hermes-claude-auth

Claude Code OAuth bypass for hermes-agent, use your Claude Code subscription (Max/Pro) with Hermes.

What this does

Patches hermes-agent at runtime to pass Anthropic's server-side OAuth content validation. It does not modify hermes-agent source files. Installation happens through a Python import hook that monkey-patches build_anthropic_kwargs on startup.

Why this exists

On 2026-04-04, Anthropic added server-side validation that rejects OAuth requests from third-party tools. This patch adds the billing header signature and system prompt structure the API expects.

Prerequisites

• hermes-agent installed (~/.hermes/hermes-agent/)
• Claude Code CLI authenticated (valid credentials at ~/.claude/.credentials.json)
• hermes-agent configured for OAuth (credential_pool has a claude_code entry in ~/.hermes/auth.json)
• Python 3.11+

Install

curl -fsSL https://raw.githubusercontent.com/kristianvast/hermes-claude-auth/main/install-remote.sh | bash

Or clone manually:

git clone https://github.com/kristianvast/hermes-claude-auth.git
cd hermes-claude-auth
./install.sh

What install.sh does:

• Copies anthropic_billing_bypass.py to ~/.hermes/patches/
• Installs the import hook as sitecustomize.py in the hermes venv's site-packages
• Restarts hermes-gateway.service if running

Uninstall

./uninstall.sh          # remove hook only
./uninstall.sh --purge  # remove hook + patch file

How it works

1. Billing header: SHA-256 signed x-anthropic-billing-header injected as system[0]
2. System prompt relocation: Non-identity system entries moved to the first user message as <system-reminder> blocks
3. Beta flag: Adds prompt-caching-scope-2026-01-05
4. Temperature fix: Strips non-default temperature on Opus 4.6 adaptive thinking, which prevents HTTP 400

Installed through a sitecustomize.py MetaPathFinder hook, so it runs at interpreter startup with no source modifications.

What gets modified

File	Action
~/.hermes/patches/anthropic_billing_bypass.py	Created
<venv>/lib/pythonX.Y/site-packages/sitecustomize.py	Created or replaced
hermes-agent source files	NOT modified

Compatibility

• Tested with hermes-agent on Python 3.11+
• Linux and macOS
• Depends on build_anthropic_kwargs(is_oauth=...) in agent.anthropic_adapter, so it may need updating if hermes-agent changes that interface

Troubleshooting

Install issues

• "hermes-agent not found": Make sure Hermes is installed at ~/.hermes/hermes-agent/
• "No virtualenv found": Set HERMES_VENV to point to your venv
• Patch not loading: Check journalctl --user -u hermes-gateway -n 50 for [anthropic_billing_bypass] or [hermes-claude-auth] messages

Auth issues

• Anthropic 401 authentication failed or No Anthropic credentials found: Hermes reads Claude subscription credentials from ~/.claude/.credentials.json. If Claude Code is authenticated (e.g. in macOS Keychain) but that file is missing or stale, Hermes fails even when Claude Code itself works. Fix:

  1. Refresh Claude subscription login:

     claude auth login --claudeai
     

  2. macOS only — mirror the Keychain Claude Code-credentials entry into the file Hermes reads:

     python3 - <<'PY'
     import subprocess
     from pathlib import Path
     
     secret = subprocess.check_output(
         ['security', 'find-generic-password', '-s', 'Claude Code-credentials', '-w'],
         text=True,
     ).strip()
     
     cred_path = Path.home() / '.claude' / '.credentials.json'
     cred_path.parent.mkdir(parents=True, exist_ok=True)
     cred_path.write_text(secret)
     cred_path.chmod(0o600)
     print(f'wrote {cred_path}')
     PY
     

  3. Remove stale ANTHROPIC_TOKEN / ANTHROPIC_API_KEY values from ~/.hermes/.env — they can override subscription auth.
  4. Reset cached credentials:

     hermes auth reset anthropic
     

  5. Retry with a smoke test:

     hermes chat -q 'Reply with exactly: AUTH TEST OK' --provider anthropic -m claude-sonnet-4-6 -Q
     

  Credit: this macOS Keychain mirror step was written up by @DrQbz in issue #5.

Billing / routing issues

• HTTP 400: "Third-party apps now draw from your extra usage, not your plan limits": Anthropic's server-side validation has classified your requests as third-party and routed them to pay-per-token credits instead of your Max/Pro plan. Make sure you're on the latest version of this patch (it tracks the upstream opencode-claude-auth fingerprint changes). Reinstall with ./install.sh and restart hermes-gateway. If the error persists after update, the bypass is currently broken upstream too — track issue #6 for status.
• HTTP 400 persists after update: The billing salt or signature format may have been rotated by Anthropic again. Check for newer commits to this repo.

Credits

• griffinmartin/opencode-claude-auth, the original TypeScript implementation for opencode (MIT)
• NousResearch/hermes-agent, the AI agent this patches (MIT)

Disclaimer

This uses Claude Code subscription credentials outside the official Claude Code CLI. It works with Anthropic's current OAuth implementation but may break if Anthropic changes their validation. Use at your own risk.

License

MIT, see LICENSE.