[BUG] API Error: max_tokens: 250001 > 128000 and thinking.budget_tokens error on Opus 4.6 and Opus 4.5 (Linux)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Claude Code sends max_tokens: 250001 to the API when using Opus 4.6, which exceeds the model's 128K limit. After switching to Opus 4.5, a second error appears: thinking.budget_tokens should not exceed maximum possible tokens in the context window. Both errors occur on the very first prompt of a fresh session (e.g., just typing "hello").

What Should Happen?

Claude Code should correctly set max_tokens and thinking.budget_tokens values within the limits supported by each model:

• Opus 4.6: max_tokens should not exceed 128,000
• Opus 4.5: max_tokens should not exceed 64,000
• thinking.budget_tokens should not exceed the available context window

A simple prompt like "hello" should receive a normal response without any API errors, regardless of the selected model.

Error Messages/Logs

Opus 4.6 error (first attempt):
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"max_tokens: 250001 > 128000, which is the maximum allowed number of output tokens for claude-opus-4-6"},"request_id":"req_011CXxgdU34AM39HkJEvgy2P"}

Opus 4.6 error (second attempt, different error on same session):
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"`thinking.budget_tokens` should not exceed maximum possible tokens in the context window."},"request_id":"req_011CXxggSrT5gxc67p6SvpUD"}

Opus 4.5 error (after switching model via /model):
API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"`thinking.budget_tokens` should not exceed maximum possible tokens in the context window."},"request_id":"req_011CXxgrRGjFKPEyYPhZs7KP"}

All three errors occur on the very first prompt of a session (typing just "hello" or any simple text). No workaround (environment variables, settings.json, full reinstall) resolves the issue.

Steps to Reproduce

1. Install Claude Code via native installer: curl -fsSL https://claude.ai/install.sh | bash
2. Start Claude Code (defaults to Opus 4.6)
3. Type any simple prompt (e.g., "hello")
4. Observe error: API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"max_tokens: 250001 > 128000, which is the maximum allowed number of output tokens for claude-opus-4-6"}}
5. Switch to Opus 4.5 via /model claude-opus-4-5-20251101
6. Type the same prompt
7. Observe error: API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"thinking.budget_tokens should not exceed maximum possible tokens in the context window."}}

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.2

Claude Code Version

2.1.37

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Terminator Terminal Emulator

Additional Information

Configuration

~/.claude/settings.json at time of error:

{
  "statusLine": {
    "type": "command",
    "command": "chroot=\"\"; [ -n \"${debian_chroot}\" ] && chroot=\"($debian_chroot)\"; printf '%s\\033[01;32m%s@%s\\033[00m:\\033[01;34m%s\\033[00m' \"$chroot\" \"$(whoami)\" \"$(hostname -s)\" \"$(pwd)\""
  },
  "enabledPlugins": {
    "frontend-design@claude-plugins-official": true
  }
}

No custom maxTokens, max_tokens, or thinking settings were configured. Default settings only.

System Info

• OS: Linux Mint (Ubuntu-based)
• Architecture: x86_64
• Claude Code: v2.1.37 (native binary, ELF 64-bit)
• Installation: curl -fsSL https://claude.ai/install.sh | bash
• Subscription: Max plan (x20)

Key Observations

1. The value 250001 is not present in any user configuration file. It appears to be hardcoded or dynamically calculated within the Claude Code binary.
2. The binary is a compiled ELF executable (~/.local/share/claude/versions/2.1.37), so the problematic value cannot be edited by users.
3. The thinking.budget_tokens error persists even after switching from Opus 4.6 to Opus 4.5, suggesting corrupted session state or incorrect budget calculation for both models.
4. Full uninstall (removing ~/.local/bin/claude, ~/.local/share/claude, ~/.claude, ~/.claude.json) and fresh reinstall did not fix the issue.
5. CLAUDE_CODE_MAX_OUTPUT_TOKENS=128000 environment variable had no effect, consistent with #24159.

Workarounds Attempted (all failed)

| Workaround | Result |
|---|---|
| CLAUDE_CODE_MAX_OUTPUT_TOKENS=128000 | No effect |
| maxTokens: 128000 in settings.json | No effect |
| Full uninstall + reinstall | Same errors |
| claude update | Already on latest |
| Clearing ~/.claude/sessions and ~/.claude/projects | Same errors |
| Switching to Opus 4.5 via /model | Different error (thinking.budget_tokens) |