diff --git a/canvas/src/components/tabs/ConfigTab.tsx b/canvas/src/components/tabs/ConfigTab.tsx
index 619cee0f5..d8a2d4f1e 100644
--- a/canvas/src/components/tabs/ConfigTab.tsx
+++ b/canvas/src/components/tabs/ConfigTab.tsx
@@ -6,6 +6,7 @@ import { useCanvasStore } from "@/store/canvas";
 import { type ConfigData, DEFAULT_CONFIG, TextInput, NumberInput, Toggle, TagList, Section } from "./config/form-inputs";
 import { parseYaml, toYaml } from "./config/yaml-utils";
 import { SecretsSection } from "./config/secrets-section";
+import { LLMBillingSection } from "./config/llm-billing-section";
 import { ExternalConnectionSection } from "./ExternalConnectionSection";
 import {
   ProviderModelSelector,
@@ -1108,6 +1109,8 @@ export function ConfigTab({ workspaceId }: Props) {
             </div>
           </Section>
 
+          <LLMBillingSection workspaceId={workspaceId} />
+
           <SecretsSection
             workspaceId={workspaceId}
             requiredEnv={config.runtime_config?.required_env}
diff --git a/canvas/src/components/tabs/config/__tests__/llm-billing-section.test.tsx b/canvas/src/components/tabs/config/__tests__/llm-billing-section.test.tsx
new file mode 100644
index 000000000..1875f8a9c
--- /dev/null
+++ b/canvas/src/components/tabs/config/__tests__/llm-billing-section.test.tsx
@@ -0,0 +1,176 @@
+// @vitest-environment jsdom
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import {
+  render,
+  screen,
+  waitFor,
+  cleanup,
+  fireEvent,
+} from "@testing-library/react";
+import { LLMBillingSection } from "../llm-billing-section";
+
+// Tests for LLMBillingSection (internal#691). Locks in:
+//  - the section renders the resolved mode + source label
+//  - the dropdown maps "inherit" → PUT {mode: null}
+//  - the dropdown maps "byok" → PUT {mode: "byok"}
+//  - a garbled override surfaces the warning banner
+//  - the post-write resolution updates the UI without a refetch
+
+const apiGet = vi.fn();
+const apiPut = vi.fn();
+
+vi.mock("@/lib/api", () => ({
+  api: {
+    get: (...args: unknown[]) => apiGet(...args),
+    put: (...args: unknown[]) => apiPut(...args),
+    post: vi.fn().mockResolvedValue({}),
+    del: vi.fn().mockResolvedValue({}),
+    patch: vi.fn().mockResolvedValue({}),
+  },
+}));
+
+// Collapsed-by-default Section wrapper would hide the content; replace
+// it with a passthrough so the dropdown is reachable in the test DOM.
+vi.mock("../form-inputs", async () => {
+  const actual = await vi.importActual<typeof import("../form-inputs")>(
+    "../form-inputs",
+  );
+  return {
+    ...actual,
+    Section: ({ children }: { children: React.ReactNode }) => (
+      <div>{children}</div>
+    ),
+  };
+});
+
+beforeEach(() => {
+  vi.clearAllMocks();
+});
+
+afterEach(() => {
+  cleanup();
+});
+
+describe("LLMBillingSection — internal#691", () => {
+  it("renders the resolved mode + source for an inherited workspace", async () => {
+    apiGet.mockResolvedValueOnce({
+      workspace_id: "ws-1",
+      resolved_mode: "platform_managed",
+      workspace_override: null,
+      org_default: "platform_managed",
+      source: "org_default",
+    });
+
+    render(<LLMBillingSection workspaceId="ws-1" />);
+
+    await waitFor(() => {
+      expect(apiGet).toHaveBeenCalledWith(
+        "/admin/workspaces/ws-1/llm-billing-mode",
+      );
+    });
+    // Resolved mode appears.
+    expect(screen.getByText(/Resolved mode:/i).textContent).toMatch(/platform_managed/);
+    // Source label appears.
