fix(chatcompletions): only repair args that look like truncated JSON

The previous repair pass rewrote any non-parseable arguments string to
"{}", which clobbered legitimate accumulator contents in
test_stream_response_yields_events_for_tool_call (concatenated raw
deltas like "arg1arg2"). Narrow the guard: only when the stripped
buffer starts with "{" or "[" *and* fails to parse do we replace it
with "{}". Everything else is left untouched.

src/cai/sdk/agents/models/chatcompletions/message_builder.py

@@ -427,11 +427,15 @@ class Converter:
                     else:
                         # Truncated/streamed-then-cut function call args (e.g. "{")
                         # are not valid JSON and the upstream proxy rejects the whole
-                        # request with HTTP 400, wedging the conversation.
+                        # request with HTTP 400, wedging the conversation. Only repair
-                        try:
+                        # half-finished JSON object/array buffers; leave anything else
-                            json.loads(arguments)
+                        # (raw concatenated deltas, provider-specific blobs) alone.
-                        except (TypeError, ValueError):
+                        stripped = arguments.lstrip() if isinstance(arguments, str) else ""
-                            arguments = "{}"
+                        if stripped and stripped[0] in "{[":
+                            try:
+                                json.loads(arguments)
+                            except (TypeError, ValueError):
+                                arguments = "{}"
                     tool_calls_param.append(
                         ChatCompletionMessageToolCallParam(
                             id=tc.get("id", "")[:40],

src/cai/sdk/agents/models/openai_chatcompletions.py

@@ -2678,12 +2678,16 @@ class OpenAIChatCompletionsModel(Model):
 
                 # Repair partially-streamed function calls before emission so a
                 # truncated args string like "{" cannot poison conversation history
-                # and trigger HTTP 400 on the next request.
+                # and trigger HTTP 400 on the next request. Only rewrite when the
+                # buffer is clearly a half-finished JSON object/array; leave other
+                # accumulator contents (raw concatenated deltas, provider quirks)
+                # untouched so the rest of the pipeline can decide.
                 for _fc in state.function_calls.values():
                     _args = _fc.arguments or ""
-                    if not _args.strip():
+                    _stripped = _args.lstrip()
+                    if not _stripped:
                         _fc.arguments = "{}"
-                    else:
+                    elif _stripped[0] in "{[":
                         try:
                             json.loads(_args)
                         except (TypeError, ValueError):