fix: prevent model/session message cascade on OpenCode session start

- Skip restart in setModel when the model hasn't actually changed
- Don't reset _lastReportedModel on process start (prevents duplicate
  model messages across restarts)
- Only broadcast "Session started" for the first start, not for restarts
  triggered by model changes
- Add debug SSE logging (behind CODEKIN_DEBUG_SSE env var) to diagnose
  thinking/text mixing with models like Kimi

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: alari76

server/opencode-process.ts

@@ -490,6 +490,9 @@ export class OpenCodeProcess extends EventEmitter<ClaudeProcessEvents> implement
         if (!this.isOwnSession(properties)) break
         const field = properties.field as string | undefined
         const delta = properties.delta as string | undefined
+        if (process.env.CODEKIN_DEBUG_SSE) {
+          console.log(`[opencode-sse] delta field=${field} len=${delta?.length ?? 0} text=${delta?.slice(0, 80)}`)
+        }
         if (field === 'text' && delta) {
           this.receivedDeltas = true
           // Buffer initial deltas to detect and strip user echo prefix.
@@ -539,6 +542,10 @@ export class OpenCodeProcess extends EventEmitter<ClaudeProcessEvents> implement
         // Only process events for our session
         if (!this.isOwnSession(properties)) break
 
+        if (process.env.CODEKIN_DEBUG_SSE) {
+          console.log(`[opencode-sse] part.updated type=${part.type} len=${part.text?.length ?? 0} text=${part.text?.slice(0, 80)} receivedDeltas=${this.receivedDeltas} emittedPartText=${this.emittedPartText}`)
+        }
+
         switch (part.type) {
           case 'text': {
             // Text may arrive via message.part.delta (streaming) or as full
@@ -709,6 +716,12 @@ export class OpenCodeProcess extends EventEmitter<ClaudeProcessEvents> implement
           parts?: OpenCodeMessagePart[]
         } | undefined
         if (!info || info.role !== 'assistant' || !info.parts) break
+        if (process.env.CODEKIN_DEBUG_SSE) {
+          console.log(`[opencode-sse] message.updated parts=${info.parts.length} types=${info.parts.map(p => p.type).join(',')}`)
+          for (const p of info.parts) {
+            console.log(`[opencode-sse]   part type=${p.type} text=${p.text?.slice(0, 120)}`)
+          }
+        }
         for (const part of info.parts) {
           this.handleSSEEvent({ type: 'message.part.updated', properties: { ...properties, part } })
         }

server/session-lifecycle.ts

@@ -172,14 +172,16 @@ export class SessionLifecycle {
 
     cp.start()
     session.claudeProcess = cp
-    // Reset so the next system_init always broadcasts the model message,
-    // even if the model hasn't changed since the previous process.
-    session._lastReportedModel = undefined
     this.deps.globalBroadcast?.({ type: 'sessions_updated' })
 
+    // Only show "Session started" for the initial start, not for restarts
+    // triggered by model/permission changes — those already show a model message.
+    const isRestart = !!session._lastReportedModel
     const startMsg: WsServerMessage = { type: 'claude_started', sessionId }
     this.deps.addToHistory(session, startMsg)
-    this.deps.broadcast(session, startMsg)
+    if (!isRestart) {
+      this.deps.broadcast(session, startMsg)
+    }
     return true
   }
 

server/session-manager.ts

@@ -1211,7 +1211,10 @@ export class SessionManager {
   setModel(sessionId: string, model: string): boolean {
     const session = this.sessions.get(sessionId)
     if (!session) return false
-    session.model = model || undefined
+    const newModel = model || undefined
+    // Skip restart if the model hasn't actually changed.
+    if (session.model === newModel) return true
+    session.model = newModel
     // Clear any pending restart timer from a prior crash to prevent a stale
     // timer from spawning a second process after we restart below.
     if (session._restartTimer) { clearTimeout(session._restartTimer); session._restartTimer = undefined }