diff --git a/open-sse/translator/concerns/thinkingUnified.js b/open-sse/translator/concerns/thinkingUnified.js
index 1cf44384..d6082724 100644
--- a/open-sse/translator/concerns/thinkingUnified.js
+++ b/open-sse/translator/concerns/thinkingUnified.js
@@ -173,6 +173,12 @@ function applyFormat(fmt, body, cfg, caps) {
     }
     case "claude-adaptive": {
       if (none && canDisable) { body.thinking = { type: "disabled" }; break; }
+      // output_config.effort alone does NOT turn thinking on: Anthropic requires
+      // an explicit thinking:{type:"adaptive"} on Opus 4.6/4.7/4.8 and Sonnet 4.6
+      // ("thinking is off unless you explicitly set it"), and Anthropic-compatible
+      // shims (e.g. GitHub Copilot /v1/messages) default thinking off even for
+      // Sonnet 5. Send both fields — the documented adaptive-thinking shape.
+      body.thinking = { type: "adaptive" };
       const level = toLevel(eff);
       body.output_config = { effort: level === "xhigh" ? "high" : level };
       break;
diff --git a/tests/translator/__snapshots__/golden-request.test.js.snap b/tests/translator/__snapshots__/golden-request.test.js.snap
index 17fa66dd..a7a0c219 100644
--- a/tests/translator/__snapshots__/golden-request.test.js.snap
+++ b/tests/translator/__snapshots__/golden-request.test.js.snap
@@ -118,6 +118,9 @@ exports[`GOLDEN request: OpenAI → Claude > reasoning_effort → adaptive outpu
       "type": "text",
     },
   ],
+  "thinking": {
+    "type": "adaptive",
+  },
 }
 `;
 
diff --git a/tests/translator/thinking-unified.test.js b/tests/translator/thinking-unified.test.js
index bae22c33..dad57622 100644
--- a/tests/translator/thinking-unified.test.js
+++ b/tests/translator/thinking-unified.test.js
@@ -55,10 +55,14 @@ describe("extractThinking", () => {
 });
 
 describe("applyThinking per provider format", () => {
-  it("claude 4.6+ → adaptive output_config (no budget_tokens)", () => {
+  it("claude 4.6+ → adaptive thinking + output_config (no budget_tokens)", () => {
     const out = apply("claude", "claude-opus-4.7", { reasoning_effort: "high" }, "claude");
     expect(out.output_config).toEqual({ effort: "high" });
-    expect(out.thinking).toBeUndefined();
+    // Anthropic: on Opus 4.6/4.7/4.8 and Sonnet 4.6 thinking stays OFF unless
+    // thinking:{type:"adaptive"} is sent explicitly; output_config alone is not
+    // enough (and Anthropic-compatible shims like Copilot default off even on
+    // Sonnet 5). Both fields together are the documented adaptive shape.
+    expect(out.thinking).toEqual({ type: "adaptive" });
   });
   it("claude haiku → enabled+budget", () => {
     const out = apply("claude", "claude-haiku-4.5", { reasoning_effort: "high" }, "claude");