diff --git a/internal/thinking/summary.go b/internal/thinking/summary.go
index 430a35d8..5cca0a9b 100644
--- a/internal/thinking/summary.go
+++ b/internal/thinking/summary.go
@@ -126,11 +126,19 @@ func applySummaryConfigForModel(body []byte, format, model string, modelInfo *re
 		body = applyOpenAIChatSummaryConfig(body, model, enabled)
 	case "claude":
 		// Anthropic documents display as invalid with thinking.type=disabled and
-		// requires it alongside adaptive or enabled thinking. An enabled source
-		// summary needs an active target thinking mode. A disabled summary only
-		// hides an already-active target thinking mode; it must not enable thinking
-		// merely to hide a summary that would not otherwise exist. Unspecified
-		// intent returns above and leaves the target's default untouched.
+		// requires it alongside adaptive or enabled thinking. Model defaults differ:
+		// Opus 5 and Sonnet 5 default to adaptive thinking; Fable/Mythos 5 are always
+		// on. Opus 4.8/4.7/4.6, Sonnet 4.6, and the 4.5 models default to thinking
+		// off. The newest models also default display to omitted. Keeping a missing
+		// thinking block absent therefore preserves both kinds of model default;
+		// absence does not mean every Claude model runs without thinking. Only an
+		// enabled summary may activate a valid target thinking mode so that summarized
+		// text can be returned. A disabled summary only adds omitted to an
+		// already-active target mode.
+		//
+		// Anthropic docs:
+		// https://platform.claude.com/docs/en/build-with-claude/thinking
+		// https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#supported-models
 		if enabled && !gjson.GetBytes(body, "thinking.type").Exists() {
 			body = enableClaudeThinkingForSummary(body, model, modelInfo)
 		}
diff --git a/internal/thinking/summary_test.go b/internal/thinking/summary_test.go
index 59bd2eb4..1fed4147 100644
--- a/internal/thinking/summary_test.go
+++ b/internal/thinking/summary_test.go
@@ -176,8 +176,9 @@ func TestApplySummaryConfigForModel_ClaudeEnabledSummaryUsesValidThinkingMode(t
 	}
 }
 
-// Disabling summaries must not activate Claude thinking. Doing so would add
-// reasoning tokens, latency, and cost to a request that asked only to hide output.
+// Disabling summaries must not make CPA add a Claude thinking block. Absence
+// preserves the per-model default: newer models may still think by default,
+// while older models remain off.
 func TestApplySummaryConfigForModel_ClaudeDisabledSummaryDoesNotEnableThinking(t *testing.T) {
 	for _, model := range []string{"claude-opus-5", "claude-haiku-4-5-20251001"} {
 		body := []byte(`{"model":"` + model + `","max_tokens":32000}`)
diff --git a/test/summary_intent_translation_test.go b/test/summary_intent_translation_test.go
index 5cb01362..b53c7ca3 100644
--- a/test/summary_intent_translation_test.go
+++ b/test/summary_intent_translation_test.go
@@ -119,9 +119,10 @@ func TestSummaryIntentFinalPipeline(t *testing.T) {
 		wantExists bool
 	}{
 		{name: "Responses summary only activates visible Claude thinking", from: sdktranslator.FormatOpenAIResponse, to: sdktranslator.FormatClaude, model: "claude-sonnet-4-6-model", body: `{"model":"claude-sonnet-4-6-model","reasoning":{"summary":"auto"},"input":"hi"}`, path: "thinking.display", want: "summarized", wantExists: true},
-		// Disabling summaries alone must not activate Claude thinking: doing so adds
-		// reasoning tokens, latency, and cost to a request with no thinking effort.
-		{name: "Responses null summary alone keeps Claude thinking disabled", from: sdktranslator.FormatOpenAIResponse, to: sdktranslator.FormatClaude, model: "claude-sonnet-4-6-model", body: `{"model":"claude-sonnet-4-6-model","reasoning":{"summary":null},"input":"hi"}`, path: "thinking"},
+		// Summary visibility must not override Claude's per-model thinking default.
+		// Sonnet 4.6 defaults off; newer default-on models remain default-on without
+		// CPA injecting an explicit thinking block.
+		{name: "Responses null summary alone preserves Claude thinking default", from: sdktranslator.FormatOpenAIResponse, to: sdktranslator.FormatClaude, model: "claude-sonnet-4-6-model", body: `{"model":"claude-sonnet-4-6-model","reasoning":{"summary":null},"input":"hi"}`, path: "thinking"},
 		{name: "Responses default keeps Claude display default", from: sdktranslator.FormatOpenAIResponse, to: sdktranslator.FormatClaude, model: "claude-sonnet-4-6-model", body: `{"model":"claude-sonnet-4-6-model","input":"hi"}`, path: "thinking.display"},
 		{name: "Chat summary alias only activates valid Claude thinking", from: sdktranslator.FormatOpenAI, to: sdktranslator.FormatClaude, model: "claude-sonnet-4-6-model", body: `{"model":"claude-sonnet-4-6-model","reasoning":{"exclude":false},"messages":[{"role":"user","content":"hi"}]}`, path: "thinking.display", want: "summarized", wantExists: true},
 		{name: "Interactions summary only activates valid Claude thinking", from: sdktranslator.FormatInteractions, to: sdktranslator.FormatClaude, model: "claude-sonnet-4-6-model", body: `{"model":"claude-sonnet-4-6-model","generation_config":{"thinking_summaries":"auto"},"input":"hi"}`, path: "thinking.display", want: "summarized", wantExists: true},