+    expect(
+      screen.getByText(/inherited from org default/i),
+    ).toBeTruthy();
+  });
+
+  it('PUTs {mode: "byok"} when user picks BYOK and reflects the new resolution', async () => {
+    apiGet.mockResolvedValueOnce({
+      workspace_id: "ws-2",
+      resolved_mode: "platform_managed",
+      workspace_override: null,
+      org_default: "platform_managed",
+      source: "org_default",
+    });
+    apiPut.mockResolvedValueOnce({
+      workspace_id: "ws-2",
+      resolved_mode: "byok",
+      workspace_override: "byok",
+      org_default: "platform_managed",
+      source: "workspace_override",
+    });
+
+    render(<LLMBillingSection workspaceId="ws-2" />);
+    await waitFor(() => expect(apiGet).toHaveBeenCalled());
+
+    const select = (await screen.findByLabelText(
+      /llm billing mode override/i,
+    )) as HTMLSelectElement;
+    fireEvent.change(select, { target: { value: "byok" } });
+
+    await waitFor(() => {
+      expect(apiPut).toHaveBeenCalledWith(
+        "/admin/workspaces/ws-2/llm-billing-mode",
+        { mode: "byok" },
+      );
+    });
+    // Post-write resolution propagated to UI.
+    await waitFor(() => {
+      expect(
+        screen.getByText(/explicit override on this workspace/i),
+      ).toBeTruthy();
+    });
+  });
+
+  it("PUTs {mode: null} when user picks Inherit (clears the override)", async () => {
+    apiGet.mockResolvedValueOnce({
+      workspace_id: "ws-3",
+      resolved_mode: "byok",
+      workspace_override: "byok",
+      org_default: "platform_managed",
+      source: "workspace_override",
+    });
+    apiPut.mockResolvedValueOnce({
+      workspace_id: "ws-3",
+      resolved_mode: "platform_managed",
+      workspace_override: null,
+      org_default: "platform_managed",
+      source: "org_default",
+    });
+
+    render(<LLMBillingSection workspaceId="ws-3" />);
+    await waitFor(() => expect(apiGet).toHaveBeenCalled());
+
+    const select = (await screen.findByLabelText(
+      /llm billing mode override/i,
+    )) as HTMLSelectElement;
+    fireEvent.change(select, { target: { value: "inherit" } });
+
+    await waitFor(() => {
+      expect(apiPut).toHaveBeenCalledWith(
+        "/admin/workspaces/ws-3/llm-billing-mode",
+        { mode: null },
+      );
+    });
+  });
+
+  it("surfaces a warning banner when the override value is garbled", async () => {
+    apiGet.mockResolvedValueOnce({
+      workspace_id: "ws-4",
+      resolved_mode: "platform_managed", // resolver fell through, default-closed
+      workspace_override: "byokk", // typo persisted somehow
+      org_default: "platform_managed",
+      source: "org_default",
+    });
+
+    render(<LLMBillingSection workspaceId="ws-4" />);
+
+    await waitFor(() => {
+      expect(
+        screen.getByText(/non-standard value/i),
+      ).toBeTruthy();
+    });
+  });
+
+  it("renders an error banner when the GET fails", async () => {
+    apiGet.mockRejectedValueOnce(new Error("network down"));
+
+    render(<LLMBillingSection workspaceId="ws-5" />);
+
+    await waitFor(() => {
+      expect(screen.getByText(/network down/i)).toBeTruthy();
+    });
+  });
+});
diff --git a/canvas/src/components/tabs/config/index.ts b/canvas/src/components/tabs/config/index.ts
index 6f89e3564..5515c2599 100644
--- a/canvas/src/components/tabs/config/index.ts
+++ b/canvas/src/components/tabs/config/index.ts
@@ -1,3 +1,4 @@
 export { type ConfigData, DEFAULT_CONFIG, TextInput, NumberInput, Toggle, TagList, Section } from "./form-inputs";
 export { parseYaml, toYaml } from "./yaml-utils";
 export { SecretsSection } from "./secrets-section";
+export { LLMBillingSection } from "./llm-billing-section";
diff --git a/canvas/src/components/tabs/config/llm-billing-section.tsx b/canvas/src/components/tabs/config/llm-billing-section.tsx
new file mode 100644
index 000000000..16498258e
--- /dev/null
+++ b/canvas/src/components/tabs/config/llm-billing-section.tsx
@@ -0,0 +1,219 @@
+"use client";
+
+// llm-billing-section.tsx — Config-tab section for the per-workspace
+// llm_billing_mode override (internal#691).
+//
+// Surfaces:
+//   - The currently RESOLVED mode for this workspace (the mode the
+//     workspace-server's strip gate will use at next provision).
+//   - The org-level default (so the user sees what they're inheriting).
+//   - A dropdown to set / clear the workspace-level override.
+//   - A "source" line so operators can answer "is this inherited or
+//     explicit?" without DB archeology (RFC Observability hot-spot).
+//
+// Hits:
+//   GET /admin/workspaces/:id/llm-billing-mode   — read resolution
+//   PUT /admin/workspaces/:id/llm-billing-mode   — write {mode: "..."|null}
+//
+// Both routes are on the per-tenant workspace-server (same origin as the
+// other canvas /admin calls). CP's proxy at /cp/admin/workspaces/:id/
+// llm-billing-mode exists for ops use; the canvas uses the per-tenant
+// path directly to keep the round-trip cheap.
+
+import { useState, useEffect, useCallback } from "react";
+import { api } from "@/lib/api";
+import { Section } from "./form-inputs";
+
+// Mirrors workspace-server/internal/handlers/llm_billing_mode.go::BillingModeResolution.
+// Kept as a literal shape (not imported) because canvas has no Go-type bridge.
+export interface BillingModeResolution {
+  workspace_id: string;
+  resolved_mode: "platform_managed" | "byok" | "disabled";
+  // Pointer-typed on the Go side: nil = inherit, non-nil = the raw
+  // workspace-level override (even if garbled and falling through).
+  workspace_override: string | null;
+  org_default: "platform_managed" | "byok" | "disabled";
+  source: "workspace_override" | "org_default" | "constant_fallback";
+}
+
+// The dropdown emits one of these values. "inherit" is the UX-only label
+// that maps to a `null` body in the PUT request.
+type DropdownChoice = "inherit" | "platform_managed" | "byok" | "disabled";
+
+interface Props {
+  workspaceId: string;
+}
+
+const MODE_LABELS: Record<DropdownChoice, string> = {
+  inherit: "Inherit from org default",
+  platform_managed: "Platform-managed (uses Molecule credits)",
+  byok: "BYOK (your own OAuth / vendor keys)",
+  disabled: "Disabled (no LLM access)",
+};
+
+const MODE_DESCRIPTIONS: Record<DropdownChoice, string> = {
+  inherit:
+    "Use whichever mode is set at the organization level. Recommended unless this specific workspace needs a different billing source.",
+  platform_managed:
+    "Strip CLAUDE_CODE_OAUTH_TOKEN and vendor API keys from the workspace; route all LLM traffic through Molecule's proxy and bill your org credits.",
+  byok:
+    "Keep CLAUDE_CODE_OAUTH_TOKEN / vendor API keys in the workspace; LLM traffic goes directly to your provider and is billed to your OAuth subscription or API account.",
+  disabled:
+    "Block all LLM access for this workspace. Useful for sandbox workspaces that should not consume credits or hit external providers.",
+};
+
+const SOURCE_LABELS: Record<BillingModeResolution["source"], string> = {
+  workspace_override: "explicit override on this workspace",
+  org_default: "inherited from org default",
+  constant_fallback:
+    "fallback (workspace + org defaults missing or unrecognized — defaulted to platform_managed)",
+};
+
+export function LLMBillingSection({ workspaceId }: Props) {
+  const [resolution, setResolution] = useState<BillingModeResolution | null>(
+    null,
+  );
+  const [loading, setLoading] = useState(true);
+  const [saving, setSaving] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+  const [success, setSuccess] = useState(false);
+
+  const load = useCallback(async () => {
+    setLoading(true);
+    setError(null);
+    try {
+      const res = await api.get<BillingModeResolution>(
+        `/admin/workspaces/${workspaceId}/llm-billing-mode`,
+      );
+      setResolution(res);
+    } catch (e) {
+      setError(e instanceof Error ? e.message : "Failed to load billing mode");
+    } finally {
+      setLoading(false);
+    }
+  }, [workspaceId]);
+
+  useEffect(() => {
+    void load();
+  }, [load]);
+
+  // Current dropdown selection is derived from the resolution. If the
+  // override is null, we show "inherit"; otherwise we mirror the raw
+  // workspace_override (NOT resolved_mode — that would conflate "explicit
+  // platform_managed override" with "inherit while org happens to be
+  // platform_managed", which has different semantics on the write side).
