feat: add Claude Code identity transforms for standalone OAuth usage

When proxying with OAuth (no auth plugin), applies the same body
transforms as opencode-anthropic-auth:
- Prepend 'You are Claude Code' identity to system prompt
- Replace 'OpenCode' with 'Claude Code' in system text
- Prefix tool names with 'mcp_' in tools and tool_use blocks
These are only applied when ANTHROPIC_AUTH_TOKEN is set.

src/mnemosyne/gateway.py

@@ -2561,6 +2561,7 @@ def create_app(
 
         adapter = app.state.adapters[provider]
         client: httpx.Client = app.state.clients[provider]
+        oauth_token = os.environ.get("ANTHROPIC_AUTH_TOKEN")
 
         request_id = str(uuid.uuid4())
         started = time.perf_counter()
@@ -2572,6 +2573,68 @@ def create_app(
         session.token_state["model"] = req.model
         duplication_score = _duplication_score(req)
         outgoing_body = adapter.denormalize_request(req)
+
+        # When proxying for OAuth (no auth plugin doing transforms),
+        # apply Claude Code identity transforms to the outbound body.
+        # Ref: rmk40/opencode-anthropic-auth transformRequestBody()
+        if oauth_token and provider == "anthropic":
+            _CLAUDE_CODE_IDENTITY = "You are Claude Code, Anthropic's official CLI for Claude."
+            system = outgoing_body.get("system")
+            if isinstance(system, list):
+                # Strip "OpenCode" identity line, replace OpenCode→Claude Code
+                for i, item in enumerate(system):
+                    if isinstance(item, dict) and item.get("type") == "text" and item.get("text"):
+                        import re as _re_sys
+
+                        text = item["text"]
+                        text = _re_sys.sub(
+                            r"^You are OpenCode, the best coding agent on the planet\.\n*",
+                            "",
+                            text,
+                            flags=_re_sys.MULTILINE,
+                        )
+                        text = text.replace("OpenCode", "Claude Code")
+                        text = _re_sys.sub(
+                            r"(?<!/)opencode", "Claude", text, flags=_re_sys.IGNORECASE
+                        )
+                        system[i] = {**item, "text": text}
+                # Prepend Claude Code identity if not already there
+                has_identity = any(
+                    isinstance(s, str) and s == _CLAUDE_CODE_IDENTITY for s in system
+                )
+                if not has_identity:
+                    system.insert(0, _CLAUDE_CODE_IDENTITY)
+            elif isinstance(system, str):
+                system = system.replace("OpenCode", "Claude Code")
+                if not system.startswith(_CLAUDE_CODE_IDENTITY):
+                    system = f"{_CLAUDE_CODE_IDENTITY}\n{system}"
+                outgoing_body["system"] = system
+
+            # Prefix tool names with mcp_
+            tools = outgoing_body.get("tools")
+            if isinstance(tools, list):
+                for tool in tools:
+                    if (
+                        isinstance(tool, dict)
+                        and tool.get("name")
+                        and not tool["name"].startswith("mcp_")
+                    ):
+                        tool["name"] = f"mcp_{tool['name']}"
+            # Prefix tool_use names in messages
+            messages = outgoing_body.get("messages")
+            if isinstance(messages, list):
+                for msg in messages:
+                    content = msg.get("content")
+                    if isinstance(content, list):
+                        for block in content:
+                            if (
+                                isinstance(block, dict)
+                                and block.get("type") == "tool_use"
+                                and block.get("name")
+                                and not block["name"].startswith("mcp_")
+                            ):
+                                block["name"] = f"mcp_{block['name']}"
+
         if endpoint == "count_tokens":
             outgoing_body.pop("stream", None)
             outgoing_body.pop("max_tokens", None)
@@ -2590,7 +2653,6 @@ def create_app(
         # Anthropic requires specific beta headers and a Claude-CLI-style
         # user-agent for OAuth Bearer auth to be accepted.
         # Ref: rmk40/opencode-anthropic-auth request-headers.mjs
-        oauth_token = os.environ.get("ANTHROPIC_AUTH_TOKEN")
         if oauth_token and provider == "anthropic":
             headers.pop("x-api-key", None)
             headers.pop("X-Api-Key", None)