diff --git a/packages/server/src/server/agent/agent-manager.test.ts b/packages/server/src/server/agent/agent-manager.test.ts
index c55150b51f..b876a35031 100644
--- a/packages/server/src/server/agent/agent-manager.test.ts
+++ b/packages/server/src/server/agent/agent-manager.test.ts
@@ -13600,6 +13600,27 @@ describe("agent options", () => {
     ]);
   });
 
+  test("a stored record with a malformed legacy speed still reads; opening it is what rejects it", () => {
+    const manager = new AgentManager({ clients: { codex: new OptionsClient() }, logger });
+    // An older daemon accepted any value here; this one rejects it on create and on change.
+    const record = parseStoredAgentRecord({
+      id: "agent-legacy-malformed",
+      provider: "codex",
+      cwd: "/repo",
+      createdAt: "2026-01-01T00:00:00.000Z",
+      updatedAt: "2026-01-01T00:00:00.000Z",
+      lastStatus: "closed",
+      config: { providerOptions: { fast_mode: "false" } },
+      persistence: { provider: "codex", sessionId: "thread-2" },
+    });
+
+    const payload = buildStoredAgentPayload(record, ["codex"], (provider, own) =>
+      manager.effectiveOptionsFor(provider, own),
+    );
+
+    expect(payload.options).toEqual({ service_tier: "default", fast_mode: "false" });
+  });
+
   test("setAgentOptions hands the provider only the keys the patch sets", async () => {
     const { manager, client, storage, agentId } = await createOptionsAgent({ web_search: true });
 
diff --git a/packages/server/src/server/agent/agent-manager.ts b/packages/server/src/server/agent/agent-manager.ts
index 4d2db2024a..880709f152 100644
--- a/packages/server/src/server/agent/agent-manager.ts
+++ b/packages/server/src/server/agent/agent-manager.ts
@@ -68,6 +68,7 @@ import {
   applyProviderOptionsPatch,
   exposeFeatureValuesToHook,
   convertLegacyOptions,
+  MalformedLegacyOptionError,
   resolveAdapter,
   foldHookFeatureValues,
   type ProviderOptionsPatch,
@@ -2578,14 +2579,28 @@ export class AgentManager {
   /**
    * What an agent of this provider runs with, given its own options: the defaults under them.
    * Stored records are read through here before they are reopened, so the legacy conversion
-   * runs here too; a record converted on open passes through unchanged.
+   * runs here too; a record converted on open passes through unchanged. A legacy value the
+   * conversion cannot read stays as stored: this describes the record, and opening or changing
+   * the agent is what rejects the value.
    */
   effectiveOptionsFor(provider: AgentProvider, own: ProviderOptions | undefined): ProviderOptions {
     const client = this.clients.get(provider);
-    const converted = this.convertLegacyOptions(provider, own);
+    const converted = this.convertStoredOptions(provider, own);
     return client ? this.launchOptionsFor({ client, own: converted }) : (converted ?? {});
   }
 
+  private convertStoredOptions(
+    provider: AgentProvider,
+    own: ProviderOptions | undefined,
+  ): ProviderOptions | undefined {
+    try {
+      return this.convertLegacyOptions(provider, own);
+    } catch (error) {
+      if (error instanceof MalformedLegacyOptionError) return own;
+      throw error;
+    }
+  }
+
   async setTitle(agentId: string, title: string): Promise<void> {
     const agent = this.requireAgent(agentId);
     const normalizedTitle = title.trim();
diff --git a/packages/server/src/server/agent/option-controls.ts b/packages/server/src/server/agent/option-controls.ts
index e8e9ef03b3..f1c4183b26 100644
--- a/packages/server/src/server/agent/option-controls.ts
+++ b/packages/server/src/server/agent/option-controls.ts
@@ -40,6 +40,14 @@ interface LegacyOptionsInput<O> {
   options: O;
 }
 
+/** A legacy option value the conversion cannot read. */
+export class MalformedLegacyOptionError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "MalformedLegacyOptionError";
+  }
+}
+
 export function convertLegacyOptions(
   input: LegacyOptionsInput<Record<string, unknown>>,
 ): Record<string, unknown>;
@@ -53,7 +61,7 @@ export function convertLegacyOptions(
   if (provider !== "codex" || !options || !("fast_mode" in options)) return options;
   const { fast_mode: legacy, ...rest } = options;
   if (legacy !== undefined && legacy !== null && typeof legacy !== "boolean") {
-    throw new Error("Codex fast_mode must be true, false or null");
+    throw new MalformedLegacyOptionError("Codex fast_mode must be true, false or null");
   }
   if ("service_tier" in rest || legacy === undefined) return rest;
   if (legacy === null) return { ...rest, service_tier: null };