OpenRouter: empty tool arguments serialized as JSON array instead of object

[timkley]

Bug Description

When using the OpenRouter provider with an Anthropic model (e.g. anthropic/claude-sonnet-4.6), multi-step tool calls fail with:

OpenRouter Bad Request: messages.3.content.3.tool_use.input: Input should be a valid dictionary

This happens when a tool is called with no arguments (empty {}).

Root Cause

In src/Providers/OpenRouter/Maps/MessageMap.php:162, tool call arguments are serialized as:

'arguments' => json_encode($toolCall->arguments()),

When a tool has no arguments:

1. $toolCall->arguments() returns [] (PHP empty array)
2. json_encode([]) produces "[]" (JSON array)
3. OpenRouter translates this to Anthropic's input: []
4. Anthropic rejects it because input must be a dictionary (object), not an array

This is a PHP limitation — json_decode("{}", true) returns [], and json_encode([]) returns "[]" instead of "{}".

The Anthropic Provider Already Handles This

The Anthropic provider in src/Providers/Anthropic/Maps/MessageMap.php:163 correctly handles this case:

'input' => $toolCall->arguments() === [] ? new \stdClass : $toolCall->arguments(),

The OpenRouter provider is missing this safeguard.

Reproduction

1. Configure Prism with OpenRouter provider routing to an Anthropic model
2. Register a tool that accepts no required arguments (e.g. a "list all users" tool)
3. Send a prompt that triggers the tool, then requires a follow-up step (multi-step tool use)
4. The second API call fails because the accumulated assistant message contains "arguments": "[]" instead of "arguments": "{}"

Suggested Fix

In src/Providers/OpenRouter/Maps/MessageMap.php, change:

'arguments' => json_encode($toolCall->arguments()),

To:

'arguments' => json_encode($toolCall->arguments() === [] ? new \stdClass : $toolCall->arguments()),

This matches the existing pattern in the Anthropic provider.

Note: The same issue likely exists in other providers that use json_encode($toolCall->arguments()):

• src/Providers/XAI/Maps/MessageMap.php
• src/Providers/Mistral/Maps/MessageMap.php
• src/Providers/Groq/Maps/MessageMap.php
• src/Providers/DeepSeek/Maps/MessageMap.php

We'd be happy to provide a PR for this fix.

Environment

• prism-php/prism: v0.99.19
• laravel/ai: ^0.1.3
• PHP: 8.4
• Provider: OpenRouter → anthropic/claude-sonnet-4.6

[vinitkadam03]

#896 This should fix the issue. Can you please check and close the issue.

[timkley]

Yes, thanks!