diff --git a/src/resources/extensions/sf/bootstrap/register-hooks.ts b/src/resources/extensions/sf/bootstrap/register-hooks.ts
index 25f587d97..d3f968b4b 100644
--- a/src/resources/extensions/sf/bootstrap/register-hooks.ts
+++ b/src/resources/extensions/sf/bootstrap/register-hooks.ts
@@ -34,7 +34,11 @@ import { initializeLearningRuntime, resetLearningRuntime, selectLearnedModel } f
 let isFirstSession = true;
 
 async function syncServiceTierStatus(ctx: ExtensionContext): Promise<void> {
-  const { getEffectiveServiceTier, formatServiceTierFooterStatus } = await import("../service-tier.js");
+  const { getEffectiveServiceTier, formatServiceTierFooterStatus, isServiceTierDisabled } = await import("../service-tier.js");
+  // Skip the footer event entirely when the feature is explicitly disabled —
+  // no setStatus call, no RPC traffic, no leak into headless stderr even if
+  // the TUI_FOOTER_STATUS_KEYS filter is bypassed.
+  if (isServiceTierDisabled()) return;
   ctx.ui.setStatus("sf-fast", formatServiceTierFooterStatus(getEffectiveServiceTier(), ctx.model?.id));
 }
 
@@ -454,7 +458,10 @@ export function registerHooks(pi: ExtensionAPI): void {
     // ── Service Tier ────────────────────────────────────────────────────
     const modelId = event.model?.id;
     if (!modelId) return payload;
-    const { getEffectiveServiceTier, supportsServiceTier } = await import("../service-tier.js");
+    const { getEffectiveServiceTier, supportsServiceTier, isServiceTierDisabled } = await import("../service-tier.js");
+    // Short-circuit on explicit disable — never inject service_tier on any
+    // setup that has opted out, regardless of model.
+    if (isServiceTierDisabled()) return payload;
     const tier = getEffectiveServiceTier();
     if (!tier || !supportsServiceTier(modelId)) return payload;
     payload.service_tier = tier;
diff --git a/src/resources/extensions/sf/preferences-types.ts b/src/resources/extensions/sf/preferences-types.ts
index 8c81c5988..084e29353 100644
--- a/src/resources/extensions/sf/preferences-types.ts
+++ b/src/resources/extensions/sf/preferences-types.ts
@@ -324,8 +324,17 @@ export interface SFPreferences {
   gate_evaluation?: GateEvaluationConfig;
   /** GitHub sync configuration. Opt-in: syncs SF events to GitHub Issues, Milestones, and PRs. */
   github?: GitHubSyncConfig;
-  /** OpenAI service tier preference. "priority" = 2x cost, faster. "flex" = 0.5x cost, slower. Only affects gpt-5.4 models. */
-  service_tier?: "priority" | "flex";
+  /**
+   * OpenAI service tier preference. Only affects gpt-5.4 models.
+   *   - "priority" — 2x cost, faster.
+   *   - "flex"     — 0.5x cost, slower.
+   *   - "off"      — explicitly disabled: no footer status event, no
+   *                  service_tier field in outbound requests. Use this on
+   *                  setups that will never run gpt-5.4 so the feature is
+   *                  fully dormant.
+   *   - undefined  — not configured (default).
+   */
+  service_tier?: "priority" | "flex" | "off";
   /** Opt-in: search existing issues and PRs before filing from /sf forensics. Uses additional AI tokens. */
   forensics_dedup?: boolean;
   /** Opt-in: show per-prompt and cumulative session token cost in the footer. Default: false. */
diff --git a/src/resources/extensions/sf/service-tier.ts b/src/resources/extensions/sf/service-tier.ts
index e1f4f8338..8e146c07d 100644
--- a/src/resources/extensions/sf/service-tier.ts
+++ b/src/resources/extensions/sf/service-tier.ts
@@ -22,6 +22,7 @@ import { ensurePreferencesFile, serializePreferencesToFrontmatter } from "./comm
 // ─── Types ───────────────────────────────────────────────────────────────────
 
 export type ServiceTierSetting = "priority" | "flex" | undefined;
+export type ServiceTierPref = "priority" | "flex" | "off" | undefined;
 
 const SERVICE_TIER_SCOPE_NOTE = "Only affects gpt-5.4 models, regardless of provider.";
 
