (path === `/workspaces/ws-test/provider`) {
+      if (opts.providerValue === "missing") return Promise.reject(new Error("404"));
+      return Promise.resolve({
+        provider: opts.providerValue ?? "",
+        source: opts.providerValue ? "workspace_secrets" : "default",
+      });
+    }
+    if (path === `/workspaces/ws-test/files/config.yaml`) {
+      return Promise.resolve({ content: "name: ws\nruntime: hermes\n" });
+    }
+    if (path === "/templates") return Promise.resolve([]);
+    return Promise.reject(new Error(`unmocked api.get: ${path}`));
+  });
+}
+
+function billingModeCalls() {
+  return apiPut.mock.calls.filter(
+    ([path]) => path === "/admin/workspaces/ws-test/llm-billing-mode",
+  );
+}
+
+beforeEach(() => {
+  apiGet.mockReset();
+  apiPatch.mockReset();
+  apiPut.mockReset();
+  storeUpdateNodeData.mockReset();
+  storeRestartWorkspace.mockReset();
+});
+
+describe("billingModeForProvider — pure mapping (internal#703 Gap 2)", () => {
+  // Platform / empty → platform_managed. Empty means "no explicit
+  // override → inherit", which resolves to platform on the backend, so
+  // it must NOT flip the workspace into byok.
+  it("maps Platform and empty to platform_managed", () => {
+    expect(billingModeForProvider("platform")).toBe("platform_managed");
+    expect(billingModeForProvider("")).toBe("platform_managed");
+    expect(billingModeForProvider("  ")).toBe("platform_managed");
+    expect(billingModeForProvider("PLATFORM")).toBe("platform_managed");
+  });
+
+  // Every non-Platform provider → byok. If this regresses to returning
+  // platform_managed for a vendor, BYOK silently no-ops again (#703).
+  it("maps non-Platform providers to byok", () => {
+    expect(billingModeForProvider("anthropic-oauth")).toBe("byok"); // Claude Code subscription
+    expect(billingModeForProvider("anthropic")).toBe("byok"); // Anthropic API key
+    expect(billingModeForProvider("minimax")).toBe("byok");
+    expect(billingModeForProvider("openrouter")).toBe("byok");
+    expect(billingModeForProvider("openai")).toBe("byok");
+  });
+});
+
+describe("ConfigTab — provider change drives billing_mode (internal#703 Gap 2)", () => {
+  // The core fix: picking a non-Platform provider (here "anthropic-oauth"
+  // = Claude Code subscription OAuth) from a fresh/empty provider must
+  // PUT mode=byok to the per-tenant llm-billing-mode endpoint. This is
+  // the exact path that was missing — the credential env saved but the
+  // billing mode never followed, so the proxy stayed engaged.
+  it("PUTs mode=byok when switching to a non-Platform provider", async () => {
+    wireApi({ providerValue: "" });
+    apiPut.mockResolvedValue({ status: "saved" });
+
+    render(<ConfigTab workspaceId="ws-test" />);
+    const input = await screen.findByTestId("provider-input");
+    fireEvent.change(input, { target: { value: "anthropic-oauth" } });
+
+    fireEvent.click(screen.getByRole("button", { name: /^save$/i }));
+
+    await waitFor(() => {
+      const calls = billingModeCalls();
+      expect(calls.length).toBe(1);
+      expect(calls[0][1]).toEqual({ mode: "byok" });
+    });
+    // Provider credential PUT still happens too (independent endpoint).
+    expect(
+      apiPut.mock.calls.some(([path]) => path === "/workspaces/ws-test/provider"),
+    ).toBe(true);
+  });
+
+  // Switching FROM a byok provider back TO Platform must PUT
+  // mode=platform_managed so the workspace re-engages the proxy and stops
+  // expecting a (now-absent) vendor key.
+  it("PUTs mode=platform_managed when switching back to Platform", async () => {
+    wireApi({ providerValue: "anthropic-oauth" });
+    apiPut.mockResolvedValue({ status: "saved" });
+
+    render(<ConfigTab workspaceId="ws-test" />);
+    const input = await screen.findByTestId("provider-input");
+    await waitFor(() => expect((input as HTMLInputElement).value).toBe("anthropic-oauth"));
+    fireEvent.change(input, { target: { value: "platform" } });
+
+    fireEvent.click(screen.getByRole("button", { name: /^save$/i }));
+
+    await waitFor(() => {
+      const calls = billingModeCalls();
+      expect(calls.length).toBe(1);
+      expect(calls[0][1]).toEqual({ mode: "platform_managed" });
+    });
+  });
+
+  // Changing between two BYOK vendors (minimax → openrouter) keeps
+  // billing_mode=byok — the implied mode is unchanged, so re-PUTing it
+  // would be a wasteful no-op that risks an extra restart. Must NOT fire.
