Reject native batch seeds outside the JSON-safe range

studio/backend/models/inference.py

@@ -2069,6 +2069,21 @@ class DiffusionGenerateRequest(BaseModel):
             raise ValueError("must be a multiple of 16")
         return value
 
+    @model_validator(mode = "after")
+    def _batch_seeds_json_safe(self) -> "DiffusionGenerateRequest":
+        # A batch derives per-image seeds as seed, seed+1, ... seed+batch_size-1 (sd.cpp and
+        # diffusers both advance the seed per image). The base seed is bounded to 2**53-1 so it
+        # round-trips through the JSON gallery recipe, but the derived top-of-batch seed is not:
+        # with an explicit seed near the cap it can exceed Number.MAX_SAFE_INTEGER, where the
+        # frontend rounds it and a restored recipe replays a different image. Reject at the
+        # boundary so an API client can't persist an unreplayable seed.
+        if self.seed is not None and self.seed + self.batch_size - 1 > 2**53 - 1:
+            raise ValueError(
+                "seed + batch_size - 1 must not exceed 2**53 - 1 so every per-image seed "
+                "stays JSON-safe (lower the seed or the batch_size)"
+            )
+        return self
+
 
 class GalleryImage(BaseModel):
     """A persisted image's full generation recipe (embedded in the PNG too)."""

studio/backend/tests/test_diffusion_routes.py

@@ -340,6 +340,26 @@ def test_generate_rejects_non_multiple_of_16(client):
     assert ok.status_code == 200
 
 
+def test_generate_rejects_batch_seed_past_json_safe_range(client):
+    client.post(
+        "/api/inference/images/load", json = {"model_path": "x/z-image", "gguf_filename": "q.gguf"}
+    )
+    # An explicit seed at the JS-safe cap with a batch derives per-image seeds
+    # (seed+1 ...) that exceed Number.MAX_SAFE_INTEGER and no longer round-trip
+    # through the gallery JSON recipe, so the request is rejected at the boundary.
+    over = client.post(
+        "/api/inference/images/generate",
+        json = {"prompt": "p", "seed": 2**53 - 1, "batch_size": 2},
+    )
+    assert over.status_code == 422
+    # The top-of-batch seed lands exactly on the cap: still JSON-safe, so accepted.
+    ok = client.post(
+        "/api/inference/images/generate",
+        json = {"prompt": "p", "seed": 2**53 - 2, "batch_size": 2},
+    )
+    assert ok.status_code == 200
+
+
 def test_non_gguf_load_restricted_to_unsloth(client):
     # gguf_filename is optional now; with none, the load is a full-pipeline kind, which
     # is gated to unsloth/* repos. A non-unsloth repo (no filename) is rejected -> 400.