chat_anthropic: Drop empty assistant messages

[gadenbuie]

If a tool is provided for side-effects, such as printing something to the console or updating a dashboard (think querychat), the model may not reply with any text after the tool is successfully called.

pkgload::load_all()
#> ℹ Loading ellmer

chat <- chat_anthropic()
#> Using model = "claude-sonnet-4-20250514".
provider_anthropic <- chat$get_provider()

show_word <- tool(
  function(word) {
    cat("The word is:", word, "\n")
    "Success. Don't say anything else."
  },
  name = "show_word",
  description = "Show the word to the user.",
  arguments = list(
    word = type_string()
  )
)


chat$register_tool(show_word)

chat$chat("Think of a random word and show it to me.")
#> I'll think of a random word and show it to you.
#> ◯ [tool call] show_word(word = "butterfly")
#> The word is: butterfly
#> ● #> Success. Don't say anything else.

Continuing the conversation leads to an error due to a final, empty assistant message.

chat$chat("What does the word mean?")
#> Error in `req_perform_connection()` at ellmer/R/httr2.R:38:5:
#> ! HTTP 400 Bad Request.
#> ℹ messages.3: all messages must have non-empty content except for the optional
#>   final assistant message [invalid_request_error]

chat$get_turns() |> as_json(provider_anthropic, x = _) |> str()
#> List of 4
#>  $ :List of 2
#>   ..$ role   : chr "user"
#>   ..$ content:List of 1
#>   .. ..$ :List of 2
#>   .. .. ..$ type: chr "text"
#>   .. .. ..$ text: chr "Think of a random word and show it to me."
#>  $ :List of 2
#>   ..$ role   : chr "assistant"
#>   ..$ content:List of 2
#>   .. ..$ :List of 2
#>   .. .. ..$ type: chr "text"
#>   .. .. ..$ text: chr "I'll think of a random word and show it to you."
#>   .. ..$ :List of 4
#>   .. .. ..$ type : chr "tool_use"
#>   .. .. ..$ id   : chr "toolu_01PBn2JcfUoQdcunFDwfHCJu"
#>   .. .. ..$ name : chr "show_word"
#>   .. .. ..$ input:List of 1
#>   .. .. .. ..$ word: chr "butterfly"
#>  $ :List of 2
#>   ..$ role   : chr "user"
#>   ..$ content:List of 1
#>   .. ..$ :List of 4
#>   .. .. ..$ type       : chr "tool_result"
#>   .. .. ..$ tool_use_id: chr "toolu_01PBn2JcfUoQdcunFDwfHCJu"
#>   .. .. ..$ content    : chr "Success. Don't say anything else."
#>   .. .. ..$ is_error   : logi FALSE
#>  $ :List of 2
#>   ..$ role   : chr "assistant"
#>   ..$ content: list()

chat$get_turns() |> lapply(\(x) x@json) |> str()
#> List of 4
#>  $ : list()
#>  $ :List of 7
#>   ..$ id         : chr "msg_01JFo5et9G4aNQUepYR7EvbD"
#>   ..$ type       : chr "message"
#>   ..$ role       : chr "assistant"
#>   ..$ model      : chr "claude-sonnet-4-20250514"
#>   ..$ content    :List of 2
#>   .. ..$ :List of 2
#>   .. .. ..$ type: chr "text"
#>   .. .. ..$ text: chr "I'll think of a random word and show it to you."
#>   .. ..$ :List of 4
#>   .. .. ..$ type : chr "tool_use"
#>   .. .. ..$ id   : chr "toolu_01PBn2JcfUoQdcunFDwfHCJu"
#>   .. .. ..$ name : chr "show_word"
#>   .. .. ..$ input: chr "{\"word\": \"butterfly\"}"
#>   ..$ stop_reason: chr "tool_use"
#>   ..$ usage      :List of 5
#>   .. ..$ input_tokens               : int 400
#>   .. ..$ cache_creation_input_tokens: int 0
#>   .. ..$ cache_read_input_tokens    : int 0
#>   .. ..$ output_tokens              : int 67
#>   .. ..$ service_tier               : chr "standard"
#>  $ : list()
#>  $ :List of 7
#>   ..$ id         : chr "msg_011UmL7GCuouMbGssgbZK1bY"
#>   ..$ type       : chr "message"
#>   ..$ role       : chr "assistant"
#>   ..$ model      : chr "claude-sonnet-4-20250514"
#>   ..$ content    : list()
#>   ..$ stop_reason: chr "end_turn"
#>   ..$ usage      :List of 5
#>   .. ..$ input_tokens               : int 486
#>   .. ..$ cache_creation_input_tokens: int 0
#>   .. ..$ cache_read_input_tokens    : int 0
#>   .. ..$ output_tokens              : int 2
#>   .. ..$ service_tier               : chr "standard"