@@ -57,8 +58,12 @@ export function supportsServiceTier(modelId: string): boolean {
  */
 export function formatServiceTierStatus(tier: ServiceTierSetting): string {
   if (!tier) {
+    const explicitlyDisabled = isServiceTierDisabled();
+    const header = explicitlyDisabled
+      ? "Service tier: disabled (service_tier: \"off\" in preferences)"
+      : "Service tier: disabled";
     return [
-      "Service tier: disabled",
+      header,
       "",
       "Usage:",
       "  /sf fast on     Set to priority (2x cost, faster)",
@@ -106,6 +111,9 @@ export function resolveServiceTierIcon(tier: ServiceTierSetting, modelId: string
 
 /**
  * Read the effective service_tier setting from preferences.
+ * Returns undefined for both "explicitly disabled" (`"off"`) and "not
+ * configured"; callers that need to distinguish the two should use
+ * {@link isServiceTierDisabled}.
  */
 export function getEffectiveServiceTier(): ServiceTierSetting {
   const prefs = loadEffectiveSFPreferences()?.preferences;
@@ -114,6 +122,17 @@ export function getEffectiveServiceTier(): ServiceTierSetting {
   return undefined;
 }
 
+/**
+ * True when the user has explicitly set `service_tier: "off"` in preferences.
+ * Callers that register UI/hooks for the feature should skip registration
+ * entirely in this case — not just no-op their behavior. That keeps the
+ * feature fully dormant on setups that will never run a gpt-5.4 model.
+ */
+export function isServiceTierDisabled(): boolean {
+  const prefs = loadEffectiveSFPreferences()?.preferences;
+  return (prefs?.service_tier as ServiceTierPref) === "off";
+}
+
 // ─── Preference Write ────────────────────────────────────────────────────────
 
 function extractBodyAfterFrontmatter(content: string): string | null {
@@ -168,10 +187,21 @@ export async function handleFast(args: string, ctx: ExtensionCommandContext): Pr
     return;
   }
 
-  if (trimmed === "on") {
-    await writeGlobalServiceTier(ctx, "priority");
-    ctx.ui.setStatus("sf-fast", formatServiceTierFooterStatus("priority", ctx.model?.id));
-    ctx.ui.notify("Service tier set to priority (2x cost, faster responses). Only affects gpt-5.4 models, regardless of provider.", "info");
+  if (trimmed === "on" || trimmed === "flex") {
+    if (isServiceTierDisabled()) {
+      ctx.ui.notify(
+        "Service tier is explicitly disabled (service_tier: \"off\" in preferences). Remove that line before enabling.",
+        "warning",
+      );
+      return;
+    }
+    const nextTier: ServiceTierSetting = trimmed === "on" ? "priority" : "flex";
+    await writeGlobalServiceTier(ctx, nextTier);
+    ctx.ui.setStatus("sf-fast", formatServiceTierFooterStatus(nextTier, ctx.model?.id));
+    const label = nextTier === "priority"
+      ? "priority (2x cost, faster responses)"
+      : "flex (0.5x cost, slower responses)";
+    ctx.ui.notify(`Service tier set to ${label}. Only affects gpt-5.4 models, regardless of provider.`, "info");
     return;
   }
 
@@ -182,13 +212,6 @@ export async function handleFast(args: string, ctx: ExtensionCommandContext): Pr
     return;
   }
 
