fix(fallback): dedupe providerless fallback errors

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin/event.model-fallback.test.ts

@@ -241,6 +241,66 @@ describe("createEventHandler - model fallback", () => {
     expect(abortCalls).toEqual([sessionID])
   })
 
+  test("does not dispatch duplicate fallback continuations when session.error omits provider after dispatch", async () => {
+    //#given
+    const sessionID = "ses_model_fallback_providerless_duplicate"
+    setMainSession(sessionID)
+    let pendingFallbackArms = 0
+    const modelFallback = unsafeTestValue({
+      setSessionFallbackChain: () => {},
+      setPendingModelFallback: () => {
+        pendingFallbackArms += 1
+        return true
+      },
+    })
+    const { handler, abortCalls, promptAsyncCalls } = createHandler({
+      hooks: { modelFallback },
+      promptAsync: async () => ({}),
+    })
+
+    const assistantError = {
+      name: "APIError",
+      data: {
+        message:
+          "Bad Gateway: {\"error\":{\"message\":\"unknown provider for model claude-opus-4-7-thinking\"}}",
+        isRetryable: true,
+      },
+    }
+
+    await handler({
+      event: {
+        type: "message.updated",
+        properties: {
+          info: {
+            id: "msg_err_providerless_duplicate_1",
+            sessionID,
+            role: "assistant",
+            error: assistantError,
+            modelID: "claude-opus-4-7-thinking",
+            providerID: "anthropic",
+            agent: "Sisyphus - Ultraworker",
+          },
+        },
+      },
+    })
+
+    //#when - same failed model arrives without provider metadata after first dispatch resolved
+    await handler({
+      event: {
+        type: "session.error",
+        properties: {
+          sessionID,
+          error: assistantError,
+        },
+      },
+    })
+
+    //#then
+    expect(pendingFallbackArms).toBe(2)
+    expect(promptAsyncCalls).toEqual([sessionID])
+    expect(abortCalls).toEqual([sessionID])
+  })
+
   test("triggers retry prompt on session.status retry events and applies fallback", async () => {
     //#given
     const sessionID = "ses_status_retry_fallback"

src/plugin/event.ts

@@ -220,7 +220,7 @@ export function createEventHandler(args: {
   const lastHandledRetryStatusKey = new Map<string, string>();
   const lastKnownModelBySession = new Map<string, { providerID: string; modelID: string }>();
   const modelFallbackContinuationsInFlight = new Set<string>();
-  const lastDispatchedModelFallbackContinuationKey = new Map<string, string>();
+  const lastDispatchedModelFallbackContinuationKeys = new Map<string, Set<string>>();
 
   const resolveFallbackProviderID = (sessionID: string, providerHint?: string): string => {
     const normalizedProviderHint = providerHint?.trim();
@@ -361,6 +361,28 @@ export function createEventHandler(args: {
     return true;
   };
 
+  const getFallbackContinuationKeys = (fallbackContext?: {
+    agentName?: string;
+    providerID?: string;
+    modelID?: string;
+  }): string[] => {
+    const agentKey = fallbackContext?.agentName
+      ? getAgentConfigKey(fallbackContext.agentName).trim().toLowerCase()
+      : "";
+    const providerID = fallbackContext?.providerID?.trim().toLowerCase() ?? "";
+    const modelID = fallbackContext?.modelID?.trim().toLowerCase() ?? "";
+
+    if (!agentKey || !modelID) {
+      return [];
+    }
+
+    const keys = [`${agentKey}:${modelID}`];
+    if (providerID) {
+      keys.push(`${agentKey}:${providerID}:${modelID}`);
+    }
+    return keys;
+  };
+
   const autoContinueAfterFallback = async (
     sessionID: string,
     source: string,
@@ -370,18 +392,15 @@ export function createEventHandler(args: {
       modelID?: string;
     },
   ): Promise<void> => {
-    const fallbackKey = [
-      fallbackContext?.agentName ? getAgentConfigKey(fallbackContext.agentName) : "",
-      fallbackContext?.providerID ?? "",
-      fallbackContext?.modelID ?? "",
-    ].join(":");
+    const fallbackKeys = getFallbackContinuationKeys(fallbackContext);
 
     if (modelFallbackContinuationsInFlight.has(sessionID)) {
       log("[event] model-fallback continuation skipped because one is already in flight", { sessionID, source });
       return;
     }
 
-    if (fallbackKey && lastDispatchedModelFallbackContinuationKey.get(sessionID) === fallbackKey) {
+    const lastDispatchedKeys = lastDispatchedModelFallbackContinuationKeys.get(sessionID);
+    if (lastDispatchedKeys && fallbackKeys.some((fallbackKey) => lastDispatchedKeys.has(fallbackKey))) {
       log("[event] model-fallback continuation skipped because matching fallback was already dispatched", {
         sessionID,
         source,
@@ -437,8 +456,12 @@ export function createEventHandler(args: {
         log("[event] model-fallback prompt failed", { sessionID, source, error });
       });
     } finally {
-      if (dispatched && fallbackKey) {
-        lastDispatchedModelFallbackContinuationKey.set(sessionID, fallbackKey);
+      if (dispatched && fallbackKeys.length > 0) {
+        const dispatchedKeys = lastDispatchedModelFallbackContinuationKeys.get(sessionID) ?? new Set<string>();
+        for (const fallbackKey of fallbackKeys) {
+          dispatchedKeys.add(fallbackKey);
+        }
+        lastDispatchedModelFallbackContinuationKeys.set(sessionID, dispatchedKeys);
       }
       modelFallbackContinuationsInFlight.delete(sessionID);
     }
@@ -561,7 +584,7 @@ export function createEventHandler(args: {
         lastHandledRetryStatusKey.delete(sessionID);
         lastKnownModelBySession.delete(sessionID);
         modelFallbackContinuationsInFlight.delete(sessionID);
-        lastDispatchedModelFallbackContinuationKey.delete(sessionID);
+        lastDispatchedModelFallbackContinuationKeys.delete(sessionID);
         if (modelFallback) {
           clearPendingModelFallback(modelFallback, sessionID);
           clearSessionFallbackChain(modelFallback, sessionID);
@@ -720,7 +743,7 @@ export function createEventHandler(args: {
       // (non-retry idle) so future failures with the same key can trigger fallback again.
       if (sessionID && status?.type === "idle") {
         lastHandledRetryStatusKey.delete(sessionID);
-        lastDispatchedModelFallbackContinuationKey.delete(sessionID);
+        lastDispatchedModelFallbackContinuationKeys.delete(sessionID);
       }
 
       if (sessionID && status?.type === "retry" && isModelFallbackEnabled && !isRuntimeFallbackEnabled) {