diff --git a/packages/engine/src/executor.ts b/packages/engine/src/executor.ts
index d82cdbfbe..b3bedf3e3 100644
--- a/packages/engine/src/executor.ts
+++ b/packages/engine/src/executor.ts
@@ -4347,11 +4347,45 @@ export class TaskExecutor {
               return;
             }
 
+            // Sase setup: Fusion only writes to origin/dev (main is human-merged from dev).
+            // Local `main` is never auto-updated, so the default `mainRef = "main"` makes the
+            // detector compare against a stale ref and mis-flag legit merged commits as foreign.
+            // Settings expose `worktreeRebaseRemote` (= "origin") and `baseBranch` (= "dev");
+            // prefer the project's deploy branch (origin/<baseBranch>) and fall back to origin/main
+            // then "main" so non-sase projects keep current behavior.
+            const settingsForContamination = await this.store.getSettings();
+            const baseBranchSetting = (settingsForContamination as { baseBranch?: string }).baseBranch;
+            const remoteSetting = (settingsForContamination as { worktreeRebaseRemote?: string }).worktreeRebaseRemote;
+            const remoteName = typeof remoteSetting === "string" && remoteSetting.trim().length > 0
+              ? remoteSetting.split(/\s+/)[0]
+              : "origin";
+            const resolveContaminationRef = async (ref: string): Promise<string | null> => {
+              try {
+                const { stdout } = await execAsync(`git -C "${this.rootDir}" rev-parse --verify ${ref}`, { timeout: 5000 });
+                return stdout.trim() ? ref : null;
+              } catch {
+                return null;
+              }
+            };
+            const contaminationMainRef =
+              (typeof baseBranchSetting === "string" && baseBranchSetting.trim().length > 0
+                ? await resolveContaminationRef(`${remoteName}/${baseBranchSetting.trim()}`)
+                : null)
+              ?? (await resolveContaminationRef(`${remoteName}/main`))
+              ?? "main";
+            await this.store.logEntry(
+              task.id,
+              `[recovery] using contamination mainRef=${contaminationMainRef}`,
+              undefined,
+              this.currentRunContext,
+            );
+
             const classified = await classifyForeignCommits({
               repoDir: this.rootDir,
               branchName: err.branchName,
               baseSha: err.baseSha,
               foreignCommits: err.foreignCommits,
+              mainRef: contaminationMainRef,
             });
 
             const alreadyShas = classified.alreadyUpstream.map((commit) => commit.sha.slice(0, 12)).join(", ") || "none";