+  const currentChoice: DropdownChoice = (() => {
+    if (!resolution) return "inherit";
+    if (resolution.workspace_override == null) return "inherit";
+    const raw = resolution.workspace_override;
+    if (raw === "platform_managed" || raw === "byok" || raw === "disabled") {
+      return raw;
+    }
+    // Garbled value persisted via some external write. Show inherit so
+    // the user can pick a clean value; on save they'll either clear it
+    // (PUT null) or overwrite it with a valid one.
+    return "inherit";
+  })();
+
+  const handleChange = async (choice: DropdownChoice) => {
+    if (!resolution) return;
+    setSaving(true);
+    setError(null);
+    setSuccess(false);
+    try {
+      // "inherit" → PUT {mode: null}; otherwise → PUT {mode: choice}.
+      const body = choice === "inherit" ? { mode: null } : { mode: choice };
+      const updated = await api.put<BillingModeResolution>(
+        `/admin/workspaces/${workspaceId}/llm-billing-mode`,
+        body,
+      );
+      setResolution(updated);
+      setSuccess(true);
+      setTimeout(() => setSuccess(false), 2000);
+    } catch (e) {
+      setError(e instanceof Error ? e.message : "Failed to update billing mode");
+    } finally {
+      setSaving(false);
+    }
+  };
+
+  return (
+    <Section title="LLM Billing" defaultOpen={false}>
+      {loading && (
+        <div className="text-[10px] text-ink-mid">Loading billing mode…</div>
+      )}
+
+      {error && (
+        <div
+          role="alert"
+          aria-live="assertive"
+          className="px-2 py-1 bg-red-900/30 border border-red-800 rounded text-[10px] text-bad mb-2"
+        >
+          {error}
+        </div>
+      )}
+
+      {resolution && (
+        <div className="space-y-2">
+          <div className="text-[10px] text-ink-mid">
+            Resolved mode: <strong className="text-ink">{resolution.resolved_mode}</strong>{" "}
+            <span className="text-ink-mid">
+              ({SOURCE_LABELS[resolution.source]})
+            </span>
+          </div>
+          <div className="text-[10px] text-ink-mid">
+            Org default: <span className="text-ink">{resolution.org_default}</span>
+          </div>
+
+          <label
+            className="block text-[10px] text-ink-mid"
+            htmlFor={`llm-billing-mode-${workspaceId}`}
+          >
+            Override
+          </label>
+          <select
+            id={`llm-billing-mode-${workspaceId}`}
+            aria-label="LLM billing mode override"
+            value={currentChoice}
+            disabled={saving}
+            onChange={(e) => void handleChange(e.target.value as DropdownChoice)}
+            className="w-full bg-surface-card border border-line rounded p-1 text-[10px] text-ink focus:outline-none focus:border-accent disabled:opacity-50"
+          >
+            {(Object.keys(MODE_LABELS) as DropdownChoice[]).map((m) => (
+              <option key={m} value={m}>
+                {MODE_LABELS[m]}
+              </option>
+            ))}
+          </select>
+
+          <div
+            className="text-[10px] text-ink-mid leading-snug"
+            aria-live="polite"
+          >
+            {MODE_DESCRIPTIONS[currentChoice]}
+          </div>
+
+          {success && (
+            <div className="mt-1 px-2 py-1 bg-green-900/30 border border-green-800 rounded text-[10px] text-good">
+              Updated. Restart the workspace to apply.
+            </div>
+          )}
+
+          {resolution.workspace_override != null &&
+            !["platform_managed", "byok", "disabled"].includes(
+              resolution.workspace_override,
+            ) && (
+              <div
+                role="alert"
+                className="mt-1 px-2 py-1 bg-yellow-900/30 border border-yellow-800 rounded text-[10px] text-warning"
+              >
+                Workspace override has a non-standard value (
+                <code>{resolution.workspace_override}</code>) and is being
+                ignored. Pick a valid mode above to clear the corrupt value.
+              </div>
+            )}
+        </div>
+      )}
+    </Section>
+  );
+}