Anthropic server tools (web_search, etc.) forwarded to third-party providers with no parameters → 400 #2754

Bug

When a request from Claude Code (Anthropic Messages API format) includes an Anthropic server tool like web_search, Manifest's toChatTools adapter converts it to chat completions format but strips parameters because it's a server tool. The tool is then forwarded to a third-party provider (e.g. MiniMax) that requires parameters on every function tool, resulting in a 400.

Reproduction

Request from Claude Code → Manifest gateway (self-hosted) → MiniMax (MiniMax-M2.7-highspeed), chat completions.

The inbound tool (Anthropic Messages format):

{ "type": "web_search_20250305", "name": "web_search" }

The outbound tool after toChatTools (chat completions format):

{ "type": "function", "function": { "name": "web_search" } }

MiniMax response:

invalid params, function parameters is empty (2013)

Phoenix issue: 99ce618b-8c80-47d0-919d-cd5ac9d994d9 (no_fix_found)

Root cause

packages/backend/src/routing/proxy/anthropic-messages-adapter.ts, toChatTools:

...(tool.input_schema !== undefined
  ? { parameters: normalizeOpenAiFunctionSchema(tool.input_schema) }
  : typeof tool.type === 'string' &&
      tool.type !== 'custom' &&
      !isAnthropicServerToolType(tool.type)
    ? { parameters: DEFAULT_CUSTOM_TOOL_INPUT_SCHEMA }
    : {}),

Anthropic server tools (web_search_, bash_, computer_, text_editor_, etc.) match isAnthropicServerToolType, so the else branch is empty — no parameters is added. The tool is then forwarded to the provider as { type: "function", function: { name: "web_search" } }, which MiniMax (and likely other strict providers) reject.

Fix options

1. Inject DEFAULT_CUSTOM_TOOL_INPUT_SCHEMA for server tools too when converting to chat completions format — so the outbound tool always has parameters. Safe because the provider ignores the schema for a tool it doesn't recognize; the error is the missing field, not the schema content.

2. Drop Anthropic server tools entirely when routing to a non-Anthropic provider — they're Anthropic-internal tools that a third-party provider can't execute anyway. The model would then get no tool and respond without web search, rather than the request failing.

Option 2 is more correct (the tool can't work on MiniMax regardless), but option 1 is the smaller change and unblocks the request. Probably both: drop server tools for non-Anthropic providers, and as a belt-and-suspenders default, always emit parameters on any function tool.