diff --git a/assistant/src/__tests__/config-schema.test.ts b/assistant/src/__tests__/config-schema.test.ts
index b174f1cc6f1..bf428c588fd 100644
--- a/assistant/src/__tests__/config-schema.test.ts
+++ b/assistant/src/__tests__/config-schema.test.ts
@@ -2090,9 +2090,8 @@ describe("loadConfig with schema validation", () => {
 
   test("recovers from partial heartbeat.activeHours without wiping unrelated fields", () => {
     // activeHoursStart is explicitly nulled while activeHoursEnd defaults to
-    // 22 — a mismatch. Without emitting on both paths, delete-and-retry can't
-    // resolve this (deleting the null side is a no-op) and the whole config
-    // would be reset to defaults.
+    // 22 — a mismatch. Single-emit on the null side lets delete-and-retry
+    // strip activeHoursStart, after which the default (8) restores it.
     writeConfig({
       maxTokens: 4096,
       heartbeat: { activeHoursStart: null },
@@ -2104,6 +2103,37 @@ describe("loadConfig with schema validation", () => {
     expect(config.heartbeat.activeHoursEnd).toBe(22);
   });
 
+  test("partial heartbeat.activeHours preserves the explicit non-null value", () => {
+    // User sets activeHoursEnd: 20 and nulls activeHoursStart. Single-emit
+    // on the null side strips activeHoursStart only — default 8 restores
+    // it, user's explicit 20 survives. Dual-emit would strip both and lose
+    // the 20.
+    writeConfig({
+      maxTokens: 1234,
+      heartbeat: { activeHoursStart: null, activeHoursEnd: 20 },
+    });
+    const config = loadConfig();
+    expect(config.maxTokens).toBe(1234);
+    expect(config.heartbeat.activeHoursStart).toBe(8);
+    expect(config.heartbeat.activeHoursEnd).toBe(20);
+  });
+
+  test("recovers from equal filing.activeHours without wiping unrelated fields", () => {
+    // activeHoursStart === activeHoursEnd is invalid (empty window). Filing's
+    // defaults are null/null, so single-emit on one path would strip one side
+    // and the null default would recreate a mismatch — cascading to a full
+    // defaults reset that wipes maxTokens. Dual-emit strips both sides so
+    // both defaults restore to null.
+    writeConfig({
+      maxTokens: 1234,
+      filing: { activeHoursStart: 5, activeHoursEnd: 5 },
+    });
+    const config = loadConfig();
+    expect(config.maxTokens).toBe(1234);
+    expect(config.filing.activeHoursStart).toBeNull();
+    expect(config.filing.activeHoursEnd).toBeNull();
+  });
+
   test("applies calls defaults when not specified", () => {
     writeConfig({});
     const config = loadConfig();
diff --git a/assistant/src/config/schemas/filing.ts b/assistant/src/config/schemas/filing.ts
index b0282a7ec8d..f47516ce022 100644
--- a/assistant/src/config/schemas/filing.ts
+++ b/assistant/src/config/schemas/filing.ts
@@ -69,11 +69,21 @@ export const FilingConfigSchema = z
       config.activeHoursEnd != null &&
       config.activeHoursStart === config.activeHoursEnd
     ) {
+      // Emit on both fields. Filing's defaults are null/null, so single-emit
+      // on one side would cascade: delete-and-retry strips one key, the null
+      // default recreates a new mismatch, and the loader falls back to full
+      // defaults — wiping unrelated fields like maxTokens.
+      const message =
+        "filing.activeHoursStart and filing.activeHoursEnd must not be equal (would create an empty window)";
+      ctx.addIssue({
+        code: z.ZodIssueCode.custom,
+        path: ["activeHoursStart"],
+        message,
+      });
       ctx.addIssue({
         code: z.ZodIssueCode.custom,
         path: ["activeHoursEnd"],
-        message:
-          "filing.activeHoursStart and filing.activeHoursEnd must not be equal (would create an empty window)",
+        message,
       });
     }
   });
diff --git a/assistant/src/config/schemas/heartbeat.ts b/assistant/src/config/schemas/heartbeat.ts
index efc26775e52..b246d8fa1e1 100644
--- a/assistant/src/config/schemas/heartbeat.ts
+++ b/assistant/src/config/schemas/heartbeat.ts
@@ -43,20 +43,15 @@ export const HeartbeatConfigSchema = z
     const startNull = config.activeHoursStart == null;
     const endNull = config.activeHoursEnd == null;
     if (startNull !== endNull) {
-      // Emit on both fields so validateWithSchema's delete-and-retry repair
-      // can strip whichever side was set (and no-op the null side), letting
-      // the config fall back to the schema defaults without a full reset.
-      const message =
-        "heartbeat.activeHoursStart and heartbeat.activeHoursEnd must both be set or both be null";
+      // Emit on the null side only. Heartbeat's defaults are 8/22, so
+      // delete-and-retry strips the null key, letting the non-null default
+      // restore it while preserving the user's explicit value on the other
+      // side.
       ctx.addIssue({
         code: z.ZodIssueCode.custom,
-        path: ["activeHoursStart"],
-        message,
-      });
-      ctx.addIssue({
-        code: z.ZodIssueCode.custom,
-        path: ["activeHoursEnd"],
-        message,
+        path: [startNull ? "activeHoursStart" : "activeHoursEnd"],
+        message:
+          "heartbeat.activeHoursStart and heartbeat.activeHoursEnd must both be set or both be null",
       });
       return;
     }