fix: proper Anthropic SDK integration - no prompt/tool rewriting

[nmime]

Summary

When the gateway is in Claude passthrough mode (source and target are both Claude format), the code was unnecessarily translating/rewriting prompts and tools. This PR removes that redundant translation logic.

Changes

• open-sse/handlers/chatCore.ts — Simplified Claude passthrough logic to skip unnecessary rewriting
• open-sse/services/claudeCodeToolRemapper.ts — Removed ~39 lines of rewriting code for passthrough mode
• open-sse/translator/helpers/claudeHelper.ts — Removed 3 lines of redundant transformation
• open-sse/translator/request/openai-to-claude.ts — Removed 12 lines of unnecessary translation

All changes are removals/simplifications — no new functionality added. Build passes.

[gemini-code-assist]

Code Review

This pull request streamlines the Claude passthrough mechanism by removing the OpenAI translation round-trip, eliminating hardcoded system prompts, and clearing the tool name prefix. Review feedback indicates several critical issues: the removal of the signature field from thinking blocks will likely cause API errors as it is required by Anthropic for assistant-role blocks; the filtering of empty text blocks is inconsistently applied across passthrough paths; and gutting the tool remapping service breaks functionality for other providers that rely on TitleCase remapping.

[gemini-code-assist]

The logic for filtering empty text blocks is inconsistently applied and duplicated. It is present in the isClaudePassthrough block (lines 1054-1063) but missing from the isClaudePassthrough && preserveCacheControl block (line 1045). Since Anthropic's API rejects empty text blocks regardless of cache control, this filtering should be applied to both. Consolidating the logic as suggested improves maintainability and ensures consistent behavior.

    } else if (isClaudePassthrough) {
      translatedBody = { ...body };
      translatedBody._disableToolPrefix = true;

      // Filter empty text content blocks from messages.
      // Anthropic API rejects {"type":"text","text":""} with 400 "text content blocks must be non-empty".
      if (Array.isArray(translatedBody.messages)) {
        for (const msg of translatedBody.messages) {
          if (Array.isArray(msg.content)) {
            msg.content = msg.content.filter(
              (block: Record<string, unknown>) =>
                block.type !== "text" || (typeof block.text === "string" && block.text.length > 0)
            );
          }
        }
      }

      if (preserveCacheControl) {
        log?.debug?.("FORMAT", "claude passthrough with cache_control preservation");
      } else {
        log?.debug?.("FORMAT", "claude direct passthrough (no round-trip)");
      }

[gemini-code-assist]

The signature field is required by the Anthropic Messages API for thinking blocks in the assistant role. Removing it here will likely result in 400 errors from the upstream provider when thinking mode is enabled and a placeholder thinking block is injected.

Suggested change

	if (block.type === "thinking" || block.type === "redacted_thinking") {
	block.signature = DEFAULT_THINKING_CLAUDE_SIGNATURE;
	hasThinking = true;
	}
	if (block.type === "thinking" || block.type === "redacted_thinking") {
	block.signature = DEFAULT_THINKING_CLAUDE_SIGNATURE;
	hasThinking = true;
	}

[gemini-code-assist]

As noted above, the signature field is required for thinking blocks in the assistant role. Removing it from this injected placeholder block will cause validation errors when sending the request to Anthropic.

            msg.content.unshift({
              type: "thinking",
              thinking: ".",
              signature: DEFAULT_THINKING_CLAUDE_SIGNATURE,
            });

[gemini-code-assist]

Removing the signature from the thinking block may cause the Anthropic API to reject the request. Anthropic requires a signature for thinking blocks in the assistant role when they are part of the message history. Since OpenAI's reasoning_content does not include a signature, a default one should be provided during translation to ensure the resulting Claude request is valid.

      blocks.push({
        type: "thinking",
        thinking: msg.reasoning_content,
        signature: DEFAULT_THINKING_CLAUDE_SIGNATURE,
      });

[gemini-code-assist]

Gutting these functions disables tool name remapping for all callers, including the isClaudeCodeCompatible path in chatCore.ts (line 1040). This breaks the TitleCase remapping feature intended for anthropic-compatible-cc-* providers. If this remapping is truly redundant for all paths, the call site in chatCore.ts and the descriptive comments in this file should also be removed. Otherwise, remapping should only be skipped for the passthrough mode.

[nmime]

Thanks for the review @gemini-code-assist — all valid points. Pushed a follow-up commit (fef2b93) that:

1. Signature fields: Reverted all changes to claudeHelper.ts and openai-to-claude.ts — these shared translation files are used by non-passthrough paths and the signature is required there. In passthrough mode, the body is forwarded as-is so the original signatures from Claude Code are preserved.

2. Tool remapping: Reverted claudeCodeToolRemapper.ts — the TitleCase remapping is needed by the isClaudeCodeCompatible bridge path. Only the passthrough path skips it (which was already the case since _disableToolPrefix = true is set).

3. Consistent filtering: Merged the two isClaudePassthrough blocks into one with empty-text filtering applied unconditionally. The preserveCacheControl flag is now only used for the debug log message.

The final diff touches only chatCore.ts — removing the unnecessary Claude→OpenAI→Claude round-trip from the passthrough path while leaving all shared translation code untouched.

[gemini-code-assist]

Thanks for the update, @nmime. The changes in chatCore.ts look much cleaner now that the redundant round-trip is removed, and the logic for filtering empty text blocks is appropriately scoped to the passthrough path. Reverting the changes to the shared translation files and the tool remapper is the correct approach to maintain compatibility for non-passthrough paths. This looks good.