diff --git a/libs/ag-ui/src/lib/reducer.spec.ts b/libs/ag-ui/src/lib/reducer.spec.ts
index cf1b5624..6684d74e 100644
--- a/libs/ag-ui/src/lib/reducer.spec.ts
+++ b/libs/ag-ui/src/lib/reducer.spec.ts
@@ -432,4 +432,25 @@ describe('ACTIVITY events (F5 subagent activities)', () => {
     // merge: status updated, text preserved
     expect(store.activities().get('tc-1')?.content()).toEqual({ status: 'complete', text: 'hello' });
   });
+
+  it('ACTIVITY_DELTA with a malformed patch (non-existent path) does not throw and leaves content unchanged', () => {
+    // Regression guard: an out-of-order ACTIVITY_DELTA (e.g. replace /messages/5/content
+    // when there are 0 messages) must be dropped — not thrown — so the stream stays usable.
+    const store = makeStore();
+    reduceEvent({ type: 'ACTIVITY_SNAPSHOT', messageId: 'tc-1', activityType: 'subagent',
+      content: { status: 'running', text: 'prior' } } as any, store);
+    // Send a patch that targets a non-existent array index — applyPatch throws without the guard.
+    expect(() =>
+      reduceEvent({ type: 'ACTIVITY_DELTA', messageId: 'tc-1', activityType: 'subagent',
+        patch: [{ op: 'replace', path: '/messages/5/content', value: 'x' }] } as any, store),
+    ).not.toThrow();
+    // Prior content must be preserved unchanged.
+    const content = store.activities().get('tc-1')?.content();
+    expect(content?.['text']).toBe('prior');
+    expect(content?.['status']).toBe('running');
+    // Subsequent valid patches must still apply (store remains usable).
+    reduceEvent({ type: 'ACTIVITY_DELTA', messageId: 'tc-1', activityType: 'subagent',
+      patch: [{ op: 'replace', path: '/text', value: 'updated' }] } as any, store);
+    expect(store.activities().get('tc-1')?.content()['text']).toBe('updated');
+  });
 });
diff --git a/libs/ag-ui/src/lib/reducer.ts b/libs/ag-ui/src/lib/reducer.ts
index 6dc2cadc..38e0818f 100644
--- a/libs/ag-ui/src/lib/reducer.ts
+++ b/libs/ag-ui/src/lib/reducer.ts
@@ -353,7 +353,15 @@ export function reduceEvent(event: BaseEvent, store: ReducerStore): void {
       };
       const entry = store.activities().get(e.messageId);
       if (!entry) return;          // unknown activity — ignore
-      entry.content.update((c) => applyPatch(c, e.patch));  // inner signal → live, no map churn
+      entry.content.update((c) => {
+        try {
+          return applyPatch(c, e.patch);
+        } catch (err) {
+          // A malformed/out-of-order ACTIVITY_DELTA must not break the stream — drop it.
+          if (typeof console !== 'undefined') console.warn('[ag-ui] dropping malformed ACTIVITY_DELTA patch', err);
+          return c;
+        }
+      });  // inner signal → live, no map churn
       return;
     }
     default: {
diff --git a/libs/ag-ui/src/lib/to-agent.spec.ts b/libs/ag-ui/src/lib/to-agent.spec.ts
index 34d0482d..4df3211f 100644
--- a/libs/ag-ui/src/lib/to-agent.spec.ts
+++ b/libs/ag-ui/src/lib/to-agent.spec.ts
@@ -677,4 +677,21 @@ describe('subagents transcript projection (F5-transcript)', () => {
     expect(sa?.messages()).toEqual([{ id: 'sub-1', role: 'assistant', content: 'partial' }]);
     expect(sa?.toolCalls!()).toEqual([]);
   });
+
+  it('coerces role:"tool" to role:"assistant" in subagent message projection', () => {
+    // Regression guard: a buggy/future emitter putting role:'tool' in messages[]
+    // must not leak into the rendered subagent card — the subagent transcript is
+    // assistant turns only; tool/system/user don't belong there.
+    const source = new StubAgent();
+    const agent = toAgent(source as never);
+    source.emit(snapshotWithContent('tc-1', {
+      status: 'running',
+      messages: [{ id: 'm1', role: 'tool', content: 'leak', toolCallIds: [] }],
+    }) as never);
+    const sa = agent.subagents!().get('tc-1');
+    expect(sa?.messages()[0].role).toBe('assistant');
+    // Content and id must pass through unchanged.
+    expect(sa?.messages()[0].content).toBe('leak');
+    expect(sa?.messages()[0].id).toBe('m1');
+  });
 });
diff --git a/libs/ag-ui/src/lib/to-agent.ts b/libs/ag-ui/src/lib/to-agent.ts
index 1917fae2..8c39d79d 100644
--- a/libs/ag-ui/src/lib/to-agent.ts
+++ b/libs/ag-ui/src/lib/to-agent.ts
@@ -251,7 +251,7 @@ export function toAgent(source: AbstractAgent, options: ToAgentOptions = {}): Ag
           if (Array.isArray(raw)) {
             return (raw as Array<Record<string, unknown>>).map((m, i) => ({
               id: (m['id'] as string) ?? `${id}-${i}`,
-              role: (m['role'] as Message['role']) ?? 'assistant',
+              role: 'assistant' as Message['role'],
               content: typeof m['content'] === 'string' ? (m['content'] as string) : (m['content'] as Message['content']) ?? '',
               ...(Array.isArray(m['toolCallIds']) ? { toolCallIds: m['toolCallIds'] as string[] } : {}),
               ...(typeof m['reasoning'] === 'string' ? { reasoning: m['reasoning'] as string } : {}),