diff --git a/internal/translator/claude/openai/chat-completions/claude_openai_request.go b/internal/translator/claude/openai/chat-completions/claude_openai_request.go
index 7d16eaf6a..e0957b2d2 100644
--- a/internal/translator/claude/openai/chat-completions/claude_openai_request.go
+++ b/internal/translator/claude/openai/chat-completions/claude_openai_request.go
@@ -305,9 +305,9 @@ func ConvertOpenAIRequestToClaude(modelName string, inputRawJSON []byte, stream
 
 				// Convert parameters schema for the tool
 				if parameters := function.Get("parameters"); parameters.Exists() {
-					anthropicTool, _ = sjson.SetRawBytes(anthropicTool, "input_schema", []byte(parameters.Raw))
+					anthropicTool, _ = sjson.SetRawBytes(anthropicTool, "input_schema", util.NormalizeClaudeToolInputSchema([]byte(parameters.Raw)))
 				} else if parameters := function.Get("parametersJsonSchema"); parameters.Exists() {
-					anthropicTool, _ = sjson.SetRawBytes(anthropicTool, "input_schema", []byte(parameters.Raw))
+					anthropicTool, _ = sjson.SetRawBytes(anthropicTool, "input_schema", util.NormalizeClaudeToolInputSchema([]byte(parameters.Raw)))
 				}
 				anthropicTool = common.AttachCacheControl(anthropicTool, tool)
 				if !gjson.GetBytes(anthropicTool, "cache_control").Exists() {
diff --git a/internal/translator/claude/openai/chat-completions/claude_openai_request_test.go b/internal/translator/claude/openai/chat-completions/claude_openai_request_test.go
index 84ae0e27c..801b8e39e 100644
--- a/internal/translator/claude/openai/chat-completions/claude_openai_request_test.go
+++ b/internal/translator/claude/openai/chat-completions/claude_openai_request_test.go
@@ -382,6 +382,57 @@ func TestConvertOpenAIRequestToClaude_PreservesToolCacheControl(t *testing.T) {
 	}
 }
 
+func TestConvertOpenAIRequestToClaude_NormalizesRootToolSchemaUnions(t *testing.T) {
+	inputJSON := `{
+		"model":"claude-sonnet-4-5",
+		"messages":[{"role":"user","content":"hi"}],
+		"tools":[
+			{
+				"type":"function",
+				"function":{
+					"name":"without_type",
+					"parameters":{
+						"anyOf":[
+							{"type":"object","properties":{"a":{"type":"string"}}},
+							{"type":"object","properties":{"b":{"type":"string"}}}
+						]
+					}
+				}
+			},
+			{
+				"type":"function",
+				"function":{
+					"name":"constraint_union",
+					"parametersJsonSchema":{
+						"type":"object",
+						"properties":{"a":{"type":"string"},"b":{"type":"string"}},
+						"anyOf":[{"required":["a"]},{"required":["b"]}]
+					}
+				}
+			}
+		]
+	}`
+
+	result := ConvertOpenAIRequestToClaude("claude-sonnet-4-5", []byte(inputJSON), false)
+	root := gjson.ParseBytes(result)
+
+	for _, toolName := range []string{"without_type", "constraint_union"} {
+		schema := root.Get(`tools.#(name=="` + toolName + `").input_schema`)
+		if got := schema.Get("type").String(); got != "object" {
+			t.Fatalf("%s input_schema.type = %q, want object. Output: %s", toolName, got, result)
+		}
+		if schema.Get("anyOf").Exists() {
+			t.Fatalf("%s input_schema should not contain root anyOf. Output: %s", toolName, result)
+		}
+		if !schema.Get("properties.a").Exists() || !schema.Get("properties.b").Exists() {
+			t.Fatalf("%s input_schema should contain properties a and b. Output: %s", toolName, result)
+		}
+		if schema.Get("required").Exists() {
+			t.Fatalf("%s input_schema should not merge alternative required fields. Output: %s", toolName, result)
+		}
+	}
+}
+
 func TestConvertOpenAIRequestToClaude_PartCacheControlWinsOverMessageLevel(t *testing.T) {
 	inputJSON := `{
 		"model": "gpt-4.1",
diff --git a/internal/translator/claude/openai/responses/claude_openai-responses_request.go b/internal/translator/claude/openai/responses/claude_openai-responses_request.go
index 57d536450..310ac488e 100644
--- a/internal/translator/claude/openai/responses/claude_openai-responses_request.go
+++ b/internal/translator/claude/openai/responses/claude_openai-responses_request.go
