fix(posit): switch API flavor when chat.model crosses model families

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Bug fixes
 
 * `ChatBedrock()` (with the default `api="converse"`) no longer sends assistant turns with an empty `content` array, which Converse rejects. This happens when a response carries no content blocks, for example when a guardrail intervenes before the model produces any. A `"[empty string]"` placeholder is sent instead, matching how empty text content is already normalized. (#426)
+* `ChatPosit()` now switches API flavors when `chat.model` is set to a model from the other family: setting a non-Claude model on a Claude-backed chat (or vice versa) swaps the underlying provider so requests use the correct wire format and gateway endpoint, rather than failing with a mismatched request. The `cache` setting is preserved across flavor switches. (Mirrors tidyverse/ellmer#1139.)
+* Anthropic-backed providers (`ChatAnthropic()`, `ChatPosit()`, etc.) now drop thinking blocks that lack a signature (e.g., reasoning emitted by a non-Claude model) when replaying conversation history, instead of failing with `Invalid signature in thinking block`.
 
 
 ## [0.23.0] - 2026-09-04

chatlas/_chat.py

@@ -536,7 +536,9 @@ def model(self) -> str:
 
         Setting this updates the model for subsequent requests. The model name
         is not validated, so make sure it's a valid model for the chat's
-        provider.
+        provider. Note that some providers (e.g., `ChatPosit()`) dispatch to a
+        different API flavor based on the model name; setting a model from
+        another flavor swaps out the underlying provider accordingly.
 
         Returns
         -------
@@ -547,7 +549,11 @@ def model(self) -> str:
 
     @model.setter
     def model(self, value: str):
-        self.provider.model = value
+        old_provider = self.provider
+        new_provider = old_provider.set_model(value)
+        if new_provider is not old_provider:
+            old_provider.close()
+        self.provider = new_provider
 
     @property
     def conversation_id(self) -> str | None:

chatlas/_content_image.py

@@ -156,7 +156,7 @@ def content_image_file(
                 "Install it with `pip install pillow-heif`, or pass "
                 "`resize='none'` to send the original bytes without resizing."
             )
-        pillow_heif.register_heif_opener()
+        pillow_heif.register_heif_opener()  # pyright: ignore[reportPrivateImportUsage]
 
     if resize == "none":
         with open(path, "rb") as image_file:

chatlas/_provider.py

@@ -197,6 +197,24 @@ def model(self):
     def model(self, value: str):
         self._model = value
 
+    def set_model(self, value: str) -> "Provider[Any, Any, Any, Any]":
+        """
+        Set the model used by the provider.
+
+        The default implementation just updates the model name and returns
+        the provider unchanged. Providers that dispatch to a different API
+        flavor based on the model name (e.g., Posit AI) override this to
+        return a different provider instance when the new model belongs to
+        another flavor.
+
+        Returns
+        -------
+        Provider
+            The provider to use for subsequent requests (possibly `self`).
+        """
+        self._model = value
+        return self
+
     def close(self) -> None:
         """
         Release resources held by this provider (e.g., HTTP connection pools,

chatlas/_provider_anthropic.py

@@ -894,8 +894,11 @@ def _as_message_params(self, turns: Sequence[Turn]) -> list["MessageParam"]:
             content = [
                 self._as_content_block(c)
                 for c in turn.contents
-                if not isinstance(c, PROVIDER_ANNOTATION_TYPES)
-                or anthropic_replayable(c)
+                if (
+                    not isinstance(c, PROVIDER_ANNOTATION_TYPES)
+                    or anthropic_replayable(c)
+                )
+                and self._is_replayable_thinking(c)
             ]
 
             # Drop empty assistant turns to avoid an API error
@@ -915,6 +918,15 @@ def _as_message_params(self, turns: Sequence[Turn]) -> list["MessageParam"]:
             messages.append({"role": role, "content": content})
         return messages
 
+    @staticmethod
+    def _is_replayable_thinking(content: Content) -> bool:
+        # Thinking blocks without a signature (e.g., reasoning emitted by a
+        # non-Claude model, replayed after a provider switch) are rejected by
+        # the API ("Invalid `signature` in `thinking` block"), so drop them.
+        if not isinstance(content, ContentThinking):
+            return True
+        return bool((content.extra or {}).get("signature"))
+
     @staticmethod
     def _as_content_block(content: Content) -> "ContentBlockParam":
         if isinstance(content, ContentText):

chatlas/_provider_posit.py

@@ -309,6 +309,18 @@ def __init__(
             ),
         )
 
+    def set_model(self, value: str) -> "PositAnthropicProvider | PositOpenAIProvider":
+        if value.startswith("claude"):
+            self._model = value
+            return self
+        return PositOpenAIProvider(
+            base_url=self._gateway_base_url,
+            model=value,
+            credentials=self._credentials,
+            cache=self._cache,
+            name=self.name,
+        )
+
     def list_models(self) -> list[ModelInfo]:
         return list_models_posit(self._gateway_base_url, self._credentials)
 
@@ -320,12 +332,16 @@ def __init__(
         base_url: str,
         model: str,
         credentials: Callable[[], str],
+        cache: Literal["5m", "1h", "none"] = "5m",
         name: str = "Posit",
     ):
         super().__init__(model=model, api_key="not-used", name=name)
 
         self._gateway_base_url = base_url.rstrip("/")
         self._credentials = credentials
+        # Inert on this flavor (caching is Claude-only); stored so the
+        # setting survives a round trip through `set_model()`.
+        self._cache: Literal["5m", "1h", "none"] = cache
 
         auth = PositHttpx2Auth(credentials)
         flavor_base_url = f"{self._gateway_base_url}/openai/v1"
@@ -349,6 +365,18 @@ def __init__(
             ),
         )
 
+    def set_model(self, value: str) -> "PositAnthropicProvider | PositOpenAIProvider":
+        if value.startswith("claude"):
+            return PositAnthropicProvider(
+                base_url=self._gateway_base_url,
+                model=value,
+                credentials=self._credentials,
+                cache=self._cache,
+                name=self.name,
+            )
+        self._model = value
+        return self
+
     def list_models(self) -> list[ModelInfo]:
         return list_models_posit(self._gateway_base_url, self._credentials)
 
@@ -445,6 +473,7 @@ def ChatPosit(
             base_url=base_url,
             model=model,
             credentials=token_provider,
+            cache=cache,
         )
 
     return Chat(provider=provider, system_prompt=system_prompt)