diff --git a/CHANGELOG.md b/CHANGELOG.md
index c4242fd7826..16ce400c773 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,6 +24,7 @@ Docs: https://docs.openclaw.ai
 ### Fixes
 
 - Auto-reply: honor explicit `silentReply.direct: "allow"` for clean empty or reasoning-only direct chat turns while keeping the default direct-chat empty-response guard conservative. Fixes #74409. Thanks @jesuskannolis.
+- OpenAI Codex: send a non-empty Responses input item when a Codex turn only has systemPrompt-backed instructions, avoiding ChatGPT backend 400s from `input: []`. Fixes #73820. Thanks @woodhouse-bot.
 - Ollama: normalize provider-prefixed tool-call names at the native stream boundary so Kimi/Ollama calls such as `functions.exec` dispatch as `exec` instead of missing configured tools. Fixes #74487. Thanks @afurm and @carreipeia.
 - Security/audit: resolve configured model aliases before model-tier and small-parameter checks, so alias-based GPT-5/Codex configs no longer report false weak-model warnings. Fixes #74455. Thanks @blaspat.
 - CLI/agent: isolate Gateway-timeout embedded fallback runs under explicit `gateway-fallback-*` sessions so accepted Gateway runs cannot race transcript locks or replace the routed conversation session. Fixes #62981. Thanks @HemantSudarshan.
diff --git a/src/agents/openai-transport-stream.test.ts b/src/agents/openai-transport-stream.test.ts
index 13bf1a7e7f3..d3f377310cf 100644
--- a/src/agents/openai-transport-stream.test.ts
+++ b/src/agents/openai-transport-stream.test.ts
@@ -1031,6 +1031,40 @@ describe("openai transport stream", () => {
     expect(params.service_tier).toBe("auto");
   });
 
+  it("adds minimal user input for Codex responses when only the system prompt is present", () => {
+    const params = buildOpenAIResponsesParams(
+      {
+        id: "gpt-5.4",
+        name: "GPT-5.4",
+        api: "openai-codex-responses",
+        provider: "openai-codex",
+        baseUrl: "https://chatgpt.com/backend-api",
+        reasoning: true,
+        input: ["text"],
+        cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+        contextWindow: 200000,
+        maxTokens: 8192,
+      } satisfies Model<"openai-codex-responses">,
+      {
+        systemPrompt: `Stable prefix${SYSTEM_PROMPT_CACHE_BOUNDARY}Dynamic suffix`,
+        messages: [],
+        tools: [],
+      } as never,
+      undefined,
+    ) as {
+      input?: Array<{ role?: string; content?: Array<{ type?: string; text?: string }> }>;
+      instructions?: string;
+    };
+
+    expect(params.instructions).toBe("Stable prefix\nDynamic suffix");
+    expect(params.input).toEqual([
+      {
+        role: "user",
+        content: [{ type: "input_text", text: " " }],
+      },
+    ]);
+  });
+
   it("does not infer high reasoning when Pi passes thinking off", () => {
     const params = buildOpenAIResponsesParams(
       {
diff --git a/src/agents/openai-transport-stream.ts b/src/agents/openai-transport-stream.ts
index 805c31a5a93..7214f9564b5 100644
--- a/src/agents/openai-transport-stream.ts
+++ b/src/agents/openai-transport-stream.ts
@@ -53,6 +53,7 @@ import { transformTransportMessages } from "./transport-message-transform.js";
 import { mergeTransportMetadata, sanitizeTransportPayloadText } from "./transport-stream-shared.js";
 
 const DEFAULT_AZURE_OPENAI_API_VERSION = "2024-12-01-preview";
+const OPENAI_CODEX_RESPONSES_EMPTY_INPUT_TEXT = " ";
 const log = createSubsystemLogger("openai-transport");
 
 type BaseStreamOptions = {
@@ -876,6 +877,22 @@ function buildOpenAICodexResponsesInstructions(context: Context): string | undef
   return sanitizeTransportPayloadText(stripSystemPromptCacheBoundary(context.systemPrompt));
 }
 
+function ensureOpenAICodexResponsesInput(messages: ResponseInput, context: Context): void {
+  if (messages.length > 0 || !context.systemPrompt) {
+    return;
+  }
+  const text = buildOpenAICodexResponsesInstructions(context);
+  if (!text) {
+    throw new Error(
+      "OpenAI Codex Responses requires non-empty input when only systemPrompt is provided.",
+    );
+  }
+  messages.push({
+    role: "user",
+    content: [{ type: "input_text", text: OPENAI_CODEX_RESPONSES_EMPTY_INPUT_TEXT }],
+  });
+}
+
 export function buildOpenAIResponsesParams(
   model: Model<Api>,
   context: Context,
@@ -892,6 +909,9 @@ export function buildOpenAIResponsesParams(
     new Set(["openai", "openai-codex", "opencode", "azure-openai-responses"]),
     { includeSystemPrompt: !isCodexResponses, supportsDeveloperRole },
   );
+  if (isCodexResponses) {
+    ensureOpenAICodexResponsesInput(messages, context);
+  }
   const cacheRetention = resolveCacheRetention(options?.cacheRetention);
   const payloadPolicy = resolveOpenAIResponsesPayloadPolicy(model, {
     storeMode: "disable",