diff --git a/anthropic_billing_bypass.py b/anthropic_billing_bypass.py
new file mode 100644
index 0000000..21778f4
--- /dev/null
+++ b/anthropic_billing_bypass.py
@@ -0,0 +1,497 @@
+"""
+Claude Code OAuth bypass for hermes-agent.
+==========================================
+
+Monkey-patches hermes-agent's ``agent.anthropic_adapter.build_anthropic_kwargs``
+at import time via a sitecustomize.py hook so that OAuth-authenticated requests
+pass Anthropic's server-side content validation.
+
+Background
+----------
+On 2026-04-04 Anthropic deployed server-side validation on OAuth requests: if
+the ``system[]`` array contains text that doesn't match Claude Code's system
+prompt structure, the request is rejected with HTTP 400 — even on accounts with
+remaining subscription quota.  Third-party tools (hermes-agent, opencode, cline,
+aider, etc.) all hit this simultaneously.
+
+opencode-claude-auth v1.4.8 (PR #148) worked around it by:
+
+  1. Injecting a cryptographically-signed ``x-anthropic-billing-header`` as
+     ``system[0]``.  The signature is derived from characters at positions 4, 7,
+     20 of the first user message, a hardcoded salt, and the Claude CLI version.
+  2. Relocating all non-Claude-Code system prompt content to the first user
+     message wrapped in ``<system-reminder>`` blocks.
+  3. Adding the ``prompt-caching-scope-2026-01-05`` beta flag.
+
+hermes-agent already implements the Claude Code identity prefix, user-agent
+spoofing, ``x-app: cli``, tool name ``mcp_`` prefixing, and the
+``oauth-2025-04-20`` / ``claude-code-20250219`` beta flags.  This patch adds
+only the three items above plus a temperature fix for Opus 4.6 adaptive thinking.
+
+Installation
+------------
+Installed automatically by ``install.sh``.  See README.md for details.
+
+The ``sitecustomize_hook.py`` loader runs at Python interpreter startup and
+hooks ``agent.anthropic_adapter``'s import so that ``apply_patches()`` runs
+immediately after the module is loaded.  No hermes-agent source modifications
+are needed.
+
+Reversal
+--------
+Run ``uninstall.sh`` or manually remove the sitecustomize hook from the venv's
+site-packages and restart hermes-gateway.
+
+References
+----------
+- https://github.com/griffinmartin/opencode-claude-auth
+- https://github.com/griffinmartin/opencode-claude-auth/pull/148
+
+Version history
+---------------
+- 1.0.0 (2026-04-09): Initial — billing header, system prompt relocation,
+  prompt-caching beta flag, aux-client temperature hook for Opus 4.6.
+"""
+
+from __future__ import annotations
+
+__version__ = "1.0.0"
+
+import hashlib
+import inspect
+import logging
+import sys
+import traceback
+from typing import Any, Dict, List
+
+logger = logging.getLogger("anthropic_billing_bypass")
+
+# ---------------------------------------------------------------------------
+# Cryptographic signing (ported from opencode-claude-auth/src/signing.ts)
+# ---------------------------------------------------------------------------
+
+# Shared secret shipped in the Claude Code CLI binary.  Anthropic's server
+# uses this salt to verify billing-header signatures.
+_BILLING_SALT = "59cf53e54c78"
+
+# Sentinel strings — entries in system[] starting with these are kept;
+# everything else is relocated to the first user message.
+_BILLING_PREFIX = "x-anthropic-billing-header"
+_SYSTEM_IDENTITY = "You are Claude Code, Anthropic's official CLI for Claude."
+
+
+def _extract_first_user_message_text(messages: List[Dict[str, Any]]) -> str:
+    """Return the text of the first user message's first text block.
+
+    Matches Claude Code's K19() exactly: find the first message with
+    role="user", then return the text of its first text content block.
+    """
+    for msg in messages:
+        if not isinstance(msg, dict) or msg.get("role") != "user":
+            continue
+        content = msg.get("content")
+        if isinstance(content, str):
+            return content
+        if isinstance(content, list):
+            for block in content:
+                if isinstance(block, dict) and block.get("type") == "text":
+                    text = block.get("text")
+                    if isinstance(text, str) and text:
+                        return text
+        return ""
+    return ""
+
+
+def _compute_cch(message_text: str) -> str:
+    """First 5 hex chars of SHA-256(message_text)."""
+    return hashlib.sha256(message_text.encode("utf-8")).hexdigest()[:5]
+
+
+def _compute_version_suffix(message_text: str, version: str) -> str:
+    """3-char version suffix: SHA-256(salt + sampled_chars + version)[:3].
+
+    Samples characters at indices 4, 7, 20 from the message text, padding
+    with "0" when the message is shorter than the index.
