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 284bd2bc85..4705cfdcec 100644
--- a/packages/server/src/server/agent/providers/claude/agent.test.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.test.ts
@@ -33,6 +33,8 @@ import type {
 } from "../../agent-sdk-types.js";
 import type { AgentAttachment } from "@getpaseo/protocol/messages";
 import { buildAgentPrompt, renderPromptAttachmentAsText } from "../../prompt-attachments.js";
+import { applyProviderOptionsPatch } from "../../option-controls.js";
+import { mergeProviderOptions } from "../../provider-options.js";
 
 interface TestClaudeSession {
   translateMessageToEvents(message: SDKMessage): Promise<AgentStreamEvent[]>;
@@ -1547,9 +1549,14 @@ describe("ClaudeAgentSession features", () => {
     await session.setModel?.("claude-fable-5");
 
     expect(queryMock.applyFlagSettings).toHaveBeenLastCalledWith({ fastMode: false });
-    expect(events.filter((event) => event.type === "options_changed")).toEqual([
-      { type: "options_changed", provider: "claude", patch: { fast_mode: null } },
+    const reported = events.filter((event) => event.type === "options_changed");
+    expect(reported).toEqual([
+      { type: "options_changed", provider: "claude", patch: { fast_mode: false } },
     ]);
+    // With a `config.json` default of `fast_mode: true`, the agent's effective options must
+    // still read Fast as off: a removed key would let the default fill `true` back in.
+    const stored = applyProviderOptionsPatch({ fast_mode: true }, reported[0].patch);
+    expect(mergeProviderOptions({ fast_mode: true }, stored)).toEqual({ fast_mode: false });
     await expect(session.applyOptions?.({ allowedTools: ["Read"] })).resolves.toBeUndefined();
     await session.close();
   });
diff --git a/packages/server/src/server/agent/providers/claude/agent.ts b/packages/server/src/server/agent/providers/claude/agent.ts
index 39e51848db..b122793b3c 100644
--- a/packages/server/src/server/agent/providers/claude/agent.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.ts
@@ -2747,10 +2747,14 @@ class ClaudeAgentSession implements AgentSession {
       const supported = claudeModelSupportsFastMode(this.config.model);
       await activeQuery.applyFlagSettings({ fastMode: supported });
       if (!supported) {
-        // Fast cannot run on this model, so the session drops the option and says so.
-        const { fast_mode: _fastMode, ...rest } = this.config.providerOptions;
-        this.config.providerOptions = rest;
-        this.pushEvent({ type: "options_changed", provider: "claude", patch: { fast_mode: null } });
+        // Fast cannot run on this model. Reporting it off rather than removed keeps a
+        // `config.json` default of `fast_mode: true` from filling it back in.
+        this.config.providerOptions = { ...this.config.providerOptions, fast_mode: false };
+        this.pushEvent({
+          type: "options_changed",
+          provider: "claude",
+          patch: { fast_mode: false },
+        });
       }
       this.cachedRuntimeInfo = null;
     }