diff --git a/internal/harness/harness_test.go b/internal/harness/harness_test.go
index ce4ba36..3b53eda 100644
--- a/internal/harness/harness_test.go
+++ b/internal/harness/harness_test.go
@@ -805,7 +805,7 @@ func TestPiFirstPartyRoutesUseNativeAPIs(t *testing.T) {
 	}
 	require.Equal(t, "anthropic-messages", models["acme/primary"]["api"])
 	require.Equal(t, testEndpoint, models["acme/primary"]["baseUrl"], "Pi's Anthropic client appends /v1")
-	require.NotContains(t, models["acme/primary"], "compat")
+	require.Equal(t, true, get(models["acme/primary"], []string{"compat", "forceAdaptiveThinking"}).Data)
 	require.Equal(t, "openai-responses", models["acme/background"]["api"])
 	require.NotContains(t, models["acme/background"], "baseUrl")
 	require.NotContains(t, models["acme/background"], "compat")
diff --git a/internal/harness/pi.go b/internal/harness/pi.go
index 44bb65d..8591f1b 100644
--- a/internal/harness/pi.go
+++ b/internal/harness/pi.go
@@ -116,6 +116,8 @@ func piModel(r Route, baseURL string) map[string]any {
 	case ProviderAnthropic:
 		// Pi's Anthropic client appends /v1/messages itself.
 		model["api"], model["baseUrl"] = "anthropic-messages", baseURL
+		// Anthropic Messages models use adaptive thinking, not a token budget.
+		model["compat"] = map[string]any{"forceAdaptiveThinking": true}
 	case ProviderOpenAI:
 		model["api"] = "openai-responses"
 	default: