fix(agents): use configured primary as fallback origin to prevent indefinite session pinning

liuhao1024 · liuhao1024 · commit 4b525c8f0b0a · 2026-06-14T09:18:35.000+08:00
When a model fallback occurs without an existing autoFallbackPrimaryProbe (first fallback), resolveFallbackSelectionOrigin falls back to selectionRun (which is the fallback candidate) instead of the configured primary. This records the wrong origin, causing the snap-back probe guard (originProvider !== primaryProvider) to always return early — the session stays pinned to the fallback model indefinitely. Fix: pass params.followupRun.run (the configured primary) as the primary reference through applyFallbackCandidateSelectionToEntry → resolveFallbackSelectionOrigin, so the origin is always the primary model even on the first fallback. Fixes #92776
diff --git a/src/auto-reply/reply/agent-runner-execution.ts b/src/auto-reply/reply/agent-runner-execution.ts
@@ -441,7 +441,11 @@ function buildFallbackSelectionState(params: {
   };
 }
 
-function resolveFallbackSelectionOrigin(params: { entry: SessionEntry; run: FollowupRun["run"] }): {
+function resolveFallbackSelectionOrigin(params: {
+  entry: SessionEntry;
+  run: FollowupRun["run"];
+  primary?: { provider: string; model: string };
+}): {
   provider: string;
   model: string;
 } {
@@ -460,6 +464,13 @@ function resolveFallbackSelectionOrigin(params: { entry: SessionEntry; run: Foll
       return { provider: persistedOriginProvider, model: persistedOriginModel };
     }
   }
+  // When a primary reference is available, use it as the origin instead of
+  // params.run — params.run may already be a fallback candidate, which would
+  // record the wrong origin and prevent the snap-back probe from ever matching
+  // the configured primary.  See #92776.
+  if (params.primary) {
+    return { provider: params.primary.provider, model: params.primary.model };
+  }
   return { provider: params.run.provider, model: params.run.model };
 }
 
@@ -470,6 +481,7 @@ export function applyFallbackCandidateSelectionToEntry(params: {
   provider: string;
   model: string;
   origin?: { provider: string; model: string };
+  primary?: { provider: string; model: string };
   force?: boolean;
   now?: number;
 }): { updated: boolean; nextState?: FallbackSelectionState } {
@@ -482,7 +494,7 @@ export function applyFallbackCandidateSelectionToEntry(params: {
   }
   const scopedAuthProfile = resolveRunAuthProfile(params.run, params.provider);
   const origin =
-    params.origin ?? resolveFallbackSelectionOrigin({ entry: params.entry, run: params.run });
+    params.origin ?? resolveFallbackSelectionOrigin({ entry: params.entry, run: params.run, primary: params.primary });
   const nextState = buildFallbackSelectionState({
     provider: params.provider,
     model: params.model,
@@ -1814,6 +1826,15 @@ export async function runAgentTurnWithFallback(params: {
       provider: persistedProvider,
       model,
       force: candidateRun !== effectiveRun && Boolean(effectiveRun.autoFallbackPrimaryProbe),
+      // Always pass the configured primary so the origin is recorded correctly
+      // even on the first fallback (before autoFallbackPrimaryProbe exists).
+      // Without this, resolveFallbackSelectionOrigin falls back to selectionRun
+      // which may already be a fallback candidate — recording the wrong origin
+      // and preventing the snap-back probe from ever matching.  See #92776.
+      primary: {
+        provider: params.followupRun.run.provider,
+        model: params.followupRun.run.model,
+      },
       ...(effectiveRun.autoFallbackPrimaryProbe
         ? {
             origin: {
