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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallIf your self-hosted, proxied, or OpenAI-compatible LLM endpoint returns HTTP 400, inspect the exact JSON request before changing it. An explicit "content": null can be rejected by some validators—but the right correction depends on whether the request uses the Responses API or Chat Completions, and on the message type. A 400 status alone does not identify the cause.
Start with the response and the request that failed
Record the response body, request method and endpoint path, model or provider, and client version. Then capture the JSON actually sent over the wire; a client’s displayed conversation may not match its serialized request.
Look for the field named in the error. If the error points to content, check whether the value is explicitly null, omitted, an empty string, or an empty array. These are distinct JSON shapes, and an endpoint may accept one while rejecting another. If the request does not contain content: null, or the error identifies another field, investigate that field instead. HTTP 400 has other causes, including a missing required parameter on a differently routed API; see this Perplexity API report involving max_tokens and the OpenClaw guide to other invalid-request conditions.
Identify which API shape and message type failed
Two reported cases involve content: null, but they concern different API surfaces and should not be treated as interchangeable.
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| Request context | What to inspect | Reported adjustment |
|---|---|---|
| Responses API: a reasoning item replayed in a later turn | Reasoning items in input[*] |
Omit content when it is absent in this reported case. |
| Chat Completions: assistant tool-call history message | messages[*].content around the assistant tool call and its result |
The report suggests an empty string for a tool-only assistant message on the affected strict endpoint class. |
Responses API: check replayed reasoning items
A Codex CLI issue opened September 24, 2026 describes a multi-turn failure using Codex CLI 0.154.0, the Responses wire format, and xAI’s grok-4.6 model. The reporter says the provider returned a reasoning item without a content key, and the next request serialized that missing value as "content": null. The issue quotes the Responses schema as allowing ReasoningItem.content to be an array when present but not requiring it or defining a null value; the sibling encrypted_content is explicitly nullable. The issue is a report of that schema detail, not an independent schema check here.
The provider’s 400 response blamed a compaction blob. According to the reporter, a local proxy that removed only content: null changed the follow-up response to HTTP 200. That makes the serialized request worth checking even when the error message points elsewhere. In this reported Responses scenario, the adjustment was to omit an absent field, not to substitute an empty string. See the Codex CLI issue #47733 for the reproduction and its qualifications.
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Chat Completions: check assistant tool-call history
A KiloCode issue opened July 17, 2026 reports 400 responses from strict OpenAI-compatible /v1/chat/completions endpoints. The backend error identified /messages/2/content and said it expected a string or array but received null. The reporter says the behavior reproduced with KiloCode 7.4.11 and 7.3.44, and suggests normalizing a tool-only assistant message’s content to an empty string for that reported endpoint class.
Inspect the assistant tool-call message and surrounding tool result in the serialized messages array. Do not apply the empty-string workaround automatically to a Responses reasoning item: the two reports describe different schemas and contexts. Consult the KiloCode issue #12330 for the reported error and suggested compatibility adjustment.
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Test a correction without breaking the conversation
- Reproduce the failing request. Preserve the endpoint, model or provider, client version, response body, and exact request JSON.
- Reduce the history carefully. Keep the reasoning item or assistant tool-call message and the associated follow-up or tool result that the failure depends on.
- Change one serialization detail. For the reported Responses case, omit an absent reasoning
contentfield. For the reported strict Chat Completions case, test the suggested empty string for a tool-only assistant message. - Retry and check both outcomes. Confirm whether the endpoint accepts the request and whether the conversation or tool-call sequence still makes sense. If the adjustment fails, restore the original request and investigate the field named by the error rather than trying other content shapes at random.
Why an empty value is not the same as an absent one
In JSON, an omitted property, null, "", and [] encode different values. A client may turn an absent value into null during serialization, while a particular endpoint accepts only a string or array. The endpoint’s contract and the message type—not the fact that each value might look “empty”—determine which shape is valid.
The issue reports establish two specific compatibility symptoms, not a universal rule for all LLM endpoints. Provider implementations and accepted request schemas can differ or change. Follow the contract for the exact API route and message type you are sending.
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