diff --git a/docs/providers.md b/docs/providers.md
index 2c954657b7..4858ec0537 100644
--- a/docs/providers.md
+++ b/docs/providers.md
@@ -41,7 +41,9 @@ or without a control.
   differs from what it was given, including a setting nobody asked for (a native default such as
   Cursor opening with Fast on). It is set before `createSession`/`resumeSession` resolves and the
   daemon stores it before the agent is published. A reopened agent resumes that state, so a
-  native default the provider later changes does not follow.
+  native default the provider later changes does not follow. `options` on the snapshot is what
+  the session opened with, and afterwards it moves only through an apply or an `options_changed`.
+  Editing a provider default in `config.json` affects agents the next time they open.
 - **Live apply.** `applyOptions` receives the keys the patch touches, resolved against the
   `config.json` defaults. A removal arrives as the default the key falls back to, or `null` when
   there is none; the provider returns the setting to its own default if it can, or throws to
diff --git a/packages/server/src/server/agent/agent-manager.test.ts b/packages/server/src/server/agent/agent-manager.test.ts
index 3278cbb631..a9236dee0f 100644
--- a/packages/server/src/server/agent/agent-manager.test.ts
+++ b/packages/server/src/server/agent/agent-manager.test.ts
@@ -12939,6 +12939,22 @@ describe("agent options", () => {
     expect(states.at(-1)?.effectiveOptions).toEqual({ service_tier: "default", fast_mode: true });
   });
 
+  test("editing a provider default does not change the options of a loaded agent", async () => {
+    const { manager, client, agentId } = await createOptionsAgent({ fast_mode: true });
+    expect(manager.getAgent(agentId)?.effectiveOptions).toEqual({
+      service_tier: "default",
+      fast_mode: true,
+    });
+
+    // The user edits the default in config.json while the agent is running.
+    client.effectiveOptions = (own) => ({ service_tier: "fast", ...own });
+    await manager.markAgentUnread(agentId);
+
+    const agent = manager.getAgent(agentId);
+    expect(agent?.effectiveOptions).toEqual({ service_tier: "default", fast_mode: true });
+    expect(toAgentPayload(agent!).options).toEqual({ service_tier: "default", fast_mode: true });
+  });
+
   test("a provider that rejects an option leaves the stored options unchanged", async () => {
     const { manager, client, agentId } = await createOptionsAgent({ fast_mode: false });
     client.session!.rejectWith = new Error("fast mode unavailable");
diff --git a/packages/server/src/server/agent/agent-manager.ts b/packages/server/src/server/agent/agent-manager.ts
index 7a76b552b0..71fe269e47 100644
--- a/packages/server/src/server/agent/agent-manager.ts
+++ b/packages/server/src/server/agent/agent-manager.ts
@@ -2284,6 +2284,7 @@ export class AgentManager {
   async setAgentOptions(agentId: string, patch: ProviderOptionsPatch): Promise<void> {
     const agent = this.requireAgent(agentId);
     let ended: ProviderOptionsPatch = {};
+    let resolvedForApply: ProviderOptions = {};
     if (Object.keys(patch).length > 0) {
       // A provider that cannot apply options to a running agent must not store them either.
       if (!agent.session.applyOptions) {
@@ -2297,6 +2298,7 @@ export class AgentManager {
       }
       const set = Object.fromEntries(Object.entries(patch).filter(([, value]) => value != null));
       const resolved = this.effectiveOptionsFor(agent.provider, set);
+      resolvedForApply = resolved;
       ended =
         (await agent.session.applyOptions(
           Object.fromEntries(Object.keys(patch).map((key) => [key, resolved[key] ?? null])),
@@ -2305,6 +2307,7 @@ export class AgentManager {
       await this.drainSessionEvents(agentId);
     }
     this.storeOptionsPatch(agent, { ...patch, ...ended });
+    this.updateEffectiveOptions(agent, { ...patch, ...ended }, resolvedForApply);
     this.touchUpdatedAt(agent);
     this.emitState(agent);
   }
@@ -2322,6 +2325,24 @@ export class AgentManager {
     agent.config.providerOptions = Object.keys(own).length > 0 ? own : undefined;
   }
 
+  /**
+   * What the running session has: the options it opened with, moved only by an apply or a change
+   * the provider reports. A removal shows the default it fell back to, as the provider was told.
+   */
+  private updateEffectiveOptions(
+    agent: ManagedAgent,
+    patch: ProviderOptionsPatch,
+    fallbacks: ProviderOptions = {},
+  ): void {
+    const moved = Object.fromEntries(
+      Object.entries(patch).map(([key, value]) => [key, value ?? fallbacks[key] ?? null]),
+    );
+    agent.effectiveOptions = applyProviderOptionsPatch({
+      options: agent.effectiveOptions,
+      patch: moved,
+    });
+  }
+
   private effectiveOptionsFor(
     provider: AgentProvider,
     own: ProviderOptions | undefined,
@@ -4068,6 +4089,8 @@ export class AgentManager {
       session,
       capabilities: session.capabilities,
       config,
+      // What the session opened with. Provider defaults edited later reach it when it next opens.
+      effectiveOptions: this.effectiveOptionsFor(config.provider, config.providerOptions),
       runtimeInfo: undefined,
       lifecycle: "initializing",
       createdAt: options?.createdAt ?? now,
@@ -4726,6 +4749,7 @@ export class AgentManager {
   }): void {
     const { agent, patch, flags } = params;
     this.storeOptionsPatch(agent, patch);
+    this.updateEffectiveOptions(agent, patch);
     flags.shouldDispatchEvent = false;
     this.emitState(agent);
   }
@@ -5285,7 +5309,6 @@ export class AgentManager {
     if ("session" in agent && agent.session?.controls) {
       agent.controls = agent.session.controls;
     }
-    agent.effectiveOptions = this.effectiveOptionsFor(agent.provider, agent.config.providerOptions);
   }
 
   private checkAndSetAttention(agent: ManagedAgent): void {