DeepSeek V4.1 Flash Responses emits non-roundtrippable reasoning.content:null

[ele-yufo]

With deepseek/deepseek-v4.1-flash on /provider/v1/responses, a tool-call response contains a reasoning item with content: null. Replaying that item (as Responses clients such as Codex CLI do) alongside its function_call and a string function_call_output returns HTTP 400 Invalid input (param: input).

Minimal reproduction

1. Send a /provider/v1/responses request to deepseek/deepseek-v4.1-flash with one function tool; the model returns a reasoning item whose content is null, followed by a function_call.
2. Send the next turn with input = [previous user message, the returned reasoning item unchanged, the function_call, a function_call_output with a string output].
3. Result: HTTP 400 Invalid input, param: input.

Controls

• Identical request with only reasoning.content changed from null to [] → HTTP 200.
• Identical request with the reasoning item removed → HTTP 200.
• Changing store, parallel_tool_calls, or call IDs does not affect the result.
• The same flow works with mimo-v2.6-flash, mimo-v2.6-pro and step-5-preview on the same endpoint.

Impact
Codex CLI (and other Responses clients that replay reasoning items) cannot complete any tool-using turn with DeepSeek V4.1 Flash on the GOAT plan.

Request
Please either emit a round-trippable reasoning item (e.g. content: []) or accept content: null on input, and ideally identify the offending field in the Invalid input error.