Responses API: undocumented reasoning+message pairing constraint breaks multi-turn conversations

Reasoning+message items must appear as consecutive pairs in input, but nothing documents this. The most common pattern — filtering response.output() to keep only messages — silently produces orphaned items → 400 on the next turn.

This broke OpenClaw (64.9k forks) on gpt-5.3-codex. They had to add downgradeOpenAIReasoningBlocks() to strip orphan reasoning items.

400: Item 'msg_...' of type 'message' was provided without its required preceding item of type 'reasoning'

Tested in Java, JS, Python, Go, .NET — identical results across all 5 SDKs. This is an API-level constraint, not SDK-specific. Confirmed via curl (see gist). The SDK types don't prevent building input arrays that violate it.

* Nondeterministic — o4-mini sometimes returns reasoning-only output (no message to orphan). Codex with reasoning=high reliably returns both items.

Workaround: previousResponseId(). For manual history, always pass reasoning+message pairs together.

Related:

• openai/openai-openapi#536 (spec root cause)
• openai/openai-node#1791 (same bug, Node SDK)
• openai/openai-python#TBD (same bug, Python SDK)
• openai/openai-dotnet#TBD (same bug, .NET SDK)
• openai/openai-go#TBD (same bug, Go SDK)
• openclaw/openclaw#49167 (64.9k forks, production breakage)

To Reproduce

import com.openai.client.OpenAIClient;
import com.openai.client.okhttp.OpenAIOkHttpClient;
import com.openai.models.responses.*;

import java.util.ArrayList;
import java.util.List;

public class PairingConstraintRepro {
    public static void main(String[] args) {
        OpenAIClient client = OpenAIOkHttpClient.builder()
            .apiKey(System.getenv("OPENAI_API_KEY"))
            .build();

        String[] prompts = {"Write a Python prime checker.", "Add type hints.", "Add docstrings."};
        List<ResponseInputUnionParam> conversation = new ArrayList<>();

        for (String msg : prompts) {
            System.out.println("\n> " + msg);
            conversation.add(ResponseInputUnionParam.ofEasyInputMessage(
                EasyInputMessageParam.builder()
                    .role(EasyInputMessageParam.Role.USER)
                    .content(msg)
                    .build()));

            try {
                Response response = client.responses().create(
                    ResponseCreateParams.builder()
                        .model("gpt-5.3-codex")
                        .input(ResponseCreateParams.Input.ofResponseInputs(conversation))
                        .maxOutputTokens(300)
                        .reasoning(Reasoning.builder().effort(Reasoning.Effort.HIGH).build())
                        .build());

                // Common pattern: keep only messages, discard reasoning
                for (ResponseOutputItem item : response.output()) {
                    if (item.isMessage()) {
                        // Convert output message to input — orphan message → 400
                        conversation.add(ResponseInputUnionParam.ofResponseInputItem(
                            ResponseInputItemParam.builder()
                                .id(item.asMessage().id())
                                .type(ResponseInputItemParam.Type.MESSAGE)
                                .build()));
                    }
                }
            } catch (Exception e) {
                System.out.println("  ERROR: " + e.getMessage().substring(0, Math.min(120, e.getMessage().length())));
                break;
            }
        }
        // Turn 2 → 400: Item 'msg_...' was provided without its required preceding item
    }
}

Note: the Java SDK's distinct input/output wrapper types make the conversion verbose, but the pairing constraint failure is the same regardless.

Reproduced on o4-mini and gpt-5.3-codex. Full cross-language repro (JS, Python, .NET, curl): https://gist.github.com/achandmsft/57886350885cec3af8ef3f456ed529cf

OS

Windows 11, also reproduced on Linux

Java version

Java 21

Library version

openai-java v4.29.0

Start with the Responses API flow shown in PairingConstraintRepro, especially ResponseInputUnionParam, ResponseCreateParams, response.output(), and previousResponseId(). Reproduce the second-turn failure with the listed models and determine whether the remedy belongs in this SDK or the API; done requires a clearly identified, testable ownership and behavior change.