fix(tools): never let a model whitelist strip the prompt / source images

_build_fal_payload and _build_fal_edit_payload assemble the request and then
filter it down to the model's supports / edit_supports whitelist. That filter
also covers prompt (and image_urls for edits), which every FAL endpoint
requires. Today all model configs happen to list those keys, but a single
config that omits one would silently produce a request with no prompt or no
source images — a broken generation with no error.

Always keep the mandatory keys regardless of the whitelist so a missing
whitelist entry can only drop optional knobs, never the prompt or the images.

tests/tools/test_image_generation_image_to_image.py

@@ -79,6 +79,40 @@ class TestFalEditPayload:
         assert FAL_MODELS["fal-ai/nano-banana-pro"].get("edit_endpoint")
 
 
+class TestMandatoryKeysSurviveWhitelist:
+    """A model whose whitelist forgets the mandatory keys must not produce a
+    request with the prompt / source images silently stripped."""
+
+    _SIZES = {"square": "1024x1024", "landscape": "1536x1024", "portrait": "1024x1536"}
+
+    def test_edit_keeps_prompt_and_image_urls(self, monkeypatch):
+        from tools import image_generation_tool as t
+
+        fake = {
+            "size_style": "image_size_preset",
+            "sizes": self._SIZES,
+            "edit_supports": {"seed"},  # intentionally omits prompt + image_urls
+        }
+        monkeypatch.setitem(t.FAL_MODELS, "test/edit-model", fake)
+        payload = t._build_fal_edit_payload(
+            "test/edit-model", "make it blue", ["https://x/y.png"], "square",
+        )
+        assert payload["prompt"] == "make it blue"
+        assert payload["image_urls"] == ["https://x/y.png"]
+
+    def test_text_keeps_prompt(self, monkeypatch):
+        from tools import image_generation_tool as t
+
+        fake = {
+            "size_style": "image_size_preset",
+            "sizes": self._SIZES,
+            "supports": {"seed"},  # intentionally omits prompt
+        }
+        monkeypatch.setitem(t.FAL_MODELS, "test/text-model", fake)
+        payload = t._build_fal_payload("test/text-model", "a cat", aspect_ratio="square")
+        assert payload["prompt"] == "a cat"
+
+
 class TestFalRouting:
     def _patch_submit(self, monkeypatch, image_tool, capture: dict):
         class _Handler:

tools/image_generation_tool.py

@@ -607,7 +607,13 @@ def _build_fal_payload(
                 payload[k] = v
 
     supports = meta["supports"]
-    return {k: v for k, v in payload.items() if k in supports}
+    # ``prompt`` is required by every FAL text-to-image endpoint; keep it even
+    # if a model's ``supports`` whitelist omits it, so a missing whitelist entry
+    # can't silently strip the prompt and send an empty request.
+    return {
+        k: v for k, v in payload.items()
+        if k in supports or k == "prompt"
+    }
 
 
 def _build_fal_edit_payload(
@@ -656,7 +662,15 @@ def _build_fal_edit_payload(
             if v is not None:
                 payload[k] = v
 
-    return {k: v for k, v in payload.items() if k in edit_supports}
+    # ``prompt`` and ``image_urls`` are required by every FAL edit endpoint;
+    # keep them even if a model's ``edit_supports`` whitelist omits them, so a
+    # missing whitelist entry can't silently drop the prompt or the source
+    # images and send a broken edit request.
+    _required = {"prompt", "image_urls"}
+    return {
+        k: v for k, v in payload.items()
+        if k in edit_supports or k in _required
+    }
 
 
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