Claude Code incorrectly estimated API costs and executed expensive download without confirmation

Description

Claude Code made critical errors that resulted in $663.59 unexpected charges:

1. Incorrect cost estimation: Initially stated "$125 free credit is completely sufficient" and estimated cost as "$3.65"
2. Actual cost: $663.59 (180x higher than estimated)
3. Executed without confirmation: Started background download task without waiting for user approval
4. Misleading reassurance: Told user the free credit would cover everything

What Claude Code Said vs Reality

| Claude's Statement | Reality |
|-------------------|---------|
| "预估成本: $3.65" (Estimated cost) | Actual: $663.59 |
| "免费额度: $125, 剩余额度: $121.35" | Credits applied: $0 |
| "最优解已实施完成" (Solution implemented) | Executed paid API calls without approval |

Root Cause

Claude Code:

• Miscalculated the cost estimate
• Did not verify if free credits were active
• Started download in background without explicit "proceed with payment?" confirmation
• Provided false reassurance about costs

Expected Behavior

Before ANY potentially paid API call:

1. Show accurate cost estimate
2. Explicitly ask: "This will cost $X. Proceed? (yes/no)"
3. Wait for user confirmation
4. Never execute paid operations in background without approval

Impact

• $663.59 unexpected charge on Databento
• User trusted Claude's cost estimate
• This is entirely Claude Code's fault, not user error

Environment

• Claude Code CLI
• Model: Claude Opus 4.5
• Date: 2026-01-08