diff --git a/open-sse/translator/request/claude-to-openai.ts b/open-sse/translator/request/claude-to-openai.ts
index f426c99857..013fd77a4d 100644
--- a/open-sse/translator/request/claude-to-openai.ts
+++ b/open-sse/translator/request/claude-to-openai.ts
@@ -136,6 +136,30 @@ export function claudeToOpenAIRequest(model, body, stream, credentials: unknown
   // Fix missing tool responses - OpenAI requires every tool_call to have a response
   fixMissingToolResponses(result.messages);
 
+  // #4385: drop orphan tool results — a role:"tool" message whose tool_call_id has no
+  // matching assistant.tool_calls (e.g. history truncation / compression removed the
+  // assistant turn that issued the call but kept the tool_result). OpenAI-compatible
+  // upstreams reject these with 502 "Messages with role 'tool' must be a response to a
+  // preceding message with 'tool_calls'". Mirrors the filter already applied on the
+  // Responses->Chat path in openai-responses.ts (#2893). Run after fixMissingToolResponses
+  // so the inserted "[No response received]" placeholders (which DO match a tool_call)
+  // are kept, and only genuinely orphaned results are removed.
+  const assistantToolCallIds = new Set<string>();
+  for (const msg of result.messages) {
+    const calls = (msg as JsonRecord).tool_calls;
+    if (Array.isArray(calls)) {
+      for (const tc of calls as { id?: string }[]) {
+        if (tc.id) assistantToolCallIds.add(String(tc.id));
+      }
+    }
+  }
+  result.messages = result.messages.filter((msg) => {
+    if ((msg as JsonRecord).role === "tool") {
+      return assistantToolCallIds.has(String((msg as JsonRecord).tool_call_id ?? ""));
+    }
+    return true;
+  });
+
   const useNativeResponsesWebSearch = shouldUseNativeResponsesWebSearch(credentials);
 
