[BUG] 'Prompt is too long' error on ~50 word prompts in new sessions despite 80K+ free context

What's wrong

Brand new chat sessions reject prompts of approximately 50+ words with "Prompt is too long" error, despite having ~80K tokens of free context available (verified via /context command).

Environment:

• Claude Code version: 2.0.75
• VS Code extension version: 2.0.75
• OS: macOS Sequoia 15.x
• Model: Claude Opus 4.5 (claude-opus-4-5-20251101)

Context breakdown from /context command:
| Category | Tokens | Percentage |
|----------|--------|------------|
| System prompt | 4.0k | 2.0% |
| System tools | 15.2k | 7.6% |
| MCP tools | 45.0k | 22.5% |
| Memory files | 7.9k | 4.0% |
| Messages | 1.7k | 0.9% |
| Free space | 81.0k | 40.5% |

What should happen

Prompts should be accepted when well within context limits. A ~50 word prompt (~75 tokens) should not be rejected when 81,000 tokens are available.

Steps to reproduce

1. Open new Claude Code session in VS Code (with MCP servers configured)
2. Run /context to verify available tokens (shows ~81K free / 200K total)
3. Send a message longer than approximately 50 words
4. Receive "Prompt is too long" error
5. Note: Shorter prompts sometimes work, longer ones consistently fail

Additional context

• Error occurs on brand new sessions with no conversation history
• Appears to be validation happening before API call (similar to #12312)
• This is a new issue that started recently
• Makes Claude Code unusable for normal development work - cannot send detailed prompts or paste code snippets