fix(anthropic): use double-underscore mcp__ prefix for OAuth tool names

[liuhao1024]

What does this PR do?

Changes the Anthropic OAuth tool-name prefix from single underscore (mcp_) to double underscore (mcp__) so that tool-using requests over Max OAuth are correctly classified as Claude Code and route to plan billing instead of being rejected as third-party (HTTP 400).

Related Issue

Fixes #46675

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• agent/anthropic_adapter.py: Changed _MCP_TOOL_PREFIX from "mcp_" to "mcp__". Updated startswith guards to check both "mcp__" and "mcp_" prefixes via a _MCP_SKIP tuple, preventing double-prefixing of native MCP server tools that use the legacy single-underscore format.
• agent/transports/anthropic.py: Changed the local _MCP_PREFIX from "mcp_" to "mcp__" in normalize_response so the response-side stripping matches the new outgoing prefix.
• tests/agent/test_anthropic_mcp_prefix_strip.py: Updated all test assertions to use mcp__ prefix. Added test_legacy_single_underscore_native_mcp_not_stripped to verify that native MCP tools with the old mcp_ format are not incorrectly stripped.

How to Test

1. Configure an Anthropic Max OAuth token: hermes auth add anthropic --type oauth
2. Start a conversation with tools enabled (default)
3. Verify the request returns HTTP 200 with service_tier: standard (plan-eligible)
4. If MCP servers are configured, verify their tools still work (not double-prefixed)
5. Run: pytest tests/agent/test_anthropic_mcp_prefix_strip.py -v

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/agent/test_anthropic_mcp_prefix_strip.py -v and all 12 tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A (pure string constant change, no platform-specific behavior)
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Code Intelligence

• Analyzed: agent/anthropic_adapter.py::_MCP_TOOL_PREFIX, agent/transports/anthropic.py::normalize_response
• Blast radius: LOW — 2 string constants, symmetric prefix/strip, no control-flow changes
• Related patterns: [Bug]: Anthropic OAuth strips mcp_ prefix from Hermes-native MCP tool names, breaking registry lookup #25255 (original mcp_ prefix guard), OAuth billing classifier

[alt-glitch]

Related: fix PR for #46675.

[fzlzjerry]

can we merge it

[kshitijk4poor]

Superseded by #47723, which cherry-picks this commit (your mcp_→mcp__ change, authorship preserved in git history) and extends it to also normalize single-underscore mcp_<server>_<tool> MCP-server tool names — the gap that left sessions with an MCP server configured still billing to extra usage. Thanks for the original fix; it's carried forward in #47723. Closing this in favor of the combined PR.