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 f31cb4d995..96a7ab0d0c 100644
--- a/packages/server/src/server/agent/providers/omp/agent.test.ts
+++ b/packages/server/src/server/agent/providers/omp/agent.test.ts
@@ -659,6 +659,29 @@ describe("OMP agent client and session", () => {
     expect(omp.runtime().setFastModeRequests).toEqual([true, false]);
   });
 
+  test("an options patch that never held fast_mode leaves the runtime's Fast setting alone", async () => {
+    const omp = new OmpHarness();
+    await omp.start({ model: "openai/gpt-5" });
+    const session = omp.requireSession();
+    expect(omp.runtime().state.fastModeEnabled).toBe(false);
+
+    // Another model family has Fast on natively.
+    omp.runtime().setModelResult = { provider: "anthropic", id: "claude-x" };
+    omp.runtime().queueStateReports([
+      {
+        ...omp.runtime().state,
+        model: omp.runtime().setModelResult,
+        fastModeEnabled: true,
+        fastModeActive: true,
+      },
+    ]);
+    await session.setModel?.("anthropic/claude-x");
+
+    await session.applyOptions?.({ rpcTimeoutMs: 120000 });
+
+    expect(omp.runtime().setFastModeRequests).toEqual([]);
+  });
+
   test("shows Fast only when OMP reports its state fields", async () => {
     const omp = new OmpHarness();
     await omp.start();
diff --git a/packages/server/src/server/agent/providers/omp/agent.ts b/packages/server/src/server/agent/providers/omp/agent.ts
index 974d56f8d4..8080e68769 100644
--- a/packages/server/src/server/agent/providers/omp/agent.ts
+++ b/packages/server/src/server/agent/providers/omp/agent.ts
@@ -834,8 +834,10 @@ export class OmpAgentSession implements AgentSession {
     if (requested !== undefined && typeof requested !== "boolean") {
       throw new Error("OMP fast mode requires a boolean");
     }
-    // An absent option means the runtime's own setting at session start.
-    const target = requested ?? this.fastModeBaseline;
+    // Removing a held option returns Fast to the runtime's own setting at session start; an
+    // option that was already absent leaves whatever the runtime reports alone.
+    const removed = requested === undefined && this.config.providerOptions?.fast_mode !== undefined;
+    const target = removed ? this.fastModeBaseline : requested;
     if (target !== undefined && target !== this.state.fastModeEnabled) {
       if (!hasOmpFastMode(this.state)) throw new Error("OMP fast mode is unavailable");
       const result = await this.runtimeSession.setFastMode(target);