[compatibility] Z.AI (Zhipu GLM) rejects any tool_choice other than "auto" with HTTP 400 — "Tool choice must be auto"

[kxlion]

Summary

Hindsight's Reflect agent loop forces a retrieval tool on its first turn by sending
tool_choice: "required" (named tool_choice is normalized to required with the tools
list pre-filtered to the single forced tool, per openai_compatible_llm.py). The Z.AI
(Zhipu) endpoint — used for GLM models, e.g. z-ai/glm-5.3-flash — rejects every
tool_choice value except "auto" with HTTP 400. This is behaviorally identical to
Meta Model API, which is already handled via _NON_AUTO_TOOL_CHOICE_UNSUPPORTED_PROVIDERS.

Reproduction

Via OpenRouter with provider.only: ["z-ai"] — the error originates from the Z.AI
backend (see metadata.provider_name in the payload):

Request	Result
tool_choice: {"type":"function","function":{"name":"recall"}} + tools	❌ HTTP 400
tool_choice: "required" + tools	❌ HTTP 400
tool_choice: "auto" + same single tool	✅ 200, tool called 5/5

Error payload:

{"error":{"message":"Tool choice must be auto","code":400,"metadata":{"provider_name":"Z.AI"}}}

The "auto" row was verified with 5/5 successful tool calls on memory-retrieval style
prompts (system prompt instructing mandatory tool use, single tool available), so the
model does not need the forced tool_choice to reliably call the tool.

Impact

Every Reflect (and any other forced-tool) operation against GLM models routed to Z.AI
fails after retries. Retain/Consolidation are unaffected (no forced tool_choice).

Tested on z-ai/glm-5.3-flash with Hindsight 0.9.2, September 2026 — still current.

Suggested fix

Add Z.AI (zai) to _NON_AUTO_TOOL_CHOICE_UNSUPPORTED_PROVIDERS. The downgrade to auto
is functionally safe here: named tool_choice is already normalized to required with the
tools list narrowed to the single forced tool, so under auto the call remains practically
forced — the same reasoning as the existing DeepSeek and Meta branches.

Workaround currently in use

HINDSIGHT_API_REFLECT_LLM_EXTRA_BODY={"tool_choice":"auto"}

Verified working (Reflect succeeds end-to-end on Z.AI), but it relies on extra_body
merge precedence and is not an API contract. Alternative: route Reflect to a
tool-forcing-compatible host (e.g. Novita).