diff --git a/internal/runtime/executor/claude_executor.go b/internal/runtime/executor/claude_executor.go
index 3dd4ca5e..82b12a2f 100644
--- a/internal/runtime/executor/claude_executor.go
+++ b/internal/runtime/executor/claude_executor.go
@@ -1266,6 +1266,10 @@ func checkSystemInstructionsWithMode(payload []byte, strictMode bool) []byte {
 			}
 			return true
 		})
+	} else if system.Type == gjson.String && system.String() != "" {
+		partJSON := `{"type":"text","cache_control":{"type":"ephemeral"}}`
+		partJSON, _ = sjson.Set(partJSON, "text", system.String())
+		result += "," + partJSON
 	}
 	result += "]"
 
diff --git a/internal/runtime/executor/claude_executor_test.go b/internal/runtime/executor/claude_executor_test.go
index ead4e299..7bf77a7a 100644
--- a/internal/runtime/executor/claude_executor_test.go
+++ b/internal/runtime/executor/claude_executor_test.go
@@ -980,3 +980,87 @@ func TestClaudeExecutor_ExecuteStream_GzipErrorBodyNoContentEncodingHeader(t *te
 		t.Errorf("error message should contain decompressed JSON, got: %q", err.Error())
 	}
 }
+
+// Test case 1: String system prompt is preserved and converted to a content block
+func TestCheckSystemInstructionsWithMode_StringSystemPreserved(t *testing.T) {
+	payload := []byte(`{"system":"You are a helpful assistant.","messages":[{"role":"user","content":"hi"}]}`)
+
+	out := checkSystemInstructionsWithMode(payload, false)
+
+	system := gjson.GetBytes(out, "system")
+	if !system.IsArray() {
+		t.Fatalf("system should be an array, got %s", system.Type)
+	}
+
+	blocks := system.Array()
+	if len(blocks) != 3 {
+		t.Fatalf("expected 3 system blocks, got %d", len(blocks))
+	}
+
+	if !strings.HasPrefix(blocks[0].Get("text").String(), "x-anthropic-billing-header:") {
+		t.Fatalf("blocks[0] should be billing header, got %q", blocks[0].Get("text").String())
+	}
+	if blocks[1].Get("text").String() != "You are a Claude agent, built on Anthropic's Claude Agent SDK." {
+		t.Fatalf("blocks[1] should be agent block, got %q", blocks[1].Get("text").String())
+	}
+	if blocks[2].Get("text").String() != "You are a helpful assistant." {
+		t.Fatalf("blocks[2] should be user system prompt, got %q", blocks[2].Get("text").String())
+	}
+	if blocks[2].Get("cache_control.type").String() != "ephemeral" {
+		t.Fatalf("blocks[2] should have cache_control.type=ephemeral")
+	}
+}
+
+// Test case 2: Strict mode drops the string system prompt
+func TestCheckSystemInstructionsWithMode_StringSystemStrict(t *testing.T) {
+	payload := []byte(`{"system":"You are a helpful assistant.","messages":[{"role":"user","content":"hi"}]}`)
+
+	out := checkSystemInstructionsWithMode(payload, true)
+
+	blocks := gjson.GetBytes(out, "system").Array()
+	if len(blocks) != 2 {
+		t.Fatalf("strict mode should produce 2 blocks, got %d", len(blocks))
+	}
+}
+
+// Test case 3: Empty string system prompt does not produce a spurious block
+func TestCheckSystemInstructionsWithMode_EmptyStringSystemIgnored(t *testing.T) {
+	payload := []byte(`{"system":"","messages":[{"role":"user","content":"hi"}]}`)
+
+	out := checkSystemInstructionsWithMode(payload, false)
+
+	blocks := gjson.GetBytes(out, "system").Array()
+	if len(blocks) != 2 {
+		t.Fatalf("empty string system should produce 2 blocks, got %d", len(blocks))
+	}
+}
+
+// Test case 4: Array system prompt is unaffected by the string handling
+func TestCheckSystemInstructionsWithMode_ArraySystemStillWorks(t *testing.T) {
+	payload := []byte(`{"system":[{"type":"text","text":"Be concise."}],"messages":[{"role":"user","content":"hi"}]}`)
+
+	out := checkSystemInstructionsWithMode(payload, false)
+
+	blocks := gjson.GetBytes(out, "system").Array()
+	if len(blocks) != 3 {
+		t.Fatalf("expected 3 system blocks, got %d", len(blocks))
+	}
+	if blocks[2].Get("text").String() != "Be concise." {
+		t.Fatalf("blocks[2] should be user system prompt, got %q", blocks[2].Get("text").String())
+	}
+}
+
+// Test case 5: Special characters in string system prompt survive conversion
+func TestCheckSystemInstructionsWithMode_StringWithSpecialChars(t *testing.T) {
+	payload := []byte(`{"system":"Use <xml> tags & \"quotes\" in output.","messages":[{"role":"user","content":"hi"}]}`)
+
+	out := checkSystemInstructionsWithMode(payload, false)
+
+	blocks := gjson.GetBytes(out, "system").Array()
+	if len(blocks) != 3 {
+		t.Fatalf("expected 3 system blocks, got %d", len(blocks))
+	}
+	if blocks[2].Get("text").String() != `Use <xml> tags & "quotes" in output.` {
+		t.Fatalf("blocks[2] text mangled, got %q", blocks[2].Get("text").String())
+	}
+}