+  it("does NOT PUT billing-mode when the implied mode is unchanged", async () => {
+    wireApi({ providerValue: "minimax" });
+    apiPut.mockResolvedValue({ status: "saved" });
+
+    render(<ConfigTab workspaceId="ws-test" />);
+    const input = await screen.findByTestId("provider-input");
+    await waitFor(() => expect((input as HTMLInputElement).value).toBe("minimax"));
+    fireEvent.change(input, { target: { value: "openrouter" } });
+
+    fireEvent.click(screen.getByRole("button", { name: /^save$/i }));
+
+    await waitFor(() => {
+      // Provider PUT fires (vendor changed)...
+      expect(
+        apiPut.mock.calls.some(([path]) => path === "/workspaces/ws-test/provider"),
+      ).toBe(true);
+    });
+    // ...but billing-mode does NOT (byok → byok is a no-op).
+    expect(billingModeCalls().length).toBe(0);
+  });
+
+  // A Save that doesn't touch the provider must not PUT billing-mode —
+  // editing tier/name shouldn't disturb the workspace's billing mode.
+  it("does NOT PUT billing-mode on a Save that leaves provider unchanged", async () => {
+    wireApi({ providerValue: "anthropic-oauth" });
+    apiPut.mockResolvedValue({ status: "saved" });
+
+    render(<ConfigTab workspaceId="ws-test" />);
+    await screen.findByTestId("provider-input");
+
+    // Dirty an unrelated field so Save is enabled.
+    const tierSelect = screen.getByLabelText(/tier/i) as HTMLSelectElement;
+    fireEvent.change(tierSelect, { target: { value: "3" } });
+
+    fireEvent.click(screen.getByRole("button", { name: /^save$/i }));
+
+    await waitFor(() => {
+      // Some PUT may fire (e.g. /model); just assert billing-mode did not.
+      expect(billingModeCalls().length).toBe(0);
+    });
+  });
+
+  // If the provider credential PUT itself fails, we must NOT set byok —
+  // flipping billing_mode while the credential write failed would leave
+  // the workspace expecting a key it doesn't have (worse than no-op).
+  it("does NOT PUT billing-mode when the provider PUT fails", async () => {
+    wireApi({ providerValue: "" });
+    apiPut.mockImplementation((path: string) => {
+      if (path === "/workspaces/ws-test/provider") return Promise.reject(new Error("boom"));
+      return Promise.resolve({ status: "saved" });
+    });
+
+    render(<ConfigTab workspaceId="ws-test" />);
+    const input = await screen.findByTestId("provider-input");
+    fireEvent.change(input, { target: { value: "anthropic-oauth" } });
+
+    fireEvent.click(screen.getByRole("button", { name: /^save$/i }));
+
+    await waitFor(() => {
+      // The provider-failure error is surfaced (getByText throws if absent).
+      expect(screen.getByText(/provider update failed/i)).toBeTruthy();
+    });
+    expect(billingModeCalls().length).toBe(0);
+  });
+
+  // If the credential saved but the billing-mode PUT is rejected, the
+  // user must be warned that BYOK may not take — a silent failure here
+  // is precisely the #703 symptom we're fixing.
+  it("surfaces an error when billing-mode PUT fails after a successful provider save", async () => {
+    wireApi({ providerValue: "" });
+    apiPut.mockImplementation((path: string) => {
+      if (path === "/admin/workspaces/ws-test/llm-billing-mode") {
+        return Promise.reject(new Error("403 forbidden"));
+      }
+      return Promise.resolve({ status: "saved" });
+    });
+
+    render(<ConfigTab workspaceId="ws-test" />);
+    const input = await screen.findByTestId("provider-input");
+    fireEvent.change(input, { target: { value: "anthropic-oauth" } });
+
+    fireEvent.click(screen.getByRole("button", { name: /^save$/i }));
+
+    await waitFor(() => {
+      expect(screen.getByText(/switching billing mode failed/i)).toBeTruthy();
+    });
+  });
+});