Send {} for empty Anthropic tool_use input instead of null

[PratikDhanave]

Problem

In buildMessageParams (provider/anthropicprovider/agent.go), a FunctionCallContent is converted to an Anthropic tool_use block:

var args map[string]any
if c.Arguments != "" {
    if err := json.Unmarshal([]byte(c.Arguments), &args); err != nil {
        return ..., err
    }
}
content = append(content, anthropic.NewToolUseBlock(c.CallID, args, c.Name))

When c.Arguments is empty, args stays nil, and a nil map serializes to "input": null. Anthropic requires a tool_use block's input to be an object and rejects null with a 400. So replaying a conversation history that contains a no-argument tool call (e.g. a tool with no parameters, or a call carried over from another provider that emits empty arguments) breaks the request.

Fix

Default args to an empty object (map[string]any{}) when it is nil — i.e. when Arguments is empty/absent, or decodes to JSON null — so the serialized input is always {}.

Test

TestToolUseEmptyArgumentsSerializeAsObject (black-box, in agent_test.go) runs the agent over a history containing an assistant FunctionCallContent{Arguments: ""}, captures the request body sent to the server, and asserts the tool_use block's input is a JSON object. Fails on main (input = null), passes with the fix.

[Copilot]

Pull request overview

This PR fixes Anthropic tool_use serialization so that no-argument tool calls send an empty object ({}) for input instead of null, which Anthropic rejects with a 400 when replaying conversation history.

Changes:

• Default args to map[string]any{} when tool arguments are empty/absent (or decode to JSON null) so tool_use.input is always an object.
• Add a black-box regression test that captures the outbound request body and asserts tool_use.input is a JSON object for empty FunctionCallContent.Arguments.
• Minor gofmt/formatting adjustments in existing test helper/setup code.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
provider/anthropicprovider/agent.go	Ensure nil tool argument maps serialize as {} to satisfy Anthropic’s tool_use.input object requirement.
provider/anthropicprovider/agent_test.go	Add regression coverage verifying empty tool arguments serialize to an object, plus small formatting changes.

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