fix(fallback): guard duplicate prompt injections

src/hooks/runtime-fallback/event-handler.ts

@@ -12,6 +12,21 @@ import { dispatchFallbackRetry } from "./fallback-retry-dispatcher"
 import { createSessionStatusHandler } from "./session-status-handler"
 import { resolveMessageEventSessionID, resolveSessionEventID } from "../../shared/event-session-id"
 
+function resolveEventModel(props: Record<string, unknown> | undefined): string | undefined {
+  const model = props?.model
+  if (typeof model === "string") {
+    return model
+  }
+
+  const providerID = props?.providerID
+  const modelID = props?.modelID
+  if (typeof providerID === "string" && typeof modelID === "string") {
+    return `${providerID}/${modelID}`
+  }
+
+  return undefined
+}
+
 export function createEventHandler(deps: HookDeps, helpers: AutoRetryHelpers) {
   const { config, pluginConfig, sessionStates, sessionLastAccess, sessionRetryInFlight, sessionAwaitingFallbackResult, sessionFallbackTimeouts, sessionStatusRetryKeys } = deps
   const sessionStatusHandler = createSessionStatusHandler(deps, helpers, sessionStatusRetryKeys)
@@ -137,6 +152,19 @@ export function createEventHandler(deps: HookDeps, helpers: AutoRetryHelpers) {
       return
     }
 
+    if (sessionAwaitingFallbackResult.has(sessionID)) {
+      const pendingFallbackModel = sessionStates.get(sessionID)?.pendingFallbackModel
+      const eventModel = resolveEventModel(props)
+      if (!pendingFallbackModel || eventModel !== pendingFallbackModel) {
+        log(`[${HOOK_NAME}] session.error skipped - awaiting fallback result`, {
+          sessionID,
+          pendingFallbackModel,
+          eventModel,
+        })
+        return
+      }
+    }
+
     sessionAwaitingFallbackResult.delete(sessionID)
     helpers.clearSessionFallbackTimeout(sessionID)