diff --git a/scripts/validate.py b/scripts/validate.py
new file mode 100644
index 0000000..586ec84
--- /dev/null
+++ b/scripts/validate.py
@@ -0,0 +1,123 @@
+#!/usr/bin/env python3
+"""Validate every season file against the schema, plus the rules a JSON Schema
+cannot express on its own.
+
+Run it locally exactly as CI does:
+
+    pip install jsonschema
+    python scripts/validate.py
+
+Exits non-zero and prints every problem it found, rather than stopping at the
+first one — a contributor fixing a hand-edited file should see the whole list in
+one pass.
+"""
+
+from __future__ import annotations
+
+import json
+import sys
+from pathlib import Path
+
+from jsonschema import Draft202012Validator, FormatChecker
+
+REPO = Path(__file__).resolve().parent.parent
+SCHEMA_PATH = REPO / "schema" / "sprite-catalog.schema.json"
+SEASONS_DIR = REPO / "seasons"
+
+
+def check_season_file(path: Path, validator: Draft202012Validator) -> list[str]:
+    """Return a list of human-readable problems with one season file."""
+    problems: list[str] = []
+
+    try:
+        data = json.loads(path.read_text(encoding="utf-8"))
+    except json.JSONDecodeError as exc:
+        return [f"{path.name}: invalid JSON — {exc}"]
+
+    # Schema first. Everything below assumes the shape is broadly right.
+    schema_errors = sorted(validator.iter_errors(data), key=lambda e: list(e.path))
+    for err in schema_errors:
+        location = "/".join(str(p) for p in err.path) or "(root)"
+        problems.append(f"{path.name}: {location}: {err.message}")
+    if schema_errors:
+        return problems
+
+    # The filename is the season identifier. Letting these drift means the bot
+    # loads a file whose contents disagree with its name.
+    if data["season"] != path.stem:
+        problems.append(
+            f"{path.name}: season is {data['season']!r} but the filename says "
+            f"{path.stem!r} — they must match"
+        )
+
+    seen_bases: dict[str, int] = {}
+    for i, sprite in enumerate(data["sprites"]):
+        base = sprite["base_name"]
+        if base in seen_bases:
+            problems.append(
+                f"{path.name}: sprites[{i}]: duplicate base_name {base!r} "
+                f"(first seen at sprites[{seen_bases[base]}])"
+            )
+        seen_bases[base] = i
+
+        seen_variants: dict[str, int] = {}
+        for j, variant in enumerate(sprite["variants"]):
+            name = variant["variant"]
+            where = f"sprites[{i}].variants[{j}]"
+
+            if name in seen_variants:
+                problems.append(
+                    f"{path.name}: {where}: duplicate variant {name!r} for base "
+                    f"{base!r} (first seen at index {seen_variants[name]})"
+                )
+            seen_variants[name] = j
+
+            # A released sprite without a release date breaks the "new sprite"
+            # giveaway trigger, which keys off released_at.
+            if variant["status"] == "released" and "released_at" not in variant:
+                problems.append(
+                    f"{path.name}: {where}: status is released but released_at "
+                    f"is missing"
+                )
+
+            # Claiming a real owner for something not yet obtainable is always
+            # a mistake, usually a copy-paste one.
+            if variant["status"] == "unreleased" and variant.get("confirmed"):
+                problems.append(
+                    f"{path.name}: {where}: confirmed is true but status is "
+                    f"unreleased — an unreleased sprite cannot have a "
+                    f"confirmed owner"
+                )
+
+    return problems
+
+
+def main() -> int:
+    if not SCHEMA_PATH.exists():
+        print(f"error: schema not found at {SCHEMA_PATH}", file=sys.stderr)
+        return 1
+
+    schema = json.loads(SCHEMA_PATH.read_text(encoding="utf-8"))
+    validator = Draft202012Validator(schema, format_checker=FormatChecker())
+
+    season_files = sorted(SEASONS_DIR.glob("*.json")) if SEASONS_DIR.exists() else []
+    if not season_files:
+        print("No season files yet — nothing to validate.")
+        return 0
+
+    all_problems: list[str] = []
+    for path in season_files:
+        all_problems.extend(check_season_file(path, validator))
+
+    if all_problems:
+        print(f"{len(all_problems)} problem(s) found:\n", file=sys.stderr)
+        for problem in all_problems:
+            print(f"  {problem}", file=sys.stderr)
+        return 1
+
+    print(f"OK — {len(season_files)} season file(s) valid.")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())