fix: validate mode at stats schema boundaries

MuxMetadata.mode is typed as string (for future custom modes), but the
stats Zod schemas only accept "plan" | "exec" | undefined. The code
was casting mode without validation, which TypeScript allowed but Zod
rejected at runtime.

This caused ZodError spam on every stream delta event, since each delta
triggers getSnapshot() which parses the schema.

Fix: validate mode before storing/emitting - treat unknown modes as
undefined for stats purposes.

Author: ammar-agent

src/common/types/message.ts

@@ -124,7 +124,7 @@ export interface MuxMetadata {
   // Readers should use helper: isCompacted = compacted !== undefined && compacted !== false
   compacted?: "user" | "idle" | boolean;
   toolPolicy?: ToolPolicy; // Tool policy active when this message was sent (user messages only)
-  mode?: string; // The mode (plan/exec/etc) active when this message was sent (assistant messages only)
+  mode?: string; // The mode active when this message was sent (assistant messages only) - plan/exec today, custom modes in future
   cmuxMetadata?: MuxFrontendMetadata; // Frontend-defined metadata, backend treats as black-box
   muxMetadata?: MuxFrontendMetadata; // Frontend-defined metadata, backend treats as black-box
 }

src/node/services/sessionTimingService.ts

@@ -482,11 +482,15 @@ export class SessionTimingService {
 
     const model = normalizeGatewayModel(data.model);
 
+    // Validate mode: stats schema only accepts "plan" | "exec" for now.
+    // Custom modes will need schema updates when supported.
+    const mode = data.mode === "plan" || data.mode === "exec" ? data.mode : undefined;
+
     const state: ActiveStreamState = {
       workspaceId: data.workspaceId,
       messageId: data.messageId,
       model,
-      mode: data.mode,
+      mode,
       startTimeMs: data.startTime,
       firstTokenTimeMs: null,
       completedToolExecutionMs: 0,

src/node/services/streamManager.ts

@@ -865,7 +865,9 @@ export class StreamManager extends EventEmitter {
       streamInfo.state = StreamState.STREAMING;
 
       // Emit stream start event (include mode from initialMetadata if available)
-      const streamStartMode = streamInfo.initialMetadata?.mode as "plan" | "exec" | undefined;
+      // Validate mode - stats schema only accepts "plan" | "exec" for now
+      const rawMode = streamInfo.initialMetadata?.mode;
+      const streamStartMode = rawMode === "plan" || rawMode === "exec" ? rawMode : undefined;
       this.emit("stream-start", {
         type: "stream-start",
         workspaceId: workspaceId as string,
@@ -1776,7 +1778,10 @@ export class StreamManager extends EventEmitter {
     await this.tokenTracker.setModel(streamInfo.model);
 
     // Emit stream-start event (include mode from initialMetadata if available)
-    const replayMode = streamInfo.initialMetadata?.mode as "plan" | "exec" | undefined;
+    // Validate mode - stats schema only accepts "plan" | "exec" for now
+    const rawReplayMode = streamInfo.initialMetadata?.mode;
+    const replayMode =
+      rawReplayMode === "plan" || rawReplayMode === "exec" ? rawReplayMode : undefined;
     this.emit("stream-start", {
       type: "stream-start",
       workspaceId,