diff --git a/internal/translator/antigravity/claude/antigravity_claude_request.go b/internal/translator/antigravity/claude/antigravity_claude_request.go
index fe2c8cde9..76bad5d60 100644
--- a/internal/translator/antigravity/claude/antigravity_claude_request.go
+++ b/internal/translator/antigravity/claude/antigravity_claude_request.go
@@ -308,6 +308,8 @@ func ConvertClaudeRequestToAntigravity(modelName string, inputRawJSON []byte, _
 			role := originalRole
 			if role == "assistant" {
 				role = "model"
+			} else if role == "system" {
+				role = "user"
 			}
 			clientContentJSON := []byte(`{"role":"","parts":[]}`)
 			clientContentJSON, _ = sjson.SetBytes(clientContentJSON, "role", role)
diff --git a/internal/translator/antigravity/claude/antigravity_claude_request_test.go b/internal/translator/antigravity/claude/antigravity_claude_request_test.go
index 017078d43..d843dd948 100644
--- a/internal/translator/antigravity/claude/antigravity_claude_request_test.go
+++ b/internal/translator/antigravity/claude/antigravity_claude_request_test.go
@@ -133,6 +133,53 @@ func TestConvertClaudeRequestToAntigravity_StripsClaudeCodeAttribution(t *testin
 	}
 }
 
+func TestConvertClaudeRequestToAntigravity_ConvertsMessageSystemRoleToUserContent(t *testing.T) {
+	inputJSON := []byte(`{
+		"model": "gemini-3.5-flash",
+		"system": [{"type": "text", "text": "Top-level rules"}],
+		"messages": [
+			{"role": "user", "content": [{"type": "text", "text": "Hello"}]},
+			{"role": "system", "content": "String mid-conversation rule"},
+			{"role": "system", "content": [{"type": "text", "text": "Array mid-conversation rule"}]}
+		]
+	}`)
+
+	output := ConvertClaudeRequestToAntigravity("gemini-3-flash-agent", inputJSON, false)
+	outputStr := string(output)
+
+	if systemContent := gjson.Get(outputStr, `request.contents.#(role=="system")`); systemContent.Exists() {
+		t.Fatalf("system role should not be emitted in request.contents: %s", systemContent.Raw)
+	}
+
+	contents := gjson.Get(outputStr, "request.contents").Array()
+	if len(contents) != 3 {
+		t.Fatalf("Expected the user and message-level system turns in request.contents, got %d: %s", len(contents), gjson.Get(outputStr, "request.contents").Raw)
+	}
+	if got := contents[0].Get("role").String(); got != "user" {
+		t.Fatalf("Expected first content role user, got %q", got)
+	}
+	if got := contents[1].Get("role").String(); got != "user" {
+		t.Fatalf("Expected message-level system content to be downgraded to user role, got %q", got)
+	}
+	if got := contents[1].Get("parts.0.text").String(); got != "String mid-conversation rule" {
+		t.Fatalf("Unexpected string message-level system content text: %q", got)
+	}
+	if got := contents[2].Get("role").String(); got != "user" {
+		t.Fatalf("Expected array message-level system content to be downgraded to user role, got %q", got)
+	}
+	if got := contents[2].Get("parts.0.text").String(); got != "Array mid-conversation rule" {
+		t.Fatalf("Unexpected array message-level system content text: %q", got)
+	}
+
+	parts := gjson.Get(outputStr, "request.systemInstruction.parts").Array()
+	if len(parts) != 1 {
+		t.Fatalf("Expected only top-level system parts, got %d: %s", len(parts), gjson.Get(outputStr, "request.systemInstruction.parts").Raw)
+	}
+	if got := parts[0].Get("text").String(); got != "Top-level rules" {
+		t.Fatalf("Unexpected first system part: %q", got)
+	}
+}
+
 func testNonAnthropicRawSignature(t *testing.T) string {
 	t.Helper()
 
diff --git a/internal/translator/gemini-cli/claude/gemini-cli_claude_request.go b/internal/translator/gemini-cli/claude/gemini-cli_claude_request.go
index b21936a95..80e942118 100644
--- a/internal/translator/gemini-cli/claude/gemini-cli_claude_request.go
+++ b/internal/translator/gemini-cli/claude/gemini-cli_claude_request.go
@@ -77,6 +77,8 @@ func ConvertClaudeRequestToCLI(modelName string, inputRawJSON []byte, _ bool) []
 			role := roleResult.String()
 			if role == "assistant" {
 				role = "model"
+			} else if role == "system" {
+				role = "user"
 			}
 
