fix(ai-proxy): keep empty anthropic tools array - #14999

Summary

An OpenAI-format chat request with tools: [], proxied by ai-proxy to the anthropic provider, leaves Kong as "tools": {}. Anthropic rejects that with 400 tools: Input should be a valid array — the same error reported in #14983.

Root cause: to_tools() in kong/llm/drivers/anthropic.lua builds the translated list as a plain {}. The request body itself is decoded with array_mt (kong.request.get_body → decode_with_array_mt), but this new table has no metatable, so when nothing is inserted cjson encodes it as an object in normalize-request → kong.service.request.set_body.

The fix creates that table with cjson.array_mt. Non-empty tools encode exactly as before.

This does not replace #14984. #14983 is about ai-prompt-decorator with the native Anthropic format, which is not in this repo's decorator schema (openai, bedrock, gemini). I found this separate path on master while reproducing it: no decorator needed, just ai-proxy + anthropic + tools: [].

Reproduction

resty in the kong:3.9 image with this repo's master sources mounted on package.path. Input decoded with kong.tools.cjson.decode_with_array_mt, then anthropic.to_format(..., "llm/v1/chat"), then cjson.safe.encode (what set_body uses):

in : {"messages":[{"role":"user","content":"ping"}],"tools":[]}
out: {"model":"claude-opus-4","max_tokens":8,"stream":false,"tools":{},"messages":[{"role":"user","content":"ping"}]}

Tests

New unit case in spec/03-plugins/38-ai-proxy/01-unit_spec.lua ("anthropic tools").

Run locally: busted 2.3.0 under resty in the kong:3.9 image (OpenResty LuaJIT + lua-cjson from that image), master sources from this branch. This is not Kong's make test dev environment.

• without the driver change: not ok — Passed in: '{..."tools":{}}', Expected: '"tools":[]'
• with the change: ok
• whole 01-unit_spec.lua: 156/156 ok with the change, 155/156 without (only the new case fails)
• luacheck on both touched Lua files: 0 warnings / 0 errors

Not run: integration specs (03-anthropic_integration_spec.lua) and the full CI matrix.

Changelog: changelog/unreleased/kong/fix-ai-proxy-anthropic-empty-tools.yml.

I built this with Claude Code as an implementation collaborator; I checked the reproduction and test runs above.

— Anton Dziatkovskii · github.com/tonydzi

🤖 Generated with Claude Code