diff --git a/pyproject.toml b/pyproject.toml
index 77cdd13db3..ac3d88b364 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -53,7 +53,10 @@ dependencies = [
 
   # Core application dependencies, pinned for reproducible builds.
   "fastapi-events",
-  "fastapi",
+  # FastAPI 0.119.0 breaks OpenAPI schema generation for the AnyInvocation class. It's not clear if this is an Invoke,
+  # FastAPI or pydantic bug. Probably Invoke's, because we are doing something unusual with AnyInvocation. 0.118.3 is
+  # the last known-good version.
+  "fastapi== 0.118.3",
   "huggingface-hub",
   "pydantic-settings",
   "pydantic",