diff --git a/backend/internal/pkg/apicompat/anthropic_responses_test.go b/backend/internal/pkg/apicompat/anthropic_responses_test.go
index 2db65572..34f5b60c 100644
--- a/backend/internal/pkg/apicompat/anthropic_responses_test.go
+++ b/backend/internal/pkg/apicompat/anthropic_responses_test.go
@@ -1008,3 +1008,114 @@ func TestAnthropicToResponses_ImageEmptyMediaType(t *testing.T) {
 	// Should default to image/png when media_type is empty.
 	assert.Equal(t, "data:image/png;base64,iVBOR", parts[0].ImageURL)
 }
+
+// ---------------------------------------------------------------------------
+// normalizeToolParameters tests
+// ---------------------------------------------------------------------------
+
+func TestNormalizeToolParameters(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    json.RawMessage
+		expected string
+	}{
+		{
+			name:     "nil input",
+			input:    nil,
+			expected: `{"type":"object","properties":{}}`,
+		},
+		{
+			name:     "empty input",
+			input:    json.RawMessage(``),
+			expected: `{"type":"object","properties":{}}`,
+		},
+		{
+			name:     "null input",
+			input:    json.RawMessage(`null`),
+			expected: `{"type":"object","properties":{}}`,
+		},
+		{
+			name:     "object without properties",
+			input:    json.RawMessage(`{"type":"object"}`),
+			expected: `{"type":"object","properties":{}}`,
+		},
+		{
+			name:     "object with properties",
+			input:    json.RawMessage(`{"type":"object","properties":{"city":{"type":"string"}}}`),
+			expected: `{"type":"object","properties":{"city":{"type":"string"}}}`,
+		},
+		{
+			name:     "non-object type",
+			input:    json.RawMessage(`{"type":"string"}`),
+			expected: `{"type":"string"}`,
+		},
+		{
+			name:     "object with additional fields preserved",
+			input:    json.RawMessage(`{"type":"object","required":["name"]}`),
+			expected: `{"type":"object","required":["name"],"properties":{}}`,
+		},
+		{
+			name:     "invalid JSON passthrough",
+			input:    json.RawMessage(`not json`),
+			expected: `not json`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := normalizeToolParameters(tt.input)
+			if tt.name == "invalid JSON passthrough" {
+				assert.Equal(t, tt.expected, string(result))
+			} else {
+				assert.JSONEq(t, tt.expected, string(result))
+			}
+		})
+	}
+}
+
+func TestAnthropicToResponses_ToolWithoutProperties(t *testing.T) {
+	req := &AnthropicRequest{
+		Model:     "gpt-5.2",
+		MaxTokens: 1024,
+		Messages: []AnthropicMessage{
+			{Role: "user", Content: json.RawMessage(`"Hello"`)},
+		},
+		Tools: []AnthropicTool{
+			{Name: "mcp__pencil__get_style_guide_tags", Description: "Get style tags", InputSchema: json.RawMessage(`{"type":"object"}`)},
+		},
+	}
+
+	resp, err := AnthropicToResponses(req)
+	require.NoError(t, err)
+
+	require.Len(t, resp.Tools, 1)
+	assert.Equal(t, "function", resp.Tools[0].Type)
+	assert.Equal(t, "mcp__pencil__get_style_guide_tags", resp.Tools[0].Name)
+
+	// Parameters must have "properties" field after normalization.
+	var params map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(resp.Tools[0].Parameters, &params))
+	assert.Contains(t, params, "properties")
+}
+
+func TestAnthropicToResponses_ToolWithNilSchema(t *testing.T) {
+	req := &AnthropicRequest{
+		Model:     "gpt-5.2",
+		MaxTokens: 1024,
+		Messages: []AnthropicMessage{
+			{Role: "user", Content: json.RawMessage(`"Hello"`)},
+		},
+		Tools: []AnthropicTool{
+			{Name: "simple_tool", Description: "A tool"},
+		},
+	}
+
+	resp, err := AnthropicToResponses(req)
+	require.NoError(t, err)
+
+	require.Len(t, resp.Tools, 1)
+	var params map[string]json.RawMessage
+	require.NoError(t, json.Unmarshal(resp.Tools[0].Parameters, &params))
+	assert.JSONEq(t, `"object"`, string(params["type"]))
+	assert.JSONEq(t, `{}`, string(params["properties"]))
+}
diff --git a/backend/internal/pkg/apicompat/anthropic_to_responses.go b/backend/internal/pkg/apicompat/anthropic_to_responses.go
index 0a747869..fca3cf1f 100644
--- a/backend/internal/pkg/apicompat/anthropic_to_responses.go
+++ b/backend/internal/pkg/apicompat/anthropic_to_responses.go
@@ -409,8 +409,41 @@ func convertAnthropicToolsToResponses(tools []AnthropicTool) []ResponsesTool {
 			Type:        "function",
 			Name:        t.Name,
 			Description: t.Description,
-			Parameters:  t.InputSchema,
+			Parameters:  normalizeToolParameters(t.InputSchema),
 		})
 	}
 	return out
 }
+
+// normalizeToolParameters ensures the tool parameter schema is valid for
+// OpenAI's Responses API, which requires "properties" on object schemas.
+//
+//   - nil/empty → {"type":"object","properties":{}}
+//   - type=object without properties → adds "properties": {}
+//   - otherwise → returned unchanged
+func normalizeToolParameters(schema json.RawMessage) json.RawMessage {
+	if len(schema) == 0 || string(schema) == "null" {
+		return json.RawMessage(`{"type":"object","properties":{}}`)
+	}
+
+	var m map[string]json.RawMessage
+	if err := json.Unmarshal(schema, &m); err != nil {
+		return schema
+	}
+
+	typ := m["type"]
+	if string(typ) != `"object"` {
+		return schema
+	}
+
+	if _, ok := m["properties"]; ok {
+		return schema
+	}
+
+	m["properties"] = json.RawMessage(`{}`)
+	out, err := json.Marshal(m)
+	if err != nil {
+		return schema
+	}
+	return out
+}