[BUG] Grossly underreported input tokens count when running through Google Vertex AI, leading to actual cost 16x higher than reported in CLI

Environment

• Platform (select one):
• [ ] Anthropic API
• [ ] AWS Bedrock
• [x] Google Vertex AI
• [ ] Other: <!-- specify -->
• Claude CLI version: v0.2.105
• Operating System: Debian
• Terminal: VS Code Terminal

Bug Description

When using Claude Code thru Vertex AI, the reported usage is very much underreported.

Steps to Reproduce

1. Set up Claude Code to use Vertex AI
2. Use it
3. After using it for a good amount, run /cost

Expected Behavior

The /cost command should report the accurate cost.

Example from a session using Claude Code through Anthropic API directly:

> /cost
  ⎿  Total cost:            $0.1556
     Total duration (API):  2m 0.3s
     Total duration (wall): 2m 19.0s
     Total code changes:    36 lines added, 2 lines removed
     Tokens:                11.7k base input, 33.2k cache write, 178.9k cache read, 1.9k base output

Notice how:

• base input has much bigger number than base output
• cache write and cache read shows an even bigger number

Actual Behavior

> /cost 
  ⎿  Total cost:            $0.4740
     Total duration (API):  16m 52.8s
     Total duration (wall): 1h 15m 51.7s
     Total code changes:    258 lines added, 120 lines removed
     Tokens:                356 base input, 0 cache write, 0 cache read, 31.5k base output

Notice how:

• base input is impossibly small
• cache write and cache read shows 0

This led to a wrong cost calculation, depicted in the below table:

| | |
| --- | --- |
| Cost reported by Claude Code | $0.47 |
| Cost billed to Google Cloud | $7.70 |

From the above comparison, Claude Code CLI underreported the actual cost by 16 times.

<img width="746" alt="Image" src="https://github.com/user-attachments/assets/d6e0d184-b1fa-401c-809f-042e50e8251d" />

The above image shows that Claude Code CLI correctly counts the base input / output tokens, but anything related to cache is not factored into cost.

Additional Context

N/A