diff --git a/open-sse/translator/request/openai-to-claude.ts b/open-sse/translator/request/openai-to-claude.ts
index 6e8d9b0058..c4cfae7a3a 100644
--- a/open-sse/translator/request/openai-to-claude.ts
+++ b/open-sse/translator/request/openai-to-claude.ts
@@ -223,7 +223,23 @@ export function openaiToClaudeRequest(model, body, stream) {
   };
 
   // Temperature
-  if (body.temperature !== undefined) {
+  //
+  // Claude's Messages API rejects `temperature` when extended thinking is active.
+  // Two cases where thinking is on:
+  //   (a) Caller passes `body.thinking` or `body.reasoning_effort` (handled later —
+  //       `result.thinking` becomes truthy, and we strip temperature at the end).
+  //   (b) The request targets Claude OAuth (claude-code), which always sends
+  //       `Anthropic-Beta: ...,interleaved-thinking-2025-05-14,...` in headers.
+  //       The model is forced into thinking server-side, but neither `body.thinking`
+  //       nor `result.thinking` will be set, so we detect this by model name. This
+  //       affects claude-opus-4.x and claude-sonnet-4.x (the families that support
+  //       extended thinking).
+  //
+  // For models that don't force thinking (haiku, older sonnets), preserve temperature.
+  // Note: Opus 4.7+/Fable 5 already drop sampling params upstream of the translator via
+  // the registry `unsupportedParams` strip; this covers the remaining 4.x families.
+  const modelForcesThinking = /claude-(?:opus|sonnet)-4/i.test(String(model));
+  if (body.temperature !== undefined && !modelForcesThinking) {
     result.temperature = body.temperature;
   }
   if (body.temperature === undefined && body.top_p !== undefined) {
@@ -527,6 +543,15 @@ export function openaiToClaudeRequest(model, body, stream) {
 
   delete result[COPILOT_REASONING_SUMMARY_MARKER];
 
+  // Final guard: Claude rejects `temperature` whenever extended thinking is
+  // enabled. If `result.thinking` was set above from `body.thinking` or
+  // `body.reasoning_effort` (manual budget or adaptive effort), drop temperature
+  // defensively. The model-name strip earlier already covers Claude OAuth's
+  // forced-thinking case (claude-opus-4.x / claude-sonnet-4.x).
+  if (result.thinking && result.temperature !== undefined) {
+    delete result.temperature;
+  }
+
   // Attach toolNameMap to result for response translation
   if (toolNameMap.size > 0) {
     result._toolNameMap = toolNameMap;
diff --git a/tests/unit/openai-to-claude-strip-temperature-thinking.test.ts b/tests/unit/openai-to-claude-strip-temperature-thinking.test.ts
new file mode 100644
index 0000000000..480c294f92
--- /dev/null
+++ b/tests/unit/openai-to-claude-strip-temperature-thinking.test.ts
@@ -0,0 +1,69 @@
+/**
+ * openaiToClaudeRequest — strip `temperature` for Claude models with extended thinking.
+ *
+ * Claude's Messages API rejects `temperature` when extended thinking is active.
+ * Two cases the translator handles:
+ *   (a) Model-name detection for forced-thinking families (/claude-(opus|sonnet)-4/),
+ *       used by Claude OAuth which always sends the interleaved-thinking beta header.
+ *   (b) A final guard that drops `temperature` whenever `result.thinking` was set from
+ *       `body.thinking` or `body.reasoning_effort`.
+ *
+ * Ported from decolua/9router PR #1264 (thanks @noestelar).
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { openaiToClaudeRequest } = await import(
+  "../../open-sse/translator/request/openai-to-claude.ts"
+);
+
+const baseBody = () => ({
+  messages: [{ role: "user", content: "hi" }],
+  temperature: 0.7,
+});
+
+test("strips temperature for claude-opus-4 family (forced thinking)", () => {
+  const result = openaiToClaudeRequest("claude-opus-4-6", baseBody(), false);
+  assert.equal(result.temperature, undefined);
+});
+
+test("strips temperature for claude-sonnet-4 family (forced thinking)", () => {
+  const result = openaiToClaudeRequest("claude-sonnet-4-5", baseBody(), false);
+  assert.equal(result.temperature, undefined);
+});
+
+test("preserves temperature for non-thinking models (haiku, older sonnets)", () => {
+  const haiku = openaiToClaudeRequest("claude-haiku-5-20251001", baseBody(), false);
+  assert.equal(haiku.temperature, 0.7);
+
+  const sonnet35 = openaiToClaudeRequest("claude-3-5-sonnet-20241022", baseBody(), false);
+  assert.equal(sonnet35.temperature, 0.7);
+});
+
+test("strips temperature when body.thinking is explicitly set (final guard)", () => {
+  // Non-thinking-forced model so the model-based strip doesn't fire; the final
+  // guard must catch this case.
+  const body = {
+    ...baseBody(),
+    thinking: { type: "enabled", budget_tokens: 4096 },
+  };
+  const result = openaiToClaudeRequest("claude-haiku-5-20251001", body, false);
+  assert.equal(result.temperature, undefined);
+  assert.ok(result.thinking, "thinking should still be present");
+});
+
+test("strips temperature when reasoning_effort triggers thinking (final guard)", () => {
+  const body = { ...baseBody(), reasoning_effort: "medium" };
+  const result = openaiToClaudeRequest("claude-haiku-5-20251001", body, false);
+  assert.equal(result.temperature, undefined);
+  assert.ok(result.thinking, "reasoning_effort should produce a thinking block");
+});
+
+test("preserves temperature when reasoning_effort is unrecognized on a non-thinking model", () => {
+  // "none" is not in the effort→budget map, so no thinking block is produced and
+  // temperature must survive on a model that does not force thinking.
+  const body = { ...baseBody(), reasoning_effort: "none" };
+  const result = openaiToClaudeRequest("claude-haiku-5-20251001", body, false);
+  assert.equal(result.temperature, 0.7);
+  assert.equal(result.thinking, undefined);
+});