[BUG] Deferred custom tools break non-Anthropic models behind custom ANTHROPIC_BASE_URL

Summary

Claude Code enables deferred tool loading / ToolSearch when used through a custom ANTHROPIC_BASE_URL.

When the backend serves a non-Anthropic model that supports normal tool calling but does not implement Anthropic's deferred custom tool protocol, Claude Code can fail with:

API Error: 400 Deferred custom tools are only supported on Anthropic models and on Anthropic-compatible
provider endpoints that implement deferral. Other endpoints cannot call tools omitted from tools[].
Received stealth/ox-alpha.

In some agentic tasks, the symptom can also look less obvious: Claude Code works for several seconds, returns to the prompt before the task is finished, and the user has to repeatedly type continue.

Environment

• Claude Code: 2.1.241
• Platform: win32-x64
• Installation: npm global
• Custom endpoint configured through ANTHROPIC_BASE_URL
• Model: stealth/ox-alpha
• Operating system: Windows
• Claude Code doctor reports:

ANTHROPIC_BASE_URL is set and does not point at api.anthropic.com

Reproduction

1. Configure Claude Code to use a custom Anthropic-compatible endpoint via ANTHROPIC_BASE_URL.
2. Use a non-Anthropic model supporting standard tool calling, e.g. stealth/ox-alpha.
3. Start Claude Code normally.
4. Send a prompt that causes Claude Code to use a deferred tool.
5. Claude Code returns:

API Error: 400 Deferred custom tools are only supported on Anthropic models and on Anthropic-compatible
provider endpoints that implement deferral. Other endpoints cannot call tools omitted from tools[].
Received stealth/ox-alpha.

Root Cause

Claude Code can use ToolSearch/deferred tool loading, meaning some tool definitions are omitted from the initial tools[] request and loaded later.

This requires support for Anthropic's deferred tool semantics.

A backend/model may support ordinary Anthropic-style tool calling while not supporting deferred custom tools. In that case, Claude Code's deferred tool mode is incompatible with the endpoint.

Workaround

Disable ToolSearch / deferred loading:

PowerShell

$env:ENABLE_TOOL_SEARCH="false"
claude

Or configure it persistently in Claude Code settings:

{
  "env": {
    "ENABLE_TOOL_SEARCH": "false"
  }
}

With ToolSearch disabled, tools are provided normally instead of relying on deferred loading.

Expected Behavior

For a custom ANTHROPIC_BASE_URL, Claude Code should either:

1. detect whether the provider supports deferred custom tools before enabling ToolSearch; or
2. automatically fall back to standard/eager tool loading if deferred tools are unsupported; or
3. provide a clear startup warning explaining that ENABLE_TOOL_SEARCH=false is required.

Ideally, users should not encounter this only after an agent attempts to invoke a deferred tool.

Additional Context

This is especially confusing because the endpoint/model may otherwise support normal tool calling correctly. The failure is specifically related to deferred tool definitions rather than tool calling in general.