@@ -684,7 +684,7 @@ func convertResponsesFunctionToolToClaude(tool gjson.Result, overrideName string
 	if d := responsesToolDescription(tool); d != "" {
 		tJSON, _ = sjson.SetBytes(tJSON, "description", d)
 	}
-	tJSON, _ = sjson.SetRawBytes(tJSON, "input_schema", normalizeClaudeToolInputSchema(responsesToolParameters(tool)))
+	tJSON, _ = sjson.SetRawBytes(tJSON, "input_schema", util.NormalizeClaudeToolInputSchema([]byte(responsesToolParameters(tool).Raw)))
 	tJSON = common.AttachCacheControl(tJSON, tool)
 	if !gjson.GetBytes(tJSON, "cache_control").Exists() {
 		tJSON = common.AttachCacheControl(tJSON, tool.Get("function"))
@@ -744,27 +744,6 @@ func responsesToolParameters(tool gjson.Result) gjson.Result {
 	return gjson.Result{}
 }
 
-func normalizeClaudeToolInputSchema(parameters gjson.Result) []byte {
-	raw := strings.TrimSpace(parameters.Raw)
-	if raw == "" || raw == "null" || !gjson.Valid(raw) {
-		return []byte(`{"type":"object","properties":{}}`)
-	}
-	result := gjson.Parse(raw)
-	if !result.IsObject() {
-		return []byte(`{"type":"object","properties":{}}`)
-	}
-	schema := []byte(raw)
-	schemaType := result.Get("type").String()
-	if schemaType == "" {
-		schema, _ = sjson.SetBytes(schema, "type", "object")
-		schemaType = "object"
-	}
-	if schemaType == "object" && !result.Get("properties").Exists() {
-		schema, _ = sjson.SetRawBytes(schema, "properties", []byte(`{}`))
-	}
-	return schema
-}
-
 func qualifyResponsesNamespaceToolName(namespaceName, childName string) string {
 	childName = strings.TrimSpace(childName)
 	if childName == "" || namespaceName == "" || strings.HasPrefix(childName, "mcp__") {
diff --git a/internal/translator/claude/openai/responses/claude_openai-responses_request_test.go b/internal/translator/claude/openai/responses/claude_openai-responses_request_test.go
index cf38ef7ee..556bb2519 100644
--- a/internal/translator/claude/openai/responses/claude_openai-responses_request_test.go
+++ b/internal/translator/claude/openai/responses/claude_openai-responses_request_test.go
@@ -284,6 +284,38 @@ func TestConvertOpenAIResponsesRequestToClaude_DropsApplyPatchCustomTool(t *test
 	}
 }
 
+func TestConvertOpenAIResponsesRequestToClaude_NormalizesRootToolSchemaUnion(t *testing.T) {
+	raw := []byte(`{
+		"model":"claude-test",
+		"input":[{"role":"user","content":[{"type":"input_text","text":"hi"}]}],
+		"tools":[{
+			"type":"function",
+			"name":"lookup",
+			"parameters":{
+				"type":"object",
+				"properties":{"query":{"type":"string"},"id":{"type":"string"}},
+				"oneOf":[{"required":["query"]},{"required":["id"]}]
+			}
+		}]
+	}`)
+
+	out := ConvertOpenAIResponsesRequestToClaude("claude-test", raw, false)
+	schema := gjson.GetBytes(out, "tools.0.input_schema")
+
+	if got := schema.Get("type").String(); got != "object" {
+		t.Fatalf("input_schema.type = %q, want object. Output: %s", got, string(out))
+	}
+	if schema.Get("oneOf").Exists() {
+		t.Fatalf("input_schema should not contain root oneOf. Output: %s", string(out))
+	}
+	if !schema.Get("properties.query").Exists() || !schema.Get("properties.id").Exists() {
+		t.Fatalf("input_schema should preserve query and id properties. Output: %s", string(out))
+	}
+	if schema.Get("required").Exists() {
+		t.Fatalf("input_schema should not merge alternative required fields. Output: %s", string(out))
+	}
+}
+
 func testClaudeResponsesThinkingSignature(t *testing.T) (string, string) {
 	t.Helper()
 	channelBlock := []byte{}
diff --git a/internal/util/claude_schema.go b/internal/util/claude_schema.go
new file mode 100644
index 000000000..c8ea0a7d2
--- /dev/null
+++ b/internal/util/claude_schema.go
@@ -0,0 +1,122 @@
+package util
+
+import "encoding/json"
+
+const emptyClaudeToolInputSchema = `{"type":"object","properties":{}}`
+
+// NormalizeClaudeToolInputSchema makes a JSON Schema compatible with Claude's
+// requirement that a tool input schema is an object without root-level unions.
