fix(prometheus): prevent agent fallback to build in background tasks (#695)

src/features/claude-code-session-state/state.ts

@@ -9,3 +9,23 @@ export function setMainSession(id: string | undefined) {
 export function getMainSessionID(): string | undefined {
   return mainSessionID
 }
+
+const sessionAgentMap = new Map<string, string>()
+
+export function setSessionAgent(sessionID: string, agent: string): void {
+  if (!sessionAgentMap.has(sessionID)) {
+    sessionAgentMap.set(sessionID, agent)
+  }
+}
+
+export function updateSessionAgent(sessionID: string, agent: string): void {
+  sessionAgentMap.set(sessionID, agent)
+}
+
+export function getSessionAgent(sessionID: string): string | undefined {
+  return sessionAgentMap.get(sessionID)
+}
+
+export function clearSessionAgent(sessionID: string): void {
+  sessionAgentMap.delete(sessionID)
+}

src/features/hook-message-injector/index.ts

@@ -1,4 +1,4 @@
-export { injectHookMessage, findNearestMessageWithFields } from "./injector"
+export { injectHookMessage, findNearestMessageWithFields, findFirstMessageWithAgent } from "./injector"
 export type { StoredMessage } from "./injector"
 export type { MessageMeta, OriginalMessageContext, TextPart } from "./types"
 export { MESSAGE_STORAGE } from "./constants"

src/features/hook-message-injector/injector.ts

@@ -48,6 +48,35 @@ export function findNearestMessageWithFields(messageDir: string): StoredMessage
   return null
 }
 
+/**
+ * Finds the FIRST (oldest) message in the session with agent field.
+ * This is used to get the original agent that started the session,
+ * avoiding issues where newer messages may have a different agent
+ * due to OpenCode's internal agent switching.
+ */
+export function findFirstMessageWithAgent(messageDir: string): string | null {
+  try {
+    const files = readdirSync(messageDir)
+      .filter((f) => f.endsWith(".json"))
+      .sort() // Oldest first (no reverse)
+
+    for (const file of files) {
+      try {
+        const content = readFileSync(join(messageDir, file), "utf-8")
+        const msg = JSON.parse(content) as StoredMessage
+        if (msg.agent) {
+          return msg.agent
+        }
+      } catch {
+        continue
+      }
+    }
+  } catch {
+    return null
+  }
+  return null
+}
+
 function generateMessageId(): string {
   const timestamp = Date.now().toString(16)
   const random = Math.random().toString(36).substring(2, 14)