diff --git a/packages/server/src/server/agent/providers/omp/agent.test.ts b/packages/server/src/server/agent/providers/omp/agent.test.ts
index 2ed4846eac..74c6558022 100644
--- a/packages/server/src/server/agent/providers/omp/agent.test.ts
+++ b/packages/server/src/server/agent/providers/omp/agent.test.ts
@@ -753,6 +753,20 @@ describe("OMP agent client and session", () => {
     ]);
   });
 
+  test("a replacement runtime starts with the Fast the session has now, not the one it opened with", async () => {
+    const omp = new OmpHarness();
+    await omp.start({ modeId: "full", providerOptions: { fast_mode: true } });
+    const session = omp.requireSession();
+    omp.runtime().fastModeResult = { enabled: false, active: false };
+    await session.applyOptions?.({ fast_mode: false });
+
+    await omp.setMode("ask");
+
+    // The approval-mode restart starts a second runtime; it must not turn Fast back on.
+    expect(omp.runtimeLaunches()).toHaveLength(2);
+    expect(omp.runtime().setFastModeRequests).toEqual([]);
+  });
+
   test("restores Fast from the initial OMP state on create and resume", async () => {
     const created = new OmpHarness();
     await created.start({ providerOptions: { fast_mode: true } });
diff --git a/packages/server/src/server/agent/providers/omp/agent.ts b/packages/server/src/server/agent/providers/omp/agent.ts
index ab0246cee4..c0e0905089 100644
--- a/packages/server/src/server/agent/providers/omp/agent.ts
+++ b/packages/server/src/server/agent/providers/omp/agent.ts
@@ -847,9 +847,17 @@ export class OmpAgentSession implements AgentSession {
         fastModeActive: result.active,
       };
     }
+    if (typeof this.state.fastModeEnabled === "boolean") {
+      this.rememberFastMode(this.state.fastModeEnabled);
+    }
     return this.fastModeDifference(requested);
   }
 
+  /** A replacement runtime starts from the Fast the session has now, not the one it opened with. */
+  private rememberFastMode(enabled: boolean): void {
+    this.config.providerOptions = { ...this.config.providerOptions, fast_mode: enabled };
+  }
+
   /** The option patch when the runtime's fast mode is not what `given` stands for; unset is off. */
   private fastModeDifference(given: unknown): Record<string, unknown> {
     if (!hasOmpFastMode(this.state)) return {};
@@ -2257,6 +2265,7 @@ export class OmpAgentSession implements AgentSession {
     const before = this.state.fastModeEnabled;
     this.state = await this.runtimeSession.getState();
     if (this.state.fastModeEnabled !== before && typeof this.state.fastModeEnabled === "boolean") {
+      this.rememberFastMode(this.state.fastModeEnabled);
       this.emit({
         type: "options_changed",
         provider: this.provider,