-  if (trimmed === "flex") {
-    await writeGlobalServiceTier(ctx, "flex");
-    ctx.ui.setStatus("sf-fast", formatServiceTierFooterStatus("flex", ctx.model?.id));
-    ctx.ui.notify("Service tier set to flex (0.5x cost, slower responses). Only affects gpt-5.4 models, regardless of provider.", "info");
-    return;
-  }
-
   ctx.ui.notify(
     "Usage: /sf fast [on|off|flex|status]\n\n  on    Priority tier (2x cost, faster)\n  off   Disable service tier\n  flex  Flex tier (0.5x cost, slower)\n  status Show current setting",
     "warning",
diff --git a/src/resources/extensions/sf/tests/service-tier.test.ts b/src/resources/extensions/sf/tests/service-tier.test.ts
index 2192c9aa7..991c12042 100644
--- a/src/resources/extensions/sf/tests/service-tier.test.ts
+++ b/src/resources/extensions/sf/tests/service-tier.test.ts
@@ -125,3 +125,65 @@ describe("resolveServiceTierIcon", () => {
     assert.equal(icon, "");
   });
 });
+
+// ─── Disable via "off" preference ────────────────────────────────────────────
+//
+// These are structural tests against the source so they don't depend on the
+// effective-preferences loader behaving the same way in a test env as in a
+// real SF install. The runtime behavior is covered by hook integration paths.
+
+import { readFileSync } from "node:fs";
+import { join, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const serviceTierSrc = readFileSync(join(__dirname, "..", "service-tier.ts"), "utf-8");
+const hooksSrc = readFileSync(join(__dirname, "..", "bootstrap", "register-hooks.ts"), "utf-8");
+
+describe("service_tier: \"off\" disable", () => {
+  test("isServiceTierDisabled is exported and keys off the preference", () => {
+    assert.match(serviceTierSrc, /export function isServiceTierDisabled\(\)/);
+    assert.match(
+      serviceTierSrc,
+      /isServiceTierDisabled[\s\S]*?prefs\?\.service_tier[\s\S]*?===\s*"off"/,
+    );
+  });
+
+  test("getEffectiveServiceTier does not treat \"off\" as a tier", () => {
+    // getEffectiveServiceTier only returns priority|flex — "off" collapses to undefined.
+    assert.match(
+      serviceTierSrc,
+      /export function getEffectiveServiceTier\(\)[\s\S]*?if \(raw === "priority" \|\| raw === "flex"\) return raw;\s*return undefined;/,
+    );
+  });
+
+  test("syncServiceTierStatus skips setStatus when disabled", () => {
+    assert.match(
+      hooksSrc,
+      /async function syncServiceTierStatus\([\s\S]*?if \(isServiceTierDisabled\(\)\) return;[\s\S]*?setStatus\("sf-fast"/,
+    );
+  });
+
+  test("before_provider_request hook short-circuits when disabled", () => {
+    // Must check isServiceTierDisabled before the normal tier/support gate.
+    const hook = hooksSrc.slice(hooksSrc.indexOf("// ── Service Tier ──"));
+    assert.match(hook, /if \(isServiceTierDisabled\(\)\) return payload/);
+    assert.ok(
+      hook.indexOf("isServiceTierDisabled") < hook.indexOf("getEffectiveServiceTier()"),
+      "disable check must run before tier resolution",
+    );
+  });
+
+  test("/sf fast on refuses when explicitly disabled", () => {
+    // handleFast("on") must bail out with a warning rather than silently
+    // overwriting the explicit "off" pref.
+    assert.match(
+      serviceTierSrc,
+      /if \(trimmed === "on" \|\| trimmed === "flex"\)[\s\S]*?if \(isServiceTierDisabled\(\)\)[\s\S]*?"warning"/,
+    );
+  });
+
+  test("preferences-types declares \"off\" as a valid service_tier value", () => {
+    const typesSrc = readFileSync(join(__dirname, "..", "preferences-types.ts"), "utf-8");
+    assert.match(typesSrc, /service_tier\?:\s*"priority"\s*\|\s*"flex"\s*\|\s*"off"/);
+  });
+});