 			contentJSON := []byte(`{"role":"","parts":[]}`)
diff --git a/internal/translator/gemini-cli/claude/gemini-cli_claude_request_test.go b/internal/translator/gemini-cli/claude/gemini-cli_claude_request_test.go
index ff0cea657..50a491fd9 100644
--- a/internal/translator/gemini-cli/claude/gemini-cli_claude_request_test.go
+++ b/internal/translator/gemini-cli/claude/gemini-cli_claude_request_test.go
@@ -61,3 +61,49 @@ func TestConvertClaudeRequestToCLI_StripsClaudeCodeAttribution(t *testing.T) {
 		t.Fatalf("Unexpected system part: %q", got)
 	}
 }
+
+func TestConvertClaudeRequestToCLI_ConvertsMessageSystemRoleToUserContent(t *testing.T) {
+	inputJSON := []byte(`{
+		"model": "gemini-3-flash-preview",
+		"system": [{"type": "text", "text": "Top-level rules"}],
+		"messages": [
+			{"role": "user", "content": [{"type": "text", "text": "Hello"}]},
+			{"role": "system", "content": "String mid-conversation rule"},
+			{"role": "system", "content": [{"type": "text", "text": "Array mid-conversation rule"}]}
+		]
+	}`)
+
+	output := ConvertClaudeRequestToCLI("gemini-3-flash-preview", inputJSON, false)
+
+	if systemContent := gjson.GetBytes(output, `request.contents.#(role=="system")`); systemContent.Exists() {
+		t.Fatalf("system role should not be emitted in request.contents: %s", systemContent.Raw)
+	}
+
+	contents := gjson.GetBytes(output, "request.contents").Array()
+	if len(contents) != 3 {
+		t.Fatalf("Expected the user and message-level system turns in request.contents, got %d: %s", len(contents), gjson.GetBytes(output, "request.contents").Raw)
+	}
+	if got := contents[0].Get("role").String(); got != "user" {
+		t.Fatalf("Expected first content role user, got %q", got)
+	}
+	if got := contents[1].Get("role").String(); got != "user" {
+		t.Fatalf("Expected message-level string system content to be downgraded to user role, got %q", got)
+	}
+	if got := contents[1].Get("parts.0.text").String(); got != "String mid-conversation rule" {
+		t.Fatalf("Unexpected string message-level system content text: %q", got)
+	}
+	if got := contents[2].Get("role").String(); got != "user" {
+		t.Fatalf("Expected message-level array system content to be downgraded to user role, got %q", got)
+	}
+	if got := contents[2].Get("parts.0.text").String(); got != "Array mid-conversation rule" {
+		t.Fatalf("Unexpected array message-level system content text: %q", got)
+	}
+
+	parts := gjson.GetBytes(output, "request.systemInstruction.parts").Array()
+	if len(parts) != 1 {
+		t.Fatalf("Expected only top-level system parts, got %d: %s", len(parts), gjson.GetBytes(output, "request.systemInstruction.parts").Raw)
+	}
+	if got := parts[0].Get("text").String(); got != "Top-level rules" {
+		t.Fatalf("Unexpected first system part: %q", got)
+	}
+}
diff --git a/internal/translator/gemini/claude/gemini_claude_request.go b/internal/translator/gemini/claude/gemini_claude_request.go
index 128dac6e0..3347eaec1 100644
--- a/internal/translator/gemini/claude/gemini_claude_request.go
+++ b/internal/translator/gemini/claude/gemini_claude_request.go
@@ -71,6 +71,8 @@ func ConvertClaudeRequestToGemini(modelName string, inputRawJSON []byte, _ bool)
 			role := roleResult.String()
 			if role == "assistant" {
 				role = "model"
+			} else if role == "system" {
+				role = "user"
 			}
 
 			contentJSON := []byte(`{"role":"","parts":[]}`)
diff --git a/internal/translator/gemini/claude/gemini_claude_request_test.go b/internal/translator/gemini/claude/gemini_claude_request_test.go
index 01bed5f17..81b06214e 100644
--- a/internal/translator/gemini/claude/gemini_claude_request_test.go
+++ b/internal/translator/gemini/claude/gemini_claude_request_test.go
@@ -107,6 +107,52 @@ func TestConvertClaudeRequestToGemini_StripsClaudeCodeAttribution(t *testing.T)
 	}
 }
 
+func TestConvertClaudeRequestToGemini_ConvertsMessageSystemRoleToUserContent(t *testing.T) {
+	inputJSON := []byte(`{
+		"model": "gemini-3-flash-preview",
+		"system": [{"type": "text", "text": "Top-level rules"}],
+		"messages": [
+			{"role": "user", "content": [{"type": "text", "text": "Hello"}]},
+			{"role": "system", "content": "String mid-conversation rule"},
+			{"role": "system", "content": [{"type": "text", "text": "Array mid-conversation rule"}]}
+		]
+	}`)
+
+	output := ConvertClaudeRequestToGemini("gemini-3-flash-preview", inputJSON, false)
+
+	if systemContent := gjson.GetBytes(output, `contents.#(role=="system")`); systemContent.Exists() {
+		t.Fatalf("system role should not be emitted in contents: %s", systemContent.Raw)
+	}
+
+	contents := gjson.GetBytes(output, "contents").Array()
+	if len(contents) != 3 {
+		t.Fatalf("Expected the user and message-level system turns in contents, got %d: %s", len(contents), gjson.GetBytes(output, "contents").Raw)
+	}
+	if got := contents[0].Get("role").String(); got != "user" {
+		t.Fatalf("Expected first content role user, got %q", got)
+	}
+	if got := contents[1].Get("role").String(); got != "user" {
+		t.Fatalf("Expected message-level string system content to be downgraded to user role, got %q", got)
+	}
+	if got := contents[1].Get("parts.0.text").String(); got != "String mid-conversation rule" {
+		t.Fatalf("Unexpected string message-level system content text: %q", got)
+	}
+	if got := contents[2].Get("role").String(); got != "user" {
+		t.Fatalf("Expected message-level array system content to be downgraded to user role, got %q", got)
+	}
+	if got := contents[2].Get("parts.0.text").String(); got != "Array mid-conversation rule" {
+		t.Fatalf("Unexpected array message-level system content text: %q", got)
+	}
+
+	parts := gjson.GetBytes(output, "system_instruction.parts").Array()
+	if len(parts) != 1 {
+		t.Fatalf("Expected only top-level system parts, got %d: %s", len(parts), gjson.GetBytes(output, "system_instruction.parts").Raw)
+	}
+	if got := parts[0].Get("text").String(); got != "Top-level rules" {
+		t.Fatalf("Unexpected first system part: %q", got)
+	}
+}
+
 func TestConvertClaudeRequestToGemini_SkipsEmptyTextParts(t *testing.T) {
 	inputJSON := []byte(`{
 		"model": "claude-3-5-sonnet",