diff --git a/packages/server/src/server/agent/providers/claude/agent.test.ts b/packages/server/src/server/agent/providers/claude/agent.test.ts
index 20fcf87003..2713ff0ac6 100644
--- a/packages/server/src/server/agent/providers/claude/agent.test.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.test.ts
@@ -1014,6 +1014,25 @@ describe("ClaudeAgentSession features", () => {
     ).resolves.toEqual([]);
   });
 
+  test("opens with Fast off on a model that has none, so unrelated options still apply", async () => {
+    const { queryFactory } = createQueryMock();
+    const client = new ClaudeAgentClient({
+      logger,
+      queryFactory,
+      resolveBinary: async () => "/test/claude/bin",
+    });
+    const session = await client.createSession({
+      provider: "claude",
+      cwd: process.cwd(),
+      model: "claude-sonnet-5",
+      providerOptions: { fast_mode: true },
+    });
+
+    expect(session.openOptionsPatch).toEqual({ fast_mode: false });
+    await expect(session.applyOptions?.({ allowedTools: ["Read"] })).resolves.toBeUndefined();
+    await session.close();
+  });
+
   test("passes initial fast mode through Claude flag settings", async () => {
     const { queryFactory, queryMock } = createQueryMock();
     const client = new ClaudeAgentClient({
diff --git a/packages/server/src/server/agent/providers/claude/agent.ts b/packages/server/src/server/agent/providers/claude/agent.ts
index 76d534aebe..846206d3df 100644
--- a/packages/server/src/server/agent/providers/claude/agent.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.ts
@@ -2397,6 +2397,7 @@ class ClaudeAgentSession implements AgentSession {
   private pendingFreshSessionId: string | null = null;
   private recentStderr = "";
   private closed = false;
+  readonly openOptionsPatch: Record<string, unknown> = {};
 
   constructor(config: ClaudeAgentConfig, options: ClaudeAgentSessionOptions) {
     this.config = config;
@@ -2440,6 +2441,13 @@ class ClaudeAgentSession implements AgentSession {
     if (this.currentMode !== "plan") {
       this.planResumeMode = this.currentMode;
     }
+
+    // Fast cannot run on this model, so the session opens with it off. Off rather than removed
+    // keeps a `config.json` default of `fast_mode: true` from filling it back in.
+    if (config.providerOptions.fast_mode === true && !claudeModelSupportsFastMode(config.model)) {
+      this.config.providerOptions = { ...config.providerOptions, fast_mode: false };
+      this.openOptionsPatch = { fast_mode: false };
+    }
   }
 
   get id(): string | null {
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 a70c017b4c..832bd208a8 100644
--- a/packages/server/src/server/agent/providers/omp/agent.test.ts
+++ b/packages/server/src/server/agent/providers/omp/agent.test.ts
@@ -792,6 +792,22 @@ describe("OMP agent client and session", () => {
     expect(omp.optionsChanges()).toEqual([{ fast_mode: false }]);
   });
 
+  test("a Fast change the runtime reports when a turn completes is published", async () => {
+    const omp = new OmpHarness();
+    omp.acceptFastOnNextRuntime();
+    await omp.start({ modeId: "full", providerOptions: { fast_mode: true } });
+    omp.runtime().state = { ...omp.runtime().state, fastModeEnabled: false, fastModeActive: false };
+
+    const { completion } = await omp.startPromptUntilProviderIdle("hello", "done", {
+      isStreaming: false,
+      isCompacting: false,
+    });
+    await completion;
+    await omp.requireSession().getRuntimeInfo();
+
+    expect(omp.optionsChanges()).toEqual([{ fast_mode: false }]);
+  });
+
   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 149e7d475e..a5832135a5 100644
--- a/packages/server/src/server/agent/providers/omp/agent.ts
+++ b/packages/server/src/server/agent/providers/omp/agent.ts
@@ -1095,7 +1095,6 @@ export class OmpAgentSession implements AgentSession {
       const old = this.runtimeSession;
       const oldHostTools = this.hostTools;
       const previousSessionId = this.state.sessionId;
-      const fastModeBefore = this.state.fastModeEnabled;
       const restarted = await this.restartRuntime(
         this.config.internal ? null : (this.state.sessionFile ?? null),
         modeId,
@@ -1111,12 +1110,11 @@ export class OmpAgentSession implements AgentSession {
         this.pendingPromptResults.clear();
         this.runtimeSession = next;
         this.hostTools = restarted.hostTools;
-        this.state = state;
+        this.installState(state);
         this.currentModeId = modeId;
         this.config.modeId = modeId;
         this.runtimeDead = false;
         this.attachRuntime(next);
-        this.publishFastModeChange(fastModeBefore);
         this.subagentIndex.clear(old);
         await oldHostTools?.close();
         await old.close().catch(() => undefined);
@@ -2250,7 +2248,7 @@ export class OmpAgentSession implements AgentSession {
       }
       try {
         const state = await this.runtimeSession.getState();
-        this.state = state;
+        this.installState(state);
         if (this.closed || !this.activeTurnStarted || this.currentTurnIdForEvent() !== turnId) {
           // An interrupt settled this turn while the state check was in flight.
           return;
@@ -2267,13 +2265,14 @@ export class OmpAgentSession implements AgentSession {
   }
 
   private async refreshState(): Promise<void> {
-    const before = this.state.fastModeEnabled;
-    this.state = await this.runtimeSession.getState();
-    this.publishFastModeChange(before);
+    this.installState(await this.runtimeSession.getState());
   }
 
-  // The runtime owns Fast: whatever it accepted is what the session now runs with.
-  private publishFastModeChange(before: boolean | undefined): void {
+  // The runtime owns Fast: whatever it reports is what the session now runs with, so every state
+  // read from the runtime is installed here and a difference is published once.
+  private installState(state: OmpSessionState): void {
+    const before = this.state.fastModeEnabled;
+    this.state = state;
     if (this.state.fastModeEnabled !== before && typeof this.state.fastModeEnabled === "boolean") {
       this.rememberFastMode(this.state.fastModeEnabled);
       this.emit({