+    """
+    sampled = "".join(
+        message_text[i] if i < len(message_text) else "0" for i in (4, 7, 20)
+    )
+    input_str = f"{_BILLING_SALT}{sampled}{version}"
+    return hashlib.sha256(input_str.encode("utf-8")).hexdigest()[:3]
+
+
+def _build_billing_header_value(
+    messages: List[Dict[str, Any]],
+    version: str,
+    entrypoint: str,
+) -> str:
+    """Build the full x-anthropic-billing-header text for system[0]."""
+    text = _extract_first_user_message_text(messages)
+    suffix = _compute_version_suffix(text, version)
+    cch = _compute_cch(text)
+    return (
+        f"x-anthropic-billing-header: "
+        f"cc_version={version}.{suffix}; "
+        f"cc_entrypoint={entrypoint}; "
+        f"cch={cch};"
+    )
+
+
+# ---------------------------------------------------------------------------
+# Bypass logic (ported from opencode-claude-auth/src/transforms.ts)
+# ---------------------------------------------------------------------------
+
+
+def _model_supports_adaptive_thinking(model: str) -> bool:
+    if not isinstance(model, str):
+        return False
+    return any(v in model for v in ("4-6", "4.6"))
+
+
+def _fix_temperature_for_oauth_adaptive(
+    api_kwargs: Dict[str, Any],
+    *,
+    site: str,
+) -> None:
+    """Strip temperature from OAuth requests on adaptive-thinking models.
+
+    Opus 4.6 with implicit adaptive thinking rejects non-1 temperature
+    values with HTTP 400.  This drops the parameter entirely so the API
+    uses its default.
+    """
+    if "temperature" not in api_kwargs:
+        return
+    temp = api_kwargs.get("temperature")
+    if temp == 1 or temp == 1.0:
+        return
+    model = api_kwargs.get("model")
+    if not _model_supports_adaptive_thinking(model or ""):
+        return
+    del api_kwargs["temperature"]
+    logger.info(
+        "Dropped temperature=%r for OAuth adaptive-thinking model %r (site=%s)",
+        temp,
+        model,
+        site,
+    )
+
+
+def _prepend_to_first_user_message(
+    messages: List[Dict[str, Any]],
+    texts: List[str],
+) -> None:
+    """Prepend each text as a <system-reminder> block to the first user message.
+
+    Mutates ``messages`` in place.
+    """
+    if not texts:
+        return
+    combined = "\n\n".join(f"<system-reminder>\n{t}\n</system-reminder>" for t in texts)
+    for i, msg in enumerate(messages):
+        if not isinstance(msg, dict) or msg.get("role") != "user":
+            continue
+        content = msg.get("content")
+        if isinstance(content, str):
+            new_text = f"{combined}\n\n{content}" if content else combined
+            messages[i] = {**msg, "content": [{"type": "text", "text": new_text}]}
+            return
+        if isinstance(content, list):
+            new_content = list(content)
+            for j, block in enumerate(new_content):
+                if isinstance(block, dict) and block.get("type") == "text":
+                    existing = block.get("text") or ""
+                    new_content[j] = {
+                        **block,
+                        "text": f"{combined}\n\n{existing}" if existing else combined,
+                    }
+                    messages[i] = {**msg, "content": new_content}
+                    return
+            new_content.insert(0, {"type": "text", "text": combined})
+            messages[i] = {**msg, "content": new_content}
+            return
+        messages[i] = {**msg, "content": [{"type": "text", "text": combined}]}
+        return
+
+
+def apply_claude_code_bypass(api_kwargs: Dict[str, Any], version: str) -> None:
+    """Mutate api_kwargs in place to pass OAuth content validation.
+
+    Only call on OAuth requests (``is_oauth=True``).  Safe to call multiple
+    times — stale billing headers are replaced, duplicate identity entries
+    are dropped.
+
+    After this runs, ``api_kwargs["system"]`` contains at most the billing
+    header and the Claude Code identity prefix.  Everything else is moved to
+    the first user message as ``<system-reminder>`` blocks.
+    """
+    messages = api_kwargs.get("messages")
+    if not isinstance(messages, list) or not messages:
+        return
+
+    raw_system = api_kwargs.get("system")
+    if raw_system is None:
+        system: List[Any] = []
+    elif isinstance(raw_system, str):
+        system = [{"type": "text", "text": raw_system}] if raw_system else []
+    elif isinstance(raw_system, list):
+        system = list(raw_system)
+    else:
+        logger.warning(
+            "Unexpected system type %s; skipping bypass", type(raw_system).__name__
+        )
+        return
+
+    # Compute billing header using ORIGINAL messages (before relocation).
+    try:
+        billing_value = _build_billing_header_value(messages, version, "cli")
+    except Exception as exc:
+        logger.warning("Failed to build billing header: %s", exc)
+        return
+    billing_entry = {"type": "text", "text": billing_value}
+
+    kept: List[Any] = []
+    moved_texts: List[str] = []
+    identity_seen = False
+
+    for entry in system:
+        if not isinstance(entry, dict):
+            kept.append(entry)
+            continue
+        entry_type = entry.get("type")
+        if entry_type != "text":
+            kept.append(entry)
+            continue
+        text = entry.get("text") or ""
+        if text.startswith(_BILLING_PREFIX):
+            continue  # stale billing header — drop
+        if text.startswith(_SYSTEM_IDENTITY):
+            if identity_seen:
+                continue  # duplicate — drop
+            identity_seen = True
+            rest = text[len(_SYSTEM_IDENTITY) :].lstrip("\n")
+            identity_entry = {k: v for k, v in entry.items() if k != "text"}
+            identity_entry["text"] = _SYSTEM_IDENTITY
+            kept.append(identity_entry)
+            if rest:
+                moved_texts.append(rest)
+            continue
+        if text:
+            moved_texts.append(text)
+
+    if not identity_seen:
+        kept.insert(0, {"type": "text", "text": _SYSTEM_IDENTITY})
+
+    # Billing header first (no cache_control — changes per request).
+    api_kwargs["system"] = [billing_entry] + kept
+
+    if moved_texts:
+        _prepend_to_first_user_message(messages, moved_texts)
+
+    _fix_temperature_for_oauth_adaptive(api_kwargs, site="build_kwargs")
+
+
+# ---------------------------------------------------------------------------
+# Monkey-patch installation
+# ---------------------------------------------------------------------------
+
+
+def _get_version_safely(aa_module: Any) -> str:
+    """Return the Claude CLI version string from the adapter module."""
+    getter = getattr(aa_module, "_get_claude_code_version", None)
+    if callable(getter):
+        try:
+            version = getter()
+            if isinstance(version, str) and version and version[0].isdigit():
+                return version
+        except Exception:
+            pass
+    fallback = getattr(aa_module, "_CLAUDE_CODE_VERSION_FALLBACK", None)
+    if isinstance(fallback, str) and fallback:
+        return fallback
+    return "2.1.90"
+
+
+def _install_aux_client_hook(force: bool = False) -> bool:
+    """Patch the auxiliary client to strip temperature on OAuth adaptive models."""
+    try:
+        from agent import auxiliary_client as ac  # type: ignore[import-not-found]
+    except Exception as exc:
+        logger.warning("aux_client_hook_failed_import: %s: %s", type(exc).__name__, exc)
+        sys.stderr.write(
+            f"[anthropic_billing_bypass] aux_client_hook_failed_import: "
+            f"{type(exc).__name__}: {exc}\n"
+        )
+        return False
+
+    adapter_cls = getattr(ac, "_AnthropicCompletionsAdapter", None)
+    if adapter_cls is None:
+        logger.warning("aux_client_hook_failed: _AnthropicCompletionsAdapter not found")
+        return False
+
+    if getattr(adapter_cls, "_AUX_CLIENT_TEMP_HOOK_APPLIED", False) and not force:
+        logger.debug("aux_client_hook already installed")
+        return True
+
+    original_create = getattr(adapter_cls, "create", None)
+    if not callable(original_create):
+        logger.warning("aux_client_hook_failed: create() not callable on adapter")
+        return False
+
+    def patched_create(self: Any, **kwargs: Any) -> Any:
+        real_client = getattr(self, "_client", None)
+        if real_client is None:
+            return original_create(self, **kwargs)
+        messages_obj = getattr(real_client, "messages", None)
+        if messages_obj is None:
+            return original_create(self, **kwargs)
+
+        is_oauth = bool(getattr(self, "_is_oauth", False))
+        if not is_oauth:
+            return original_create(self, **kwargs)
+
+        inner_original = messages_obj.create
+
+        def fixed_messages_create(**inner_kwargs: Any) -> Any:
+            try:
+                _fix_temperature_for_oauth_adaptive(inner_kwargs, site="aux_client")
+            except Exception as exc:
+                logger.warning(
+                    "aux_client_hook: temperature fix raised %s: %s",
+                    type(exc).__name__,
+                    exc,
+                )
+            return inner_original(**inner_kwargs)
+
+        try:
+            messages_obj.create = fixed_messages_create
+            rebind_ok = True
+        except (AttributeError, TypeError):
+            rebind_ok = False
+        try:
+            if rebind_ok:
+                return original_create(self, **kwargs)
+
+            class _ShimMessages:
+                create = staticmethod(fixed_messages_create)
+
+            class _ShimClient:
+                messages = _ShimMessages()
+
+            self._client = _ShimClient()
+            try:
+                return original_create(self, **kwargs)
+            finally:
+                self._client = real_client
+        finally:
+            if rebind_ok:
+                try:
+                    del messages_obj.create
+                except (AttributeError, TypeError):
+                    messages_obj.create = inner_original
+
+    patched_create.__name__ = original_create.__name__
+    patched_create.__qualname__ = getattr(
+        original_create, "__qualname__", original_create.__name__
+    )
+    patched_create.__doc__ = original_create.__doc__
+    patched_create.__wrapped__ = original_create  # type: ignore[attr-defined]
+
+    adapter_cls.create = patched_create
+    adapter_cls._AUX_CLIENT_TEMP_HOOK_APPLIED = True
+    logger.info(
+        "Aux client temperature hook installed on _AnthropicCompletionsAdapter.create"
+    )
+    sys.stderr.write(
+        "[anthropic_billing_bypass] Aux client temperature hook installed\n"
+    )
+    return True
+
+
+def apply_patches(anthropic_adapter_module: Any = None) -> bool:
+    """Install the bypass on ``agent.anthropic_adapter``.
+
+    Called by the sitecustomize hook after the module is imported.  Returns
+    ``True`` on success, ``False`` if the target module is incompatible.
+    Idempotent — safe to call multiple times.
+    """
+    aa = anthropic_adapter_module
+    if aa is None:
+        try:
+            from agent import anthropic_adapter as aa  # type: ignore[import-not-found,no-redef]
+        except ImportError as exc:
+            logger.warning("Cannot import agent.anthropic_adapter: %s", exc)
+            return False
+
+    if getattr(aa, "_CLAUDE_CODE_BYPASS_APPLIED", False):
+        logger.debug("Claude Code bypass already installed")
+        return True
+
+    # 1. Add the missing beta flag.
+    new_beta = "prompt-caching-scope-2026-01-05"
+    oauth_betas = getattr(aa, "_OAUTH_ONLY_BETAS", None)
+    if isinstance(oauth_betas, list) and new_beta not in oauth_betas:
+        oauth_betas.append(new_beta)
+        logger.info("Appended beta flag: %s", new_beta)
+
+    # 2. Verify the target function exists with the expected signature.
+    original_build = getattr(aa, "build_anthropic_kwargs", None)
+    if not callable(original_build):
+        logger.warning(
+            "agent.anthropic_adapter.build_anthropic_kwargs not found — "
+            "skipping monkey-patch (incompatible hermes-agent version?)"
+        )
+        return False
+
+    try:
+        sig = inspect.signature(original_build)
+        if "is_oauth" not in sig.parameters:
+            logger.warning(
+                "build_anthropic_kwargs lacks 'is_oauth' param — "
+                "skipping monkey-patch (incompatible hermes-agent version?)"
+            )
+            return False
+    except (TypeError, ValueError) as exc:
+        logger.warning("Cannot introspect build_anthropic_kwargs: %s", exc)
+        return False
+
+    # 3. Wrap build_anthropic_kwargs to apply the bypass on OAuth requests.
+    def patched_build_anthropic_kwargs(*args: Any, **kwargs: Any) -> Dict[str, Any]:
+        result = original_build(*args, **kwargs)
+
+        try:
+            bound = sig.bind_partial(*args, **kwargs)
+            bound.apply_defaults()
+            is_oauth = bool(bound.arguments.get("is_oauth", False))
+        except TypeError:
+            is_oauth = bool(kwargs.get("is_oauth", False))
+
+        if is_oauth and isinstance(result, dict):
+            try:
+                apply_claude_code_bypass(result, _get_version_safely(aa))
+            except Exception as exc:
+                logger.warning(
+                    "apply_claude_code_bypass raised %s: %s",
+                    type(exc).__name__,
+                    exc,
+                )
+                traceback.print_exc(file=sys.stderr)
+        return result
+
+    patched_build_anthropic_kwargs.__name__ = original_build.__name__
+    patched_build_anthropic_kwargs.__qualname__ = getattr(
+        original_build, "__qualname__", original_build.__name__
+    )
+    patched_build_anthropic_kwargs.__doc__ = original_build.__doc__
+    patched_build_anthropic_kwargs.__module__ = getattr(
+        original_build, "__module__", __name__
+    )
+    patched_build_anthropic_kwargs.__wrapped__ = original_build  # type: ignore[attr-defined]
+
+    aa.build_anthropic_kwargs = patched_build_anthropic_kwargs
+    aa._CLAUDE_CODE_BYPASS_APPLIED = True  # type: ignore[attr-defined]
+    logger.info("Claude Code OAuth bypass installed (build_anthropic_kwargs)")
+    sys.stderr.write("[anthropic_billing_bypass] Claude Code OAuth bypass installed\n")
+
+    _install_aux_client_hook()
+
+    return True