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If your self-hosted, proxied or OpenAI-compatible LLM endpoint returns HTTP 400, inspect the actual request JSON before changing your prompt. In two reported cases, an assistant item serialized with "content": null was rejected—but the correct compatibility fix differed between the Responses API and Chat Completions. A 400 status alone does not identify the cause.
First confirm what the endpoint rejected
Capture the response body and the exact outgoing request. Record the HTTP method, endpoint path, model or provider, and client version. The server’s validation message and the serialized JSON are more useful than the status code by itself.
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Check whether the request actually contains "content": null, and identify which item contains it. An omitted field, null, an empty string ("") and an empty array ([]) are distinct JSON values; an endpoint may accept some and reject others.
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- For a Responses API request, inspect reasoning items in
input[*]. - For a Chat Completions request, inspect
messages[*].content, especially the assistant message associated with a tool call.
If the error names another field, or no relevant content: null appears in the request, investigate the named field and the endpoint routing instead.
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Identify the API surface and message type
The two reported failures involve different schemas. Do not apply a fix for a Chat Completions tool-call message to a Responses reasoning item—or assume that one provider’s OpenAI-compatible endpoint validates requests exactly like another.
| Request context | Reported failure | Reported adjustment |
|---|---|---|
| Responses API reasoning item replayed in a later turn | A client serialized an absent reasoning-item field as "content": null; the provider returned HTTP 400. |
Omit the absent content field, as described in the report. |
| Chat Completions assistant tool-call history | A strict compatible endpoint rejected messages[*].content: null, reporting that it expected a string or array. |
The report suggests normalizing tool-only assistant content to an empty string for that endpoint class. |
Responses API: check replayed reasoning items
A Codex CLI issue opened September 24, 2026 describes a multi-turn reproduction using Codex CLI 0.154.0, the Responses wire format and xAI’s grok-4.6 model. According to the reporter, the provider returned a reasoning item without a content key, but Codex sent "content": null when replaying it in the next request. The report quotes the Responses schema as allowing content to be absent and, when present, to be an array; it says the sibling encrypted_content field is explicitly nullable. Read the Codex issue report.
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The reporter says xAI’s 400 response blamed a compaction blob, while a local proxy that removed only content: null let the follow-up request return HTTP 200. That result is the reporter’s control, not an independently reproduced guarantee. It does show why inspecting the serialized request can be useful even when the error text points elsewhere.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsFor this specific case, test omission of the absent reasoning-item field rather than replacing it with an empty string. Keep the rest of the request unchanged so you can tell whether the serialization change affects the response.
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Chat Completions: check assistant tool-call history
A KiloCode issue opened July 17, 2026 reports HTTP 400 errors from strict OpenAI-compatible /v1/chat/completions endpoints when a previous assistant tool-call message was sent with "content": null. The reported backend error points to /messages/2/content and says it expected a string or array but received null. The author reports reproducing the issue with KiloCode 7.4.11 and 7.3.44. Read the KiloCode issue report.
For the endpoint class described in that report, the author suggests normalizing a tool-only assistant message’s content to an empty string. Treat that as a reported compatibility workaround, not a universal rule: confirm the endpoint’s contract and preserve the assistant tool-call and tool-result sequence when testing it.
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Test a fix without changing unrelated request data
- Save the failing request and response. Keep the response body, endpoint path, provider or model, and client version alongside the JSON you captured.
- Reduce the conversation history. Retain the assistant item and any tool result needed to reproduce the failure, then remove unrelated turns.
- Change one serialization detail. For the reported Responses case, omit an absent reasoning
contentfield. For the reported strict Chat Completions case, test the author’s empty-string normalization for tool-only assistant content. - Send the same request again. Compare the HTTP result and response body, and confirm that the tool-call sequence or conversation state remains valid.
- Check the exact endpoint contract. If the change does not resolve the error, or the validator identifies another field, investigate that field rather than continuing to alter
content.
Why HTTP 400 is not a diagnosis
HTTP 400 means a request was rejected; it does not by itself establish that content is at fault. A Perplexity API forum report, for example, describes intermittent 400s associated with an omitted max_tokens value when routing to an Anthropic Messages API. That report concerns a different request field and route. OpenClaw’s OpenAI HTTP API guide also documents other invalid-request conditions. Consult the guide for its documented cases.
The two content: null examples are issue reports, not population-level evidence that this is a common cause of 400s. Endpoint implementations and schemas can differ or change; use the error details and the live contract for the endpoint you are calling.
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