fix(backends): reject empty user content before sending chat request

mellea/backends/litellm.py

@@ -369,6 +369,30 @@ async def _generate_from_chat_context_standard(
             case _:
                 messages.extend(self.formatter.to_chat_messages([action]))
 
+        # Issue #1597: refuse to send an empty user prompt. `SimpleContext`
+        # intentionally discards recorded turns from `view_for_generation()`,
+        # so a caller who chains `.add(...)` and then passes an empty action
+        # would otherwise hit the model with no user-role content at all.
+        # Some chat models (e.g. Granite 4.2, see #1587) spin on empty prompts
+        # and burn tokens silently. Fail fast instead.
+        if not any(
+            m.role == "user"
+            and (
+                (m.content and m.content.strip())
+                or m.images
+                or m.audio
+                or getattr(m, "_docs", None)
+            )
+            for m in messages
+        ):
+            raise ValueError(
+                "Refusing to call the model: no user-role content in the assembled "
+                "conversation. This usually means a stateless context (e.g. "
+                "SimpleContext) was combined with an empty or whitespace-only "
+                "action; recorded turns are not forwarded to the model. See "
+                "issue #1597."
+            )
+
         # TODO: the supports_vision function is not reliably predicting if models support vision. E.g., ollama/llava is not a vision model?
         # if any(m.images is not None for m in messages):
         #     # check if model can handle images

mellea/backends/ollama.py

@@ -494,6 +494,30 @@ async def generate_from_chat_context(
                 )
             case _:
                 messages.extend(self.formatter.to_chat_messages([action]))
+
+        # Issue #1597: refuse to send an empty user prompt. `SimpleContext`
+        # intentionally discards recorded turns from `view_for_generation()`,
+        # so a caller who chains `.add(...)` and then passes an empty action
+        # would otherwise hit the model with no user-role content at all.
+        # Some chat models (e.g. Granite 4.2, see #1587) spin on empty prompts
+        # and burn tokens silently. Fail fast instead.
+        if not any(
+            m.role == "user"
+            and (
+                (m.content and m.content.strip())
+                or m.images
+                or m.audio
+                or getattr(m, "_docs", None)
+            )
+            for m in messages
+        ):
+            raise ValueError(
+                "Refusing to call the model: no user-role content in the assembled "
+                "conversation. This usually means a stateless context (e.g. "
+                "SimpleContext) was combined with an empty or whitespace-only "
+                "action; recorded turns are not forwarded to the model. See "
+                "issue #1597."
+            )
         # construct the conversation from our messages, adding a system prompt at the first message if one was provided.
         conversation: list[dict] = []
         # We use system prompt None/empty-string semantics in a way that is consistent with Hugging Face and other libraries.

mellea/backends/openai.py

@@ -1095,6 +1095,30 @@ async def _generate_from_chat_context_standard(
         # ALoraRequirement may arrive here when no adapter is registered;
         # _generate is responsible for logging a warning in that case.
 
+        # Issue #1597: refuse to send an empty user prompt. `SimpleContext`
+        # intentionally discards recorded turns from `view_for_generation()`,
+        # so a caller who chains `.add(...)` and then passes an empty action
+        # would otherwise hit the model with no user-role content at all.
+        # Some chat models (e.g. Granite 4.2, see #1587) spin on empty prompts
+        # and burn tokens silently. Fail fast instead.
+        if not any(
+            m.role == "user"
+            and (
+                (m.content and m.content.strip())
+                or m.images
+                or m.audio
+                or getattr(m, "_docs", None)
+            )
+            for m in messages
+        ):
+            raise ValueError(
+                "Refusing to call the model: no user-role content in the assembled "
+                "conversation. This usually means a stateless context (e.g. "
+                "SimpleContext) was combined with an empty or whitespace-only "
+                "action; recorded turns are not forwarded to the model. See "
+                "issue #1597."
+            )
+
         conversation: list[dict] = []
 
         system_prompt = model_opts.get(ModelOption.SYSTEM_PROMPT, "")

mellea/stdlib/context/simple.py

@@ -9,11 +9,29 @@
 
 
 class SimpleContext(Context):
-    """A `SimpleContext` is a context in which each interaction is a separate and independent turn. The history of all previous turns is NOT saved.."""
+    """A `SimpleContext` is a context in which each interaction is a separate and independent turn. The history of all previous turns is NOT saved..
+
+    Note:
+        Because `view_for_generation` always returns an empty list, anything
+        passed to `SimpleContext.add` is **never forwarded to the model** — it
+        is recorded only on the in-memory context chain. The action passed to
+        `generate_from_context` (or `MelleaSession.chat`) is the *only* thing
+        that reaches the model. Combining `.add(...)` with an empty/whitespace
+        action therefore produces an empty user prompt; the OpenAI, LiteLLM,
+        and Ollama backends now reject such calls with a `ValueError` (see
+        issue #1597) rather than sending an empty conversation to the model.
+    """
 
     def add(self, c: Span) -> SimpleContext:
         """Add a new component or CBlock to the context and return the updated context.
 
+        The added span is stored on the context chain but is **not forwarded
+        to the model on subsequent generations** — `SimpleContext.view_for_generation`
+        always returns an empty list, so each generation is treated as a
+        stateless, independent turn. To actually talk to the model, pass the
+        prompt as the `action` argument to `MelleaSession.chat` /
+        `Backend.generate_from_context`, not via `add`.
+
         Args:
             c (Span): The component, content
                 block, or model output to record.
@@ -28,7 +46,9 @@ def view_for_generation(self) -> list[Span] | None:
         """Return an empty list, since `SimpleContext` does not pass history to the model.
 
         Each call to the model is treated as a stateless, independent exchange.
-        No prior turns are forwarded.
+        No prior turns are forwarded. Spans recorded via `add` are kept on
+        the in-memory chain (`as_list`) for inspection but discarded for
+        generation.
 
         Returns:
             list[Span] | None: Always an empty list.