   // Tools
diff --git a/tests/unit/translator-claude-to-openai-orphan-tool-4385.test.ts b/tests/unit/translator-claude-to-openai-orphan-tool-4385.test.ts
new file mode 100644
index 0000000000..67ac2857af
--- /dev/null
+++ b/tests/unit/translator-claude-to-openai-orphan-tool-4385.test.ts
@@ -0,0 +1,94 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+// #4385: routing a Claude-protocol conversation (e.g. via /v1/messages) to an
+// OpenAI-compatible provider (command-code, custom openai-compatible) returned
+// 502 "Messages with role 'tool' must be a response to a preceding message with
+// 'tool_calls'". Cause: claudeToOpenAIRequest emits a role:"tool" message for every
+// Claude tool_result block, but never drops an ORPHAN one whose tool_call_id has no
+// matching assistant.tool_calls (e.g. when history truncation / compression removed
+// the assistant turn but kept the tool_result). OpenAI-compatible upstreams reject it.
+// This mirrors the orphan filter already on the Responses->Chat path (#2893).
+
+const { claudeToOpenAIRequest } = await import(
+  "../../open-sse/translator/request/claude-to-openai.ts"
+);
+
+type Msg = { role: string; tool_call_id?: string; tool_calls?: { id?: string }[] };
+
+test("#4385 drops an orphan tool_result with no preceding assistant tool_call", () => {
+  const result = claudeToOpenAIRequest(
+    "deepseek/deepseek-v4-pro",
+    {
+      messages: [
+        { role: "user", content: "start the task" },
+        {
+          role: "user",
+          content: [
+            // orphan: the assistant turn that issued tool_use "orphan_tu" was dropped
+            { type: "tool_result", tool_use_id: "orphan_tu", content: "stale output" },
+            { type: "text", text: "please continue" },
+          ],
+        },
+      ],
+    },
+    false
+  );
+
+  const toolMsgs = (result.messages as Msg[]).filter((m) => m.role === "tool");
+  assert.equal(toolMsgs.length, 0, "orphan role:'tool' must be filtered out");
+  // The user's accompanying text is preserved.
+  const userTexts = (result.messages as Msg[]).filter((m) => m.role === "user");
+  assert.equal(userTexts.length, 2);
+});
+
+test("#4385 preserves a tool_result paired with its assistant tool_call", () => {
+  const result = claudeToOpenAIRequest(
+    "deepseek/deepseek-v4-pro",
+    {
+      messages: [
+        { role: "user", content: "list files" },
+        { role: "assistant", content: [{ type: "tool_use", id: "tu_1", name: "ls", input: {} }] },
+        { role: "user", content: [{ type: "tool_result", tool_use_id: "tu_1", content: "a.ts" }] },
+      ],
+    },
+    false
+  );
+
+  const toolMsgs = (result.messages as Msg[]).filter((m) => m.role === "tool");
+  assert.equal(toolMsgs.length, 1);
+  assert.equal(toolMsgs[0].tool_call_id, "tu_1");
+  // The assistant.tool_calls is still present and precedes the tool message.
+  const assistantIdx = (result.messages as Msg[]).findIndex(
+    (m) => m.role === "assistant" && Array.isArray(m.tool_calls)
+  );
+  const toolIdx = (result.messages as Msg[]).findIndex((m) => m.role === "tool");
+  assert.ok(assistantIdx >= 0 && assistantIdx < toolIdx);
+});
+
+test("#4385 keeps valid tool_results and drops orphans in the same user turn", () => {
+  const result = claudeToOpenAIRequest(
+    "deepseek/deepseek-v4-pro",
+    {
+      messages: [
+        {
+          role: "assistant",
+          content: [{ type: "tool_use", id: "tu_valid", name: "fn", input: {} }],
+        },
+        {
+          role: "user",
+          content: [
+            { type: "tool_result", tool_use_id: "tu_valid", content: "ok" },
+            { type: "tool_result", tool_use_id: "tu_orphan", content: "stale" },
+            { type: "text", text: "done" },
+          ],
+        },
+      ],
+    },
+    false
+  );
+
+  const toolMsgs = (result.messages as Msg[]).filter((m) => m.role === "tool");
+  assert.equal(toolMsgs.length, 1);
+  assert.equal(toolMsgs[0].tool_call_id, "tu_valid");
+});
diff --git a/tests/unit/translator-claude-to-openai.test.ts b/tests/unit/translator-claude-to-openai.test.ts
index e2d4dc2ee4..23354d6e3b 100644
--- a/tests/unit/translator-claude-to-openai.test.ts
+++ b/tests/unit/translator-claude-to-openai.test.ts
@@ -264,6 +264,14 @@ test("Claude -> OpenAI converts tool_result blocks into tool messages and preser
     "gpt-4o",
     {
       messages: [
+        // #4385: a tool_result must be paired with a preceding assistant tool_call,
+        // otherwise it is an orphan that OpenAI-compatible upstreams reject (and that
+        // claudeToOpenAIRequest now filters). Pair it so this test still exercises the
+        // content-extraction mechanics (array [text, image] → "20C") on a valid sequence.
+        {
+          role: "assistant",
+          content: [{ type: "tool_use", id: "tu_1", name: "weather", input: {} }],
+        },
         {
           role: "user",
           content: [
@@ -287,11 +295,15 @@ test("Claude -> OpenAI converts tool_result blocks into tool messages and preser
   );
 
   assert.deepEqual(result.messages[0], {
+    role: "assistant",
+    tool_calls: [{ id: "tu_1", type: "function", function: { name: "weather", arguments: "{}" } }],
+  });
+  assert.deepEqual(result.messages[1], {
     role: "tool",
     tool_call_id: "tu_1",
     content: "20C",
   });
-  assert.deepEqual(result.messages[1], {
+  assert.deepEqual(result.messages[2], {
     role: "user",
     content: "Thanks",
   });