+func NormalizeClaudeToolInputSchema(schema []byte) []byte {
+	var root map[string]json.RawMessage
+	if len(schema) == 0 || json.Unmarshal(schema, &root) != nil || root == nil {
+		return []byte(emptyClaudeToolInputSchema)
+	}
+
+	properties := claudeSchemaObject(root["properties"])
+	for _, unionName := range []string{"anyOf", "oneOf", "allOf"} {
+		unionRaw, exists := root[unionName]
+		if !exists {
+			continue
+		}
+		delete(root, unionName)
+
+		var branches []json.RawMessage
+		if json.Unmarshal(unionRaw, &branches) != nil {
+			continue
+		}
+		for _, branchRaw := range branches {
+			var branch map[string]json.RawMessage
+			if json.Unmarshal(branchRaw, &branch) != nil || !claudeSchemaCanBeObject(branch) {
+				continue
+			}
+			for name, property := range claudeSchemaObject(branch["properties"]) {
+				if _, exists = properties[name]; !exists {
+					properties[name] = property
+				}
+			}
+			if unionName == "allOf" {
+				mergeClaudeSchemaRequired(root, branch["required"])
+			}
+		}
+	}
+
+	root["type"] = json.RawMessage(`"object"`)
+	propertiesRaw, errMarshalProperties := json.Marshal(properties)
+	if errMarshalProperties != nil {
+		return []byte(emptyClaudeToolInputSchema)
+	}
+	root["properties"] = propertiesRaw
+
+	normalized, errMarshalRoot := json.Marshal(root)
+	if errMarshalRoot != nil {
+		return []byte(emptyClaudeToolInputSchema)
+	}
+	return normalized
+}
+
+func claudeSchemaObject(raw json.RawMessage) map[string]json.RawMessage {
+	object := make(map[string]json.RawMessage)
+	if len(raw) == 0 {
+		return object
+	}
+	if errUnmarshal := json.Unmarshal(raw, &object); errUnmarshal != nil || object == nil {
+		return make(map[string]json.RawMessage)
+	}
+	return object
+}
+
+func claudeSchemaCanBeObject(schema map[string]json.RawMessage) bool {
+	typeRaw, exists := schema["type"]
+	if !exists {
+		return true
+	}
+
+	var schemaType string
+	if json.Unmarshal(typeRaw, &schemaType) == nil {
+		return schemaType == "object"
+	}
+
+	var schemaTypes []string
+	if json.Unmarshal(typeRaw, &schemaTypes) != nil {
+		return false
+	}
+	for _, candidate := range schemaTypes {
+		if candidate == "object" {
+			return true
+		}
+	}
+	return false
+}
+
+func mergeClaudeSchemaRequired(root map[string]json.RawMessage, branchRequired json.RawMessage) {
+	var required []string
+	if rootRequired, exists := root["required"]; exists {
+		if errUnmarshal := json.Unmarshal(rootRequired, &required); errUnmarshal != nil {
+			required = nil
+		}
+	}
+
+	var branchNames []string
+	if json.Unmarshal(branchRequired, &branchNames) != nil {
+		return
+	}
+
+	seen := make(map[string]struct{}, len(required)+len(branchNames))
+	for _, name := range required {
+		seen[name] = struct{}{}
+	}
+	for _, name := range branchNames {
+		if _, exists := seen[name]; exists {
+			continue
+		}
+		required = append(required, name)
+		seen[name] = struct{}{}
+	}
+	if len(required) == 0 {
+		return
+	}
+	requiredRaw, errMarshal := json.Marshal(required)
+	if errMarshal == nil {
+		root["required"] = requiredRaw
+	}
+}
diff --git a/internal/util/claude_schema_test.go b/internal/util/claude_schema_test.go
new file mode 100644
index 000000000..b10836c1f
--- /dev/null
+++ b/internal/util/claude_schema_test.go
@@ -0,0 +1,114 @@
+package util
+
+import "testing"
+
+func TestNormalizeClaudeToolInputSchema(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name: "root anyOf without type",
+			input: `{
+				"anyOf": [
+					{"type":"object","properties":{"a":{"type":"string"}}},
+					{"type":"object","properties":{"b":{"type":"integer"}}}
+				]
+			}`,
+			expected: `{
+				"type":"object",
+				"properties":{
+					"a":{"type":"string"},
+					"b":{"type":"integer"}
+				}
+			}`,
+		},
+		{
+			name: "root oneOf keeps nested union",
+			input: `{
+				"type":"object",
+				"properties":{
+					"nested":{"oneOf":[{"type":"string"},{"type":"number"}]}
+				},
+				"oneOf":[
+					{"properties":{"a":{"type":"string"}},"required":["a"]},
+					{"properties":{"b":{"type":"string"}},"required":["b"]}
+				]
+			}`,
+			expected: `{
+				"type":"object",
+				"properties":{
+					"nested":{"oneOf":[{"type":"string"},{"type":"number"}]},
+					"a":{"type":"string"},
+					"b":{"type":"string"}
+				}
+			}`,
+		},
+		{
+			name: "root anyOf drops alternative required fields",
+			input: `{
+				"type":"object",
+				"properties":{"a":{"type":"string"},"b":{"type":"string"}},
+				"anyOf":[{"required":["a"]},{"required":["b"]}]
+			}`,
+			expected: `{
+				"type":"object",
+				"properties":{"a":{"type":"string"},"b":{"type":"string"}}
+			}`,
+		},
+		{
+			name: "root allOf merges properties and required fields",
+			input: `{
+				"type":"object",
+				"properties":{"base":{"type":"boolean"}},
+				"required":["base"],
+				"allOf":[
+					{"type":"object","properties":{"a":{"type":"string"}},"required":["a"]},
+					{"properties":{"b":{"type":"integer"}},"required":["a","b"]}
+				]
+			}`,
+			expected: `{
+				"type":"object",
+				"properties":{
+					"base":{"type":"boolean"},
+					"a":{"type":"string"},
+					"b":{"type":"integer"}
+				},
+				"required":["base","a","b"]
+			}`,
+		},
+		{
+			name: "ordinary object schema",
+			input: `{
+				"type":"object",
+				"properties":{"query":{"type":"string"}},
+				"required":["query"],
+				"additionalProperties":false
+			}`,
+			expected: `{
+				"type":"object",
+				"properties":{"query":{"type":"string"}},
+				"required":["query"],
+				"additionalProperties":false
+			}`,
+		},
+		{
+			name:     "invalid schema",
+			input:    `{"type":`,
+			expected: `{"type":"object","properties":{}}`,
+		},
+		{
+			name:     "boolean schema",
+			input:    `true`,
+			expected: `{"type":"object","properties":{}}`,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			actual := NormalizeClaudeToolInputSchema([]byte(test.input))
+			compareJSON(t, test.expected, string(actual))
+		})
+	}
+}