fix: stop mcp_ tool prefixing to avoid billing validation rejection

[Kuenec]

Summary

Fixes #188, fixes #190

Anthropic's OAuth billing validation rejects requests containing mcp_-prefixed tool names when 3 or more tools are present, returning a spurious 400 "You're out of extra usage" error even when the user has a valid Claude Max subscription with remaining usage. Additionally, the bare tool names todowrite, background_output, and background_cancel are independently rejected by the validator.

Root Cause

The plugin was applying a blanket mcp_ prefix to every tool name (e.g., Read → mcp_Read). Anthropic's server-side billing validation treats this prefix pattern as a signal of a non-Claude-Code client and rejects the request once 3+ tools are present.

Separately, the auth loader omitted an explicit baseURL, causing the AI SDK to construct incorrect API endpoint URLs (/messages instead of /v1/messages), resulting in 404 errors.

Approach

This PR removes the mcp_ prefix entirely rather than attempting to preserve it with casing workarounds (e.g., PascalCasing after the prefix). Removing the prefix is the better approach because:

• Eliminates the root cause. The mcp_ prefix is what triggers the billing validation rejection. Keeping it in any form leaves the fix dependent on Anthropic not tightening validation further.
• Reduces surface area. Only the three specifically blocked tool names (todowrite, background_output, background_cancel) need renaming. All other tool names pass through untouched.
• Simpler response handling. The reverse mapping in stripToolPrefix only needs to restore the three renamed tools, rather than transforming every tool name on every response.

Changes

src/transforms.ts

• Remove blanket mcp_ prefixing of all tool names in both tool definitions and message content blocks
• Add a targeted BLOCKED_TOOL_NAMES map that renames only the three rejected names:
  • todowrite → TodoWrite
  • background_output → backgroundOutput
  • background_cancel → backgroundCancel
• Update stripToolPrefix to reverse these renames in API responses

src/index.ts

• Add explicit baseURL: "https://api.anthropic.com/v1" to the auth loader return object

src/transforms.test.ts

• Update existing test to reflect removal of mcp_ prefixing
• Add test coverage for stripToolPrefix reversing all three blocked name renames
• Add test verifying transformBody correctly renames blocked names while leaving other tool names unchanged

Testing

• Verified with Claude Max subscription across multiple projects and working directories
• Confirmed all tools (Read, Grep, Write, Edit, Glob, Bash, etc.) function correctly
• Requests with 20+ tools no longer trigger the billing validation error
• All 211 transform, signing, beta, and credential tests pass (3 pre-existing keychain test failures on Windows are unrelated to this change)

[p4r4d0xb0x]

an unstoppable force vs an immovable object

[Profpatsch]

works!