diff --git a/apps/server/src/core/ai-chat/ai-chat.service.spec.ts b/apps/server/src/core/ai-chat/ai-chat.service.spec.ts
index 31c985c4..44840acc 100644
--- a/apps/server/src/core/ai-chat/ai-chat.service.spec.ts
+++ b/apps/server/src/core/ai-chat/ai-chat.service.spec.ts
@@ -16,6 +16,7 @@ import {
   rowToUiMessage,
   prepareAgentStep,
   flushAssistant,
+  stripNulChars,
   chatStreamMetadata,
   accumulateStepUsage,
   isInterruptResume,
@@ -448,6 +449,45 @@ describe('flushAssistant', () => {
   });
 });
 
+/**
+ * stripNulChars: a NUL (U+0000) is rejected by Postgres in BOTH the `content`
+ * (text) and `toolCalls`/`metadata` (jsonb) columns, so it must be stripped from
+ * every persisted string. String.fromCharCode(0) avoids embedding a raw NUL byte
+ * in this source file.
+ */
+describe('stripNulChars', () => {
+  const NUL = String.fromCharCode(0);
+
+  it('deep-strips NUL from strings in nested objects/arrays', () => {
+    const out = stripNulChars({
+      content: `a${NUL}b`,
+      parts: [{ type: 'text', text: `x${NUL}${NUL}y` }],
+      nested: [`p${NUL}q`, 42, null],
+    });
+    expect(out.content).toBe('ab');
+    expect((out.parts[0] as { text: string }).text).toBe('xy');
+    expect(out.nested[0]).toBe('pq');
+    expect(out.nested[1]).toBe(42);
+    expect(out.nested[2]).toBeNull();
+    expect(JSON.stringify(out).includes(NUL)).toBe(false);
+  });
+
+  it('returns the SAME reference when there is no NUL (no needless clone)', () => {
+    const input = { a: 'clean', b: [1, 2, { c: 'ok' }] };
+    expect(stripNulChars(input)).toBe(input);
+  });
+
+  it('flushAssistant produces a NUL-free row even when the turn text carries one', () => {
+    const f = flushAssistant([], `partial${NUL}answer`, 'error', {
+      error: `bo${NUL}om`,
+    });
+    expect(f.content).toBe('partialanswer');
+    const serialized =
+      f.content + JSON.stringify(f.toolCalls) + JSON.stringify(f.metadata);
+    expect(serialized.includes(NUL)).toBe(false);
+  });
+});
+
 /**
  * chatStreamMetadata: attach metadata to the streamed assistant UI message per
  * part type — `chatId` on `start` (so the client adopts the real created chat id
diff --git a/apps/server/src/core/ai-chat/ai-chat.service.ts b/apps/server/src/core/ai-chat/ai-chat.service.ts
index 797d0746..f0793867 100644
--- a/apps/server/src/core/ai-chat/ai-chat.service.ts
+++ b/apps/server/src/core/ai-chat/ai-chat.service.ts
@@ -1876,6 +1876,45 @@ export async function applyFinalize(
   });
 }
 
+/**
+ * Deep-strip NUL characters (`\u0000`) from every string in a value, returning
+ * the SAME reference when nothing changed (so the no-NUL common case allocates
+ * nothing). Postgres rejects a NUL in BOTH `text` and `jsonb` columns ("invalid
+ * input syntax for type json" / "unsupported Unicode escape sequence"), so a
+ * stray NUL in model output or a tool result — e.g. a truncated multibyte read
+ * of a web page — otherwise fails EVERY persist of the assistant row, silently
+ * dropping that turn's content from the DB while the live stream still shows it.
+ * Applied at the flushAssistant choke point so content + toolCalls + metadata are
+ * all covered. Exported for the unit test.
+ */
+export function stripNulChars<T>(value: T): T {
+  if (typeof value === 'string') {
+    return (value.includes('\u0000')
+      ? value.replace(/\u0000/g, '')
+      : value) as T;
+  }
+  if (Array.isArray(value)) {
+    let changed = false;
+    const out = value.map((v) => {
+      const s = stripNulChars(v);
+      if (s !== v) changed = true;
+      return s;
+    });
+    return (changed ? out : value) as T;
+  }
+  if (value && typeof value === 'object') {
+    let changed = false;
+    const out: Record<string, unknown> = {};
+    for (const [k, v] of Object.entries(value as Record<string, unknown>)) {
+      const s = stripNulChars(v);
+      if (s !== v) changed = true;
+      out[k] = s;
+    }
+    return (changed ? out : value) as T;
+  }
+  return value;
+}
+
 /**
  * PURE assistant-row builder (#183 step-granular durability). Given the turn's
  * accumulated steps + the in-progress (not-yet-finished) text + the lifecycle
@@ -1945,12 +1984,16 @@ export function flushAssistant(
     };
   }
 
-  return {
+  // Strip NUL chars from the whole row before persisting: Postgres rejects a NUL
+  // in both the `content` (text) and `toolCalls`/`metadata` (jsonb) columns, and a
+  // single stray NUL in model/tool output would otherwise fail EVERY write of this
+  // row and silently drop the turn's content from the DB (see stripNulChars).
+  return stripNulChars({
     content: stepsText + trailing,
     toolCalls: serializeSteps(finished),
     metadata,
     status,
-  };
+  });
 }
 
 /**