🤖 fix: show nested tool calls during code_execution streaming (#1366)

## Problem

When using `code_execution` (PTC), nested tool calls (e.g.,
`mux.bash()`, `mux.file_read()`) didn't appear in the UI until they
completed. Users expected to see "executing..." while long-running
nested tools were in progress, just like regular top-level tool calls.

## Root Cause

Two issues combined:

### 1. MessageId Mismatch

The backend was emitting nested tool events with the wrong `messageId`:

- `aiService.ts` created `assistantMessageId` for history and the
`emitNestedEvent` callback
- `streamManager.ts` created a **separate** `messageId` for all stream
events
- Frontend aggregator looked up messages by `messageId` - nested events
with `assistantMessageId` couldn't find the message stored under
`streamInfo.messageId`
- Events were silently dropped

### 2. React Reactivity

Even after fixing the messageId, the UI didn't update when nested tools
completed:

- `handleToolCallEnd` mutated `nestedCall` objects in place
- React didn't detect changes because object references stayed the same
- Fix: use immutable update pattern (create new array + new objects)

## Solution

1. **Thread messageId through**: Pass `assistantMessageId` from
`aiService.ts` to `streamManager.startStream()` so both use the same ID

2. **Immutable updates**: Replace in-place mutation with immutable
update pattern in `handleToolCallEnd`

## Testing

- All existing tests pass
- Manual testing confirms nested tools now show "executing..." and
update to completed state in real-time

---
_Generated with `mux` • Model: `anthropic:claude-opus-4-5` • Thinking:
`high`_

Author: ethanndickson

src/browser/utils/messages/StreamingMessageAggregator.ts

@@ -1269,15 +1269,23 @@ export class StreamingMessageAggregator {
     if (message) {
       // If nested, update in parent's nestedCalls array
       if (data.parentToolCallId) {
-        const parentPart = message.parts.find(
+        const parentIndex = message.parts.findIndex(
           (part): part is DynamicToolPart =>
             part.type === "dynamic-tool" && part.toolCallId === data.parentToolCallId
         );
+        const parentPart = message.parts[parentIndex] as DynamicToolPart | undefined;
         if (parentPart?.nestedCalls) {
-          const nestedCall = parentPart.nestedCalls.find((nc) => nc.toolCallId === data.toolCallId);
-          if (nestedCall) {
-            nestedCall.state = "output-available";
-            nestedCall.output = data.result;
+          const nestedIndex = parentPart.nestedCalls.findIndex(
+            (nc) => nc.toolCallId === data.toolCallId
+          );
+          if (nestedIndex !== -1) {
+            // Create new objects to trigger React re-render (immutable update pattern)
+            const updatedNestedCalls = parentPart.nestedCalls.map((nc, i) =>
+              i === nestedIndex
+                ? { ...nc, state: "output-available" as const, output: data.result }
+                : nc
+            );
+            message.parts[parentIndex] = { ...parentPart, nestedCalls: updatedNestedCalls };
             this.invalidateCache();
             return;
           }

src/node/services/aiService.ts

@@ -1735,6 +1735,7 @@ export class AIService extends EventEmitter {
         historySequence,
         systemMessage,
         runtime,
+        assistantMessageId, // Shared messageId ensures nested tool events match stream events
         abortSignal,
         tools,
         {

src/node/services/streamManager.test.ts

@@ -90,6 +90,7 @@ describe("StreamManager - Concurrent Stream Prevention", () => {
         1,
         "You are a helpful assistant",
         runtime,
+        "test-msg-1",
         undefined,
         {}
       );
@@ -108,6 +109,7 @@ describe("StreamManager - Concurrent Stream Prevention", () => {
         2,
         "You are a helpful assistant",
         runtime,
+        "test-msg-2",
         undefined,
         {}
       );
@@ -280,6 +282,7 @@ describe("StreamManager - Concurrent Stream Prevention", () => {
         1,
         "system",
         runtime,
+        "test-msg-1",
         undefined,
         {}
       ),
@@ -291,6 +294,7 @@ describe("StreamManager - Concurrent Stream Prevention", () => {
         2,
         "system",
         runtime,
+        "test-msg-2",
         undefined,
         {}
       ),
@@ -302,6 +306,7 @@ describe("StreamManager - Concurrent Stream Prevention", () => {
         3,
         "system",
         runtime,
+        "test-msg-3",
         undefined,
         {}
       ),

src/node/services/streamManager.ts

@@ -620,6 +620,7 @@ export class StreamManager extends EventEmitter {
     abortSignal: AbortSignal | undefined,
     system: string,
     historySequence: number,
+    messageId: string,
     tools?: Record<string, Tool>,
     initialMetadata?: Partial<MuxMetadata>,
     providerOptions?: Record<string, unknown>,
@@ -720,8 +721,6 @@ export class StreamManager extends EventEmitter {
       throw error;
     }
 
-    const messageId = `assistant-${Date.now()}-${Math.random().toString(36).substr(2, 9)}`;
-
     const streamInfo: WorkspaceStreamInfo = {
       state: StreamState.STARTING,
       streamResult,
@@ -1512,6 +1511,7 @@ export class StreamManager extends EventEmitter {
     historySequence: number,
     system: string,
     runtime: Runtime,
+    messageId: string,
     abortSignal?: AbortSignal,
     tools?: Record<string, Tool>,
     initialMetadata?: Partial<MuxMetadata>,
@@ -1568,6 +1568,7 @@ export class StreamManager extends EventEmitter {
         abortSignal,
         system,
         historySequence,
+        messageId,
         tools,
         initialMetadata,
         providerOptions,