[BUG] Incorrect total_cost_usd being returned by the SDK.

[narayanan-anirudh]

Problem:

1. I'm using Sonnet 4.6,
2. The agent is infact using Sonnet 4.6,
3. The usage values are right (token consumption)
4. I am being charged for Sonnet 4.6
5. But the total_cost_usd aligns with that of Opus 4.6.

How I've verified

I've created a new API key on platform.claude.com, and I've monitored a run for a very simple "hello good morning" message. My system prompt's too large, so that's the reason why the cache_creation_tokens are through the roof.

Before (New API key):

The usage & cost values:

"value": {
            "usage": {
                "input_tokens": 3,
                "cache_creation_input_tokens": 60710,
                "cache_read_input_tokens": 2170,
                "output_tokens": 122,
                "server_tool_use": {
                    "web_search_requests": 0,
                    "web_fetch_requests": 0
                },
                "service_tier": "standard",
                "cache_creation": {
                    "ephemeral_1h_input_tokens": 0,
                    "ephemeral_5m_input_tokens": 60710
                },
                "inference_geo": "",
                "iterations": [],
                "speed": "standard"
            },
            "total_cost_usd": 0.38358749999999997
        }

After run (Claude Platform):

For that usage, here's how the cost looks like for Sonnet 4.6 and Opus 4.6:

As you can see, the Agent SDK's total_cost_usd aligns with Opus 4.6 despite the agent actually using Sonnet 4.6.

What I've tried:
Apart from passing the model as a parameter to the agent, I've also defined it in the settings.json (assuming some CLI setting is messing it all up), but it was all in vain.

How do I get the right cost values from Claude Agent SDK? Is this a bug in the CLI binary / the SDK, or a problem with my configuration?
I'm passing in the model as a parameter to the agent, and the agent is running all fine, but why is the cost value wrong?
Please